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THE AM BULLETIN BOARD => Technical Forum => Topic started by: K1JJ on February 19, 2014, 12:02:23 AM



Title: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 12:02:23 AM
** This thread is a continuation of the 4-1000A rig built here:

http://amfone.net/Amforum/index.php?topic=33816.0

---------------------------------------------------------------------

I wonder if anyone has come across this problem or can suggest where to look?

This is Fabio II, a 4-1000A plate modulated by a pair.  If I sweep it with a sinewave from 30 Hz to 2500 Hz, the waveform looks perfect from 1% to 120% positive modulation. The crossover point looks clean and hits -100% nicely.  At 3KV, 100% modulated,  the modulator current is 400ma and the regulated modulator  screen current JUST begins to flow.  

But, when I sweep it above 2500Hz, it takes progressively more plate current as the frequency is increased and has a problem going more than 50% modulation without severe crossover distortion. At 5,000 Hz, 400 ma of plate current is barely 40% modulation.  It takes more and more current to do the same thing at a higher freq..

* But at 20% modulation, the waveform looks perfect from 30 Hz to 6,000hz.  This frequency limitation problem seems to start when serious audio power is taken.

The solid state audio driver signal looks clean going into the modulator grids, so it is after this point. I looked at the RF screens  during modulation and they appear to look OK and clean for audio.  

I have used this 1KW RCA mod xfmr in a few rigs and it has performed well in the past to 6 KHz. I am using electronically regulated mod screens and reg grids.

I am using .001 screen bypass for each RF screen.  Also have .002 for the plate choke bypass and .002 for the plate coupling caps. Maybe 500pf wud be better, but I would think that 3500Hz should not be affected so badly with .002.

Tmw I'll look at the modulator audio output at the tube plates and xfmr output.


Any ideas what could be causing this problem?

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W7TFO on February 19, 2014, 12:23:28 AM
HI Tom,

I'll take a stab at it.

>>>The higher the audio frequency, the higher the duty cycle for any given waveform.

>>>Don't forget your modulators pull grid current..gots to be able to supply that.

>>>There might be a bad turn or two in your mod iron.

Somewhere to look and/or think.

73DG


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: steve_qix on February 19, 2014, 12:55:41 AM
Hi Tom,

You said crossover distortion, and I just want to verify that you are indeed getting crossover distortion and not some other kind of distortion.

If it is crossover distortion, that indicates a change in the operating point of the modulator.  Do you have a lot of inductance in the cathode of the modulators?  This could possibly cause a problem at high frequencies which might manifest itself as crossover distortion.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 01:09:36 AM
Geez... I hope it's not the mod xfmr, Dennis.  That wud be a deal breaker.  What baffles me is that it will do 140%+ easily and cleanly at the lower frequencies.  (30Hz to 2500 Hz)     Would a shorted turn affect the highs so drastically and leave the lows alone?


Steve, the modulator filaments (cathodes) are connected to the fil xfmr and the CT to ground on key up.  I think I used .01's for fil to ground bypasses, but will have to look.  I know some say they don't bother with bypass caps there.

Do you think .002 for both plate bypass and plate coupling is too high?  I used to use 500 pf there, but these were available in the junk box.

The distortion I see (at 4Khz) begins at about 30% modulation and starts at the crossover point and then gets a ripply second harmonic look to it.  I've seen tubes that were biased too hard or with plate voltage too low do this, but I am idling them at 200ma.    I use diodes in the fil center tap as bias, so they are not sagging.

The triangle wave looks perfect at 400 Hz up to 120% modulation.  It's just when I go above 2500Hz when it all  falls apart.

If I run the audio with the D-esser working hard at about 3500 Hz and up, the audio sounds FB, but who wants to run a band-aid like that?


T




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 19, 2014, 02:12:47 AM
The .002 bypass cap seems too big.  The coupling cap or caps, how many are there? By coupling you're referring to the DC blocking cap I assume.  I would reduce them.  For a plate choke bypass cap .001 should be enough.

What size bypass caps are you using on the PA screens??  More than .001-.002 total and that can be a problem.  Add together the screen bypass caps for both tubes.  More than .002? since both screens are tied together.

I think you're using a separate PA screen supply and a screen choke to modulate the screens.  The bypass caps between the choke and the tubes should be less than .002 total.  Another problem could be the screen choke itself.  May be too much capacitance to ground or even too much capacitance through the choke back to the supply where it gets grounded by any filter caps in the supply.

If you modulate the PA screens through a dropping resistor off the modulated B+ you don't have this problem.  This may be something to try if you have the right size resistor, probably a few hundred watters are needed.

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: steve_qix on February 19, 2014, 08:46:44 AM
Very important question - what is the DC resistance (voltage / current) of the final RF amplifier?  I'm thinking it is low, but please give us the number.

The two .002 uF caps in parallel (total of .004uF - RF bypass and DC blocking together) could be an issue.  I say in parallel because the DC blocking cap usually looks into a DC short circuit to ground as a protection device (in case the DC blocking cap fails shorted) so the blocking cap appears in parallel with the RF bypass as far as the modulator is concerned.

Ok, if that is true your .004uF cap has a capacitive reactance of 3980 ohms at 10kHz, essentially across the mod transformer.  Maybe this is low and maybe it isn't.  Depends on the DC resistance of that RF amp.

Another issue *could* be, if the final RF amp's DC resistance is very low, that the mod transformer is designed to look into a much higher resistance and may have too much inductance for the DC load.  This can be verified by running the RF amplifier such that it is representing the DC resistance for which the mod transformer was actually designed, and rerunning the test.

Just ideas... no real solution yet.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 19, 2014, 09:20:04 AM
I tend to go small with bypass caps.

I have an issue with one of my rigs, it tests fine with a sine wave out past 8 KHz, sine wave injected at the same point as the mic, just at line level. With me speaking into the mic, I can not get past 2500 Hz, it rolls off badly even if I boost the highs.
The exact same audio chain works fine with other rigs.

I can boost the highs as much as I want, but they do not pass through the transmitter.

???



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: w1vtp on February 19, 2014, 09:23:47 AM
Why would a symmetrical source have a higher positive modulation level than the negative side?  Is it the result of a negative diode limiter circuit?

Male voice = asymmetrical modulation

Sine wave = symmetrical modulation

Al


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 19, 2014, 09:35:00 AM
Tom

I re-read your info, I see that you are using .001 caps for the PA screens.  They should be OK, but is there any other circuits between the screen choke and the tubes??  Like meter circuits or over current sensors or what, if anything??

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 12:31:12 PM
Good suggestions guys, thanks!

Steve, at 3500V, I am running the rig about like I have in the past and this mod iron swept FB.  I figure about 5000 ohms plate impedance.  The mod xfmr came out of a BC xmitter using a pair of 4-400s X 4-400s. So this is reasonably close in impedance the way I am running it now.    I will try substituting in 500 pf for both plate caps and see what happens.  Tron always suggested 500 pf for these class rigs, but that was 10Khz - I would have thought the effects of .002 wud not show up until maybe 5Khz at least.

You may have hit upon something else...  to the best of my memory, I thought the rig swept FB when I first built it. It would have been a red flag right away, since I always sweep them looking for perfection.  Since then I added Hall effect shutdown circuitry for both the final and modulators in the cathodes and screens. This problem may have started then and I did not sweep it afterwards, just went on to another project.  I have long leads to bring the Hall effect devices out to a central point for convenience.    I probably should have heavily bypassed the filaments after this addition.

Question:  If I have long fil CT leads can I simply bypass the fils with 1uf or even electrolytic caps to eliminate the long inductance path?   These leads also involve the metering.


Fred:  I measured the screen modulation choke to have 180 pf to ground.  There is a  .001 at each screen tube pin.  On the other side of the choke it shud not matter, cuz there is the power supply.  I have the Hall effect and metering on the PS side of the screens.

Jeff/ W2NBC suggested maybe an audio parasitic.  I already have parasitic chokes in the modulator tube plate leads. Maybe I shud add some to the grids too. But I will bypass the fils as mentioned above.  The GFZ solid state audio driver input to the mod grids looks clean when the distortion appears, so hopefully it is not that.

I'll run some of these tests today and see if things improve.  My gut feeling is there is some lump capacitance shunt or series inductance that once cut out will let the highs pass cleanly.  Hopefully not mod xfmr damage. The fact that the rig sweeps very well below 2500 is encouraging.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: steve_qix on February 19, 2014, 12:41:47 PM
Question:  If I have long fil CT leads can I simply bypass the fils with 1uf or even electrolytic caps to eliminate the long inductance path?   These leads also involve the metering.


*ANY* high inductance in series with anything (grids (DC side) screens, cathodes, plates, etc.) will impact the high frequency response.  I had the problem 40 years ago with my pair of 304-TLs modulated by 833As.  I had a relay coil in series with something (I don't remember - probably the cathodes), but it added inductance and caused a lot of high frequency distortion.  A bypass cap across the inductance fixed the problem.  1uF may not be enough. 

You'll have to calculate the capacitive reactance with respect to the impedance of the circuit being bypassed and see if it's sufficiently low for the frequencies in question.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on February 19, 2014, 12:46:01 PM
My transformer is also an RCA 1KW unit. Older though. The response is best around 1000Hz and diminishes a lot going up in frequency. The TX it is probably from used audio feedback to fix it. I've swept it but never recorded the result.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 19, 2014, 12:49:08 PM
Bypass the PA filament leads with .01 ceramic caps, you can use a few on each side plus the CT.  Can't hurt to also use one .001 ceramic on each side plus the CT.  Never use lytics they have too much inductance.  All the caps should be at the sockets.

Anything on the PS side of the screen choke shouldn't matter.

Make sure your regulated screen and grid supplies are not going haywire in the presents of strong RF.  I have seen this happen with some Lambda supplies.

Bypassing the mod filament leads probably needs more capacitance.  I don't remember bypassing my 811s fil leads.  Mod filament leads may not need bypassing but it can't hurt.

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K7MCG on February 19, 2014, 03:30:04 PM
Check the regulation bandwidth of your regulated power supplies.  They may be providing low source impedance only at lower audio frequencies.

--Chuck K7MCG


Title: A-HA!! Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 04:11:43 PM
Well, I did all the suggested stuff.  Measured the regulated supplies, bypassed the cathodes, disconnected the Hall effect shutdowns and some more stuff. NO difference. The rig still gets a very bad crossover / harmonic distortion above about 2500 Hz.


And wouldn't ya know it - the one thang I assumed  was rock solid - the solid state audio driver....   On the scope, the SS driver audio signal coming into the 4X1 grids looked fine at 5Kc and higher. But I didn't measure the DC offset that it used to bias the grids.  Under full modulation below 2500 Hz it was rock soild at 21 Volts DC no matter how heavily modulated - which is what we want.  But as I progressively went above 2500 Hz, this 21 V bias started to creep higher and higher until it went above 44 VDC!   So it appears it may be an audio parasitic that is charging up the SS driver or something related to audio instability getting into the DC bias of the audio driver.  

Steve / QIX was correct on his first post about crossover distortion being created by a DC offset of the modulator. And Jeff/ NBC  was on the right track saying it might be an audio parasitic.  More detective work is needed now.  I added some ferrite cores to the SS driver leads, but no change.  I must admit the SS driver leads are long, about 5' long. Frank warned me about this before, but it seemed to work until now.  I just don't have any room in the mod cabinet to put it, so it sits on a shelf.

More observations:

1) Disconnecting the audio negative feedback from the mod xfmr to the SS input has doesn't affect the problem.

2) Running the rig at full power or minimum power on low tap does not seem to affect the levels where the DC offset starts. It's like the RF does not affect the DC offset, rather it's based solely on audio frequency.  Maybe that's why the ferrite beads had no effect.

Just wanted to post this as I  zero into what may be the problem.  The 5 to 8Khz KC audio actually looks good up until about 30% modulation, but then when the DC offset moves up, it's like biasing the modulator into class C and the waveform goes to hell.   Got some hope and getting closer.

More ideas?

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 04:42:58 PM
A small breakthrough:

The highs get better and the DC offset drifts less when I ramp up the  +- 300V supply on the SS driver.  I brought it to +- 325VDC and now up to 4KHZ is clean.  Still see DC drift > 4Khz.    I don't dare bring it higher with fear of blowing the MOSFETS.


This is the SS driver - look at post 3 for a schematic:
http://amfone.net/Amforum/index.php?topic=23632.0

It uses 11N90 MOSFETS. I'll axe Frank how high it can be raised.  But there must be some other reason for the DC drift based on my installation / instability.


Steve, in a post in that thread you said:

"I'm absolutely *AMAZED* you don't get a parasitic with the gate zeners connected directly from gate to source without a small resistor (usually a few hundred ohms is plenty) in series with the gate, between the gate and everything else (including the zener).  I have seen parasitics without the resistors (which is why I use them in all analog designs like the class H modulators).

A tube arc-back to the grid can cascade through the circuit.  I would humbly suggest 3 things:  a) strategically placed transzorbs,   b) back biased diodes going from the output to each rail and   c) fuses (as suggested on the circuit)."


Does this sounds like what is happening?


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 19, 2014, 04:46:35 PM
Do the electrons flowing from the cathode to the plate that get stuck on the grid have anyplace to go?

It sure sounds like a charge is building up on the grids, charging up the driver.
At lower frequencies maybe they have time to leak off, but at higher frequencies they do not, like an R/C circuit?




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 05:07:18 PM
Do the electrons flowing from the cathode to the plate that get stuck on the grid have anyplace to go?

It sure sounds like a charge is building up on the grids, charging up the driver.
At lower frequencies maybe they have time to leak off, but at higher frequencies they do not, like an R/C circuit?


Good point, Brett -

I just checked and see I have 100K from each grid to ground that the SS driver feeds into. There is also a .001 from grid to ground there.  Maybe those need to be experimented with.  Maybe the resistor can be brought down to 22K to bleed off the grids better.  Or maybe it will get worse by creating a grid leak bias effect based on drive?  Or maybe the 100K is already doing that?  :-)

I will give it a go after supper and see if it has any effect...

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 07:07:09 PM
OK, getting closer...

Adding resistor shunts to the modulator grid inputs had no effect.

What did help a lot was to snip out the .001 disc bypass caps that went from modulator grids to ground. I have a bad habit of sprinkling them around thinking I will kill all RF where it shud not be.

The highs are now clean up to about 4500 HZ.

Then I found that simply by idling the modulators higher via less diodes in the fil CT, I can hold off the effects of the SS driver bias creeping up.  Now the highs are clean to about 5000Hz, but I still see the effects of the parasitic.

I looked closely at the audio signal at the grids and for the first time I see the audio parasitic mixed in with the cross-over area.  It looks to be maybe  15 Khz or so - just a guess.   I need to find a way to kill it.  Gonna try parasitic suppressors in series with the grid leads next...

The audio below 4.5 kHz is now rock solid and looks pristine up to 150% positive. Just need a little more stability above 4.5 Khz..

Any more ideas are appreciated, as they have helped me immensely so far.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 09:54:50 PM
For those of you following this exciting saga, we have some late breaking news...

Part of the problem is now identified...

One of the SS audio driver channels is putting out spikes when goosed with highs.   When running alone without the transmitter, S’s cause  random spikes twice the height of normal..  Probably a bad FET in there.  The other channel acts normally.  Tron told me last night I had some highs spitting. He always knows.

Well, anyway, I’ll get the scope in there tmw and try to find the leaky FET and replace. Hopefully this will solve the high freq distortion problem, maybe not.

In this shakedown I also found a few weak points that caused arc overs. One spot showed soot where a bolt was arcing to the cabinet. Plus a small input tuning band switch had a melted wafer. This was causing havoc by shorting the RF grids to ground... yikes!

I haven’t had a flashover in the last day despite a lot of testing into the dummy load.  I'm pretty sure the mod xfmr is OK... whew.

More tmw.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 19, 2014, 10:03:16 PM
Fun with home brew, eh?

Are the modulators running AB1 or are they taking current?

I ask because many circuits have a resistor in series with the grids, maybe to kill parasitic's?

Higher resting current is better for crossover distortion, maybe that was just masking the real problem?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 19, 2014, 10:15:02 PM
Fun with home brew, eh?

Are the modulators running AB1 or are they taking current?

I ask because many circuits have a resistor in series with the grids, maybe to kill parasitic's?

Higher resting current is better for crossover distortion, maybe that was just masking the real problem?


Hola Brett -

Yep, who knows if the overall design doesn't have a flaw, thus the challenge of a homebrew vs: commercial rig. But the commercial rigs also have human designers... :-)

Normally I run the modulators in AB1, but don't have a grid limiting resistor. The grids are directly driven by 11N90 MOSFETS.   I see maybe 10 ma of screen current, even though it is regulated stiffly. The grids draw no current in normal operation.

I think this "parasitic" problem was a masquerading fooler and simply a driver MOSFET that was breaking down.  Whenever it heard high freqs it went into arcing or feedback internally. I think the transmitter was OK all along. But that's what it takes sometimes.

In hindsight I should have done the old divide and conquer and looked at the input system - one at a time.

It's probably a bad output FET according to Frank.  We'll see tmw and hopefully the rig will sweep without problems up to 7Khz.

Yes, you know all about troubleshooting from a look at your shack... ;-)

T



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 19, 2014, 11:18:29 PM
Well, I often have problems getting something to work as it should, mostly because I overlooked something, but I have had great luck with no problems once something is done.
I built my 813 rig 25 years ago and have yet to have a problem with it, nor have I had any crap outs in anything else I built.

I do not do a lot with solid state components at high voltages though.

Just yesterday I had the first problem with my 32V3 in 30 years, the plate current meter started sticking at 200 ma.

On the other hand, I never got my last push pull 812a rig to work and gave up.



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on February 20, 2014, 02:33:14 AM
The site those things came from shows a new 1200V one with otherwise similar ratings "coming soon": STW12N120K5 1200V 12A.  It would be interesting to find out, if one of the FETs is bad, what damaged it. Also if you add more protection that would be interesting. Would it make sense to put clamps across each FET? x-employer used to sell these for lighting protection:
SMP100LC-400 or 1.5KE440A/CA (no affiliation with co.).

For now I'm sticking with the transformer drive but if I go direct to a large grid tube it will probably be 6080s. The grid of a 4-1000 is a dangerous place for SS to live.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: flintstone mop on February 20, 2014, 06:54:48 AM
Thanks Tom
for the updates and reports on your progress. It is a learning experience and gets down to the nuts and voltz of what goes on in experimenting.
At least we learn something from your replies.
Some other threads go no where and the OP never returns with a victory or defeat reply...I guess they give up and put the project on a shelf.
Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WD5JKO on February 20, 2014, 08:27:01 AM

Looking at that audio driver, capacitor C12 is used to roll off the high frequency negative feedback. I'm sure this is for a good reason, but increasing the gain as the frequency rises as part of a NFB loop can be tricky. Could this be part of the issue? What is the NFB gain reduction in DB at 1 Khz, and then 5 Khz?

Jim
Wd5JKO


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 20, 2014, 12:35:29 PM

Looking at that audio driver, capacitor C12 is used to roll off the high frequency negative feedback. I'm sure this is for a good reason, but increasing the gain as the frequency rises as part of a NFB loop can be tricky. Could this be part of the issue? What is the NFB gain reduction in DB at 1 Khz, and then 5 Khz?

Jim
Wd5JKO


Thanks for the thoughtful replies, Brett, Fred, Opcom and Jim.

Yes, I try to make these threads a learning experience, even if it makes me look like a dummy sometimes... :-)

Jim:  I disconnected the negative feedback completely and the problem remained.  At this point in time it is definitely the SS driver. I have it on the bench alone and ONE channel  is showing a strange distortion, like the bias is going crazy whenever the audio gen goes above 3 KHz.   Below 3Khz it runs fine. Exactly what the transmitter has been doing.

I replaced the phase splitter MOSFET as well as the two output MOSFETS. No difference. Checked all the zeners and resistor values - all within tolerance.  The caps check good with a cap checker but at this point I think one may be leaking and causing the splitter to increase bias, causing the distortion. I will need to sub-replace them.  

Is there a way to test for leaky caps in circuit, like using a 1 meg VOM range??

This is actually good news cuz it looks like the transmitter is fine - and it's now down to finding the bad component in the driver.  The SS driver has worked well for several years now, so the design is good.
I'll bet it got damaged when I was doing the HV shakedown run a few days ago. The rig flashed over at least five times the other night on 75M. I fixed the flashover problems, but the damage was probably already done to the SS driver..

More troubleshooting today.  I'm sending voltage readings to Frank today and maybe we can zero in on the bad stage.

T



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 20, 2014, 12:59:59 PM
If its not M1/M2, then the next things that don't like high voltage are D3 and D4, and C2 and C3....



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 20, 2014, 01:00:57 PM
Oh, and if you do not have spares, maybe swap parts from one channel to the other and see if the trouble moves?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 20, 2014, 02:52:16 PM
I don’t get it.  Today I fired it up and it’s working like it shud. Perfect sine wave  I put a heat gun to it and then froze all the components individually. No change.  Vibration  / slam test - no change.    It will be baaaack.

So maybe an intermittent somewhere?  The design was always sound.
 
Now I don’t see any change in the DC outputs when I go from full signal to none like before above 3 KHz, which is a good thang.  It stays at about 20V on both channels.  I was getting that big DC offset change before that was killing my modulator bias and causing class C distortion.
 
With the ss driver alone on the bench, I ran the sig gen from  30Hz  to 25Khz and it is solid now...  Triangle wave starts to deform below 30 Hz cuz it is capacitively coupled. Triangle looks very good up to 10 KHz and then starts to slope slightly.    The sinewave starts to fall apart below 20Hz. I will be fattening up the capacitor coupling caps to bring this lower.
 
Maybe I knocked loose a solder blob somewhere, I dunno...  Gonna get out the magnifying glass and inspect every solder connection carefully.  

I’ll let it run on the bench and beat it up all day to see what happens.  It would be nice if this really fixes the problem.
 
T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 20, 2014, 05:34:19 PM
So far the SS driver is running on the bench FB with no crap outs.   We'll have to wait until the big rig is back on the air for more excitement.

While on the bench I padded the five 600V coupling caps from 2.2 to 5 uF each.  Big difference in the triangle and sine wave ability to hit 10 to 20 Hz cleanly.  A clean audio driver that will handle required load power is as important as a good mic, maybe more so.  I suppose now the limiting factor is the modulation transformer.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on February 20, 2014, 07:56:09 PM
  I suppose now the limiting factor is the modulation transformer.


T

Road trip? Bring a truck.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 20, 2014, 09:46:56 PM
Road trip? Bring a truck.

Ya never know, Dave.  Vely appealing.  Finding the room to shoehorn it in is the problem, of course...   ;)

Well, tonight I gots the noove to fire the rig up with the SS driver.  I worked it like a rented mule. It behaved like Tiny Tim - good as gold and better.  It swept like a textbook sinewave - flat from 10 Hz to about 6500Hz before started to roll off. I couldn't be more pleased.

Because I added bigger coupling caps to the audio driver, IE, less phase shift, I was able to increase the negative feedback a few more dB.   I hear a noticeable difference in quality with the NFB in vs: out.  The reduction in gain seems to be at least 10dB or so.

After running triangle tests at 50, 400Hz  and 1K, I was satisfied the overall system is playing well again. The only problem I see is the roll off at 6500 where a slight harmonic distortion begins. It seems to need more and more audio drive to hit 100% above 6500Hz. I'm wondering if it's the .002 coupling and .002 plate bypass caps I have in there.  With the tank impedance at about 5K, I can see a meaningful path to ground above 6500Hz.  Think I'll try 500pf for each.  I have a few caps somewhere around.  Could also be the mod transformer topping out too. I don't plan to run bandwidth more than 5-6Khz, but it is still important to have a clean headroom so the audio has room to roll off cleanly. Even if it's down 20 dB at 7500Hz, there is still a potential for splatter if the waveform turns to crap up there.

So, back to more tests...

T

Fabio II in test mode:


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: steve_qix on February 20, 2014, 11:08:41 PM
The combination of those 2 caps represents about little under 4000 ohms at 10kHz.

That's probably enough to mess with the high frequencies a bit!


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 20, 2014, 11:55:30 PM
In this transmitter, the effect of the total screen bypass capacitance, on the rolloff of the higher modulation frequencies, is likely to be greater than the effect of the capacitance bypassing the modulated B+; but, as has been already stated, both are important.

Stu


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 21, 2014, 12:33:05 AM
In this transmitter, the effect of the total screen bypass capacitance, on the rolloff of the higher modulation frequencies, is likely to be greater than the effect of the capacitance bypassing the modulated B+; but, as has been already stated, both are important.

Stu

Interesting...

Stu, I reduced the total screen bypass capacitance of the two tubes to 1000 pf. (500 pf per tube)

Right now I'm reducing the plate bypass to about 500 pf and the same for the coupling.  I see that 1000 pf at 7000 Hz is several times the 5K tank impedance, so that should be good.

One thing that concerns me:  I just measured my plate choke and see it's about 303 uh.  At 7Khz audio freqs, this is under 20 ohms, which is good.   But at 1.9 Mhz RF, 303 uh is only 3.6 K. At 3.8 Mhz is it only 7.2K.    With a 5K plate impedance, the choke seems  too small in value.   However, on 75 and 160M, the amplifier tunes sharp and I feel no heat coming off this plate choke.  I always thought that we like to see  the plate choke inductive reactance to be X 5 to 10 times the tube impedance.  

Essentially, on 160M don't we have a 3.6K path across the tube, shunted to ground thru a 20 ohm capacitor to ground at the end?  I would think that would draw a lot of power off the 5K tank circuit.   Or is the power factor of inductance and capacitance the key here...What am I missing?


BTW, 500 pf at 1.8 Mhz is  167 ohms - at 3.9 Mhz this is 81 ohms.  Even this capacitive reactance at the bottom of the plate choke seems a little high for bypassing a 5K circuit, or is it?

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 21, 2014, 09:49:34 AM
Tom

This is my current thinking/understanding of the issues you asked about:

1. With respect to the modulated B+

In this traditional type of modulator... with the output resistance of the modulator matched (via the modulation transformer) to the modulation resistance of the output RF stage... the equivalent circuit of the modulator itself is a voltage source in series with a resistor whose value is equal to the modulation resistance of the RF stage. The Norton equivalent of the combined circuit (modulator + the modulated RF stage) is a current source in parallel with a resistor whose value is 0.5 x the modulation resistance of the modulated RF stage. Therefore, assuming a 5k ohm modulation resistance,  we want the total capacitance to ground to have an impedance at 10kHz that is higher than -j2.5k ohms in order to minimize high frequency rolloff.

[Note: when using a backward-connected output transformer in conjunction with a modern, very low output impedance audio amplifier, the acceptable impedance of the total capacitance to ground can be less than 0.5 x the modulation resistance of the modulated RF stage]

2. With respect to the screen voltage:

Note that it is the modulation of the screen voltage that modulates the plate current. The modulated plate voltage has much less direct effect on the plate current. The real purpose of modulating the plate voltage (with a tetrode or a pentode) is to increase the efficiency of the output RF stage. In a self modulated configuration, the modulation of the plate voltage causes the screen voltage to be modulated... and it is the resulting modulated screen voltage that modulates the plate current.

For self modulated screen voltage using a choke between the screen power supply and the screen: you want the impedance of the bypass capacitor, at 10kHz (i.e. the high audio frequency screen-to-ground impedance), to be larger than the impedance of the choke at 50Hz (i.e. low audio frequency screen-to-ground impedance). For example, a 10 Henry choke has an impedance of j3142 ohms at 50Hz. In this example, you would want the total screen bypass capacitance to be smaller than .005uF.

When using a resistor from the modulated B+ to the screen (or from the unmodulated B+ to the screen), you want the impedance of the bypass capacitor at 10kHz to be larger than: the screen voltage at carrier / the screen current at carrier.

3. With respect to the plate choke

If the choke's impedance at the lowest RF frequency of interest is not high enough, this will have an effect on the efficiency of the RF stage that can be obtained at that RF frequency. The relevant impedance to compare to, for the purpose of evaluating the plate choke's efficacy in delivering a constant current to the RF stage during each RF cycle, is the impedance looking into the output "tank" circuit... which is typically 0.5 x the plate voltage / the plate current for a class C amplifier. That is: the question you are trying to answer is: how long is the "L/R" time constant, when R= 0.5 x the plate voltage/ the plate current. If this time constant is less than 1/the lowest RF frequency of interest, then the efficiency of the amplifier will begin to drop. Putting aside its role in providing a current source to the plate of the amplifier, the effect of this inductance on the tank circuit will be a small change in the setting of the tuning capacitor at resonance


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 21, 2014, 11:26:59 AM
I have a question along these lines.
If you use a big wire wound pot to drop the screen voltage from a fixed (but adjustable) source, should you bypass around the resistor for audio?

Being a wire wound resistor, it will have some inductance.

Would there be any interaction with a choke in series with the screen?
If using a resistor, do you need the choke?



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 21, 2014, 12:40:52 PM
Good question, Brett.

I suppose the sure way to do it is sweep it -  try bypassing and not bypassing experimentally.  Much will depend on the specific circuit, loads and interactions of the components.  I think fine tune tweaking ala Dean/ WA1KNX's screen technique is good.  Maybe Stu will have some comments...


Thanks for the info Stu.  I will rewind my plate choke with thinner #24 enamelled wire and bring it up to 1.5 mH.

Question:  I believe in testing a plate choke for series resonance installed IN the circuit.  What would be the proper procedure to do this... connections for the RF sig gen and scope?

I also have an MFJ 259B analyzer - could this be of help?

T




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 21, 2014, 12:52:47 PM
Brett

This resistor will add impedance at all audio frequencies between the screen and ground (the output of the power supply looks like ground for audio frequencies of interest), which is what you want for self modulation. The resistor's inductance is also helpful, in this regard, but probably has a negligible effect at all audio frequencies of interest.

You should not put an audio bypass capacitor across the resistor.

If this resistor has a large enough value (i.e. equal to the impedance of the choke you would be using, at 50Hz audio frequency), then you probably can leave out the choke, and still get the self modulation of the screen that you need.

The key is that you want additional negative charge (electrons) to accumulate on the screen when the plate voltage is modulated downward ... therefore modulating the screen voltage downward as well. Likewise, when the plate voltage is modulated upward, negative charge will be removed from the screen (i.e. some of the electrons that have accumulated on the screen will leave the screen, and flow to the plate), and the screen voltage will be modulated upward as well (less negative charge on the screen). In order for this self-modulation of the screen to work properly, you don't want the varying (modulated) charge on the screen to discharge too quickly to ground via a resistor or an inductor (or both) between the screen and ground. Therefore, the sum of the impedances of the series inductor (choke) and the series resistor between the screen and ground (again, the power supply output looks like ground for audio frequencies) must be sufficiently high for all audio frequencies of interest.

Stu

I have a question along these lines.
If you use a big wire wound pot to drop the screen voltage from a fixed (but adjustable) source, should you bypass around the resistor for audio?

Being a wire wound resistor, it will have some inductance.

Would there be any interaction with a choke in series with the screen?
If using a resistor, do you need the choke?




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 21, 2014, 04:00:13 PM
Tom

Check out the various articles on the web regarding the design of RF plate chokes for high power amplifiers. You may decide that designing/constructing a viable plate choke with a higher inductance (1500uH v. 303uH) will be difficult, because of resonances, that creep into the design, that are associated with distributed capacitance.

You might be better off sticking with what you have.

Stu





Good question, Brett.

I suppose the sure way to do it is sweep it -  try bypassing and not bypassing experimentally.  Much will depend on the specific circuit, loads and interactions of the components.  I think fine tune tweaking ala Dean/ WA1KNX's screen technique is good.  Maybe Stu will have some comments...


Thanks for the info Stu.  I will rewind my plate choke with thinner #24 enamelled wire and bring it up to 1.5 mH.

Question:  I believe in testing a plate choke for series resonance installed IN the circuit.  What would be the proper procedure to do this... connections for the RF sig gen and scope?

I also have an MFJ 259B analyzer - could this be of help?

T





Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 21, 2014, 04:28:24 PM
When I built my 4x150 rf deck, I used a plate choke out of an old FT102, and when I added a 2nd tube, the choke wanted to melt down one of its pie windings (just one of 5).
I switched to a very old one (4 pie) and it works fine.
Its likely just pot luck with all the variables...

Thanks for the info Stu.


Title: ANOTHER A-HA! Re: What is causing this audio highs attenuation?
Post by: K1JJ on February 21, 2014, 06:45:09 PM
Breakthru!  Just what I had hoped for...


For the last test I ran yesterday, both plate bypass and coupling caps were 2000 pf and I saw problems starting at 6000 Hz. It took an excessive 800 mA of modulator current to hit 100%.

I changed the plate choke bypass cap from 2000 pf to 500 pf.  The coupling cap is still 2000 pf.

Now with the 500 pf in place, I can modulate cleanly to 7,000 Hz. At  7500 Hz it takes only 600 mA. to hit 100%.  The waveform starts to get harmonic distortion starting at about 7000.  This is quite a nice step in the right direction..  (It takes only 400 ma to hit 100% at 1Khz)

Now I will change out the 2000 pf plate coupling cap for 500 pf and see what happens.  I'm hoping it will modulate cleanly to 8000 Hz.   At least the modulator is not straining like it was above 4500 Hz before.

I think the modulator was getting shunted by the 2000 pf, thus the screen, no matter how clean it was, was not getting its cues from the plate due to its distortion.

I used to run 500 pf in both spots in the past, but I had such nice 2000 pf caps available, I just couldn't resist using them.

Bottom line is use the minimum values you can in those positions for AM plate modulation. For a linear amp, it doesn't matter and might as well go large.  I haven't tried it up on 160M yet, but the Tron has recommended 500 pf to cover 160M in the past for a 4-1000A final.  Of course, if the tube impedance is lower, it will require larger values.


T



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 21, 2014, 08:21:07 PM
I wonder what the plate cap will do.
I have 1000 pf at 15kv in the 3x4D32 rig, 1200 volts, 300 ma, 4000 ohms.

I also have a 1000 PF in the 4x250 rig, about the same plate impedance.

I thought larger was better there for some reason.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 21, 2014, 10:25:43 PM
Brett

At audio frequencies: the path through the pi network's inductor, and from there to ground via the safety choke (as an example), has essentially zero impedance. Therefore the plate coupling capacitor is (at audio frequencies) essentially a bypass capacitor to ground. The same is true for most other types of output network configurations.

Stu


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 22, 2014, 02:33:08 AM
Brett

At audio frequencies: the path through the pi network's inductor, and from there to ground via the safety choke (as an example), has essentially zero impedance. Therefore the plate coupling capacitor is (at audio frequencies) essentially a bypass capacitor to ground. The same is true for most other types of output network configurations.

Stu

Yes, the path through the tank inductor, thru the loading cap certainly is low impedance.  Especially the last safety RF choke shunting the tank output to ground.  Using a small coupling cap will minimize all that.


Question:  If an RF stage is series modulated thru its cathode, (like a tube PDM rig) does the plate coupling and RF choke bypass caps have the same shunting effect on audio as modulating thru the plate choke?    

In the early stages of testing my tube PDM rig, I reduced the coupling cap from 3900 pF to 500 pF and thought I saw no difference. But this is when I had filter problems, so not sure what wud happen if I tried it again.

T

Check out the new coupling caps mounted on a Plexiglas shelf. They are actually two 200 pF in parallel = 400 pF @ 14KV.   They are super high quality that I picked up at Deerfield for $5.  I used another set for the plate bypass.  Never thought I would use them anywhere.  Plate modulation demands large, good quality coupling caps or they will sing or even explode sometimes.

I still need to finish cutting that bolt and run a new sweep test tmw..




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: w8khk on February 22, 2014, 07:48:39 AM

Check out the various articles on the web regarding the design of RF plate chokes for high power amplifiers. You may decide that designing/constructing a viable plate choke with a higher inductance (1500uH v. 303uH) will be difficult, because of resonances, that creep into the design, that are associated with distributed capacitance.


There is an article on pages 30-33 of the May 1954 issue of QST by Vernon Chambers, W1JEQ,  entitled "R.F. Chokes for High-Power Parallel Feed" An answer to the Multiband Choke Problem.  This article details the series and parallel resonance issues and provides some design and testing insight.  I studied this article prior to winding the choke for my 80M - 10M dual  4-400A amplifier in 1968.  The article is downloadable from the ARRL Periodicals Archive.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 22, 2014, 09:32:08 AM
Tom

I think the answer is no.

Looking into the RF stage from the cathode (cathode and grid voltage being modulated up and down, at audio frequencies, by the same amount with respect to ground), I think the tube looks like a resistive load to ground (from the perspective of the modulator), whose value is the normal modulation resistance of the RF stage. Audio frequency bypasses from the RF stage's plate(s) to ground (e.g. via the plate coupling capacitor) should be irrelevant... since the audio frequency path from the RF stage's plate to ground is already essentially 0 ohms (via the plate choke and the plate B+ supply's output).

In this case, you would be concerned about any RF bypass capacitors from the RF tube's  (tubes') cathode(s) to ground...unless they are part of the PDM output filter.

Stu


[/quote]


Question:  If an RF stage is series modulated thru its cathode, (like a tube PDM rig) does the plate coupling and RF choke bypass caps have the same shunting effect on audio as modulating thru the plate choke?    

T


[/quote]


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 22, 2014, 12:53:25 PM
Tom

I agree with Stu,  the plate choke bypass cap doesn't matter with a PDM type rig.

I did mention, two pages back, the caps should be reduced. Seems like it is having a positive effect on the audio.

OK that's great,  now here's another one of my award winning tips.  Before you cut that long bolt, shut the rig off. ;D

Fred


Title: Near Completion - Re: What is causing this audio highs attenuation?
Post by: K1JJ on February 22, 2014, 03:44:24 PM
Yep, youse was right, Fred.  Those caps are certainly a bottleneck.  I got an email from Jeff/ NBC who said he runs 250 pf in both positions. His sweeps up to 12K.  

OK on the PDM series modulated audio NOT being affected by the caps, Stu, I will keep what I have then.


ANOTHER BREAKTHRU:

Today running 400 pF for both coupling and bypass, I saw another improvement.  Now the modulator plate current remains down at 400 ma @ 100% modulation even at 7000 Hz!   The top end is about 8K before I see some harmonic distortion creep in when I go above 100%. The modulator current hit 600 ma at 100% when doing 8,000 Hz.  This tells me there still is a limitation in there - maybe the mod xfmr.  Maybe I'll disconnect the modulator lead and measure the total capacitance going into the RF final. That should tell me where it's coming from. Hopefully I will see no more than 1000 pF or less total into the rig.  (Coupling + bypass + stray capacitance)

The difference on voice is quite dramatic for highs. The S's don't spit anymore and the CH's are much cleaner.  I rolled off my hi-cut to about 6500 Hz and it limits well.  

The triangle looks pretty decent at 50 Hz and near perfect at 100 Hz.   The triangle at 4K is now very good - it used to have a slope up there before.

I added in some more negative feedback and see no tendency to take off.  The RF final seems stable with all the changes and haven't had a flashover for days now.

So gonna button it up for now and run it on the air to see what breaks.  Need to install a Hall effect shutdown circuit in  the Dual Quads tube PDM rig, so that will be the next focus.

Thanks for all the help, guys. It probably prevented me from getting frustrated and tearing it down. I was tempted to build up a big PDM tube rig, but now that Fabio II is running so well, I will stick with it.

Seez ya on the airwaves.


T


Title: On-Air Recording - Re: What is causing this audio highs attenuation?
Post by: K1JJ on February 22, 2014, 09:54:41 PM
Jeff/ W2NBC made a recording of the 4-1000 rig tonight running at about  75% power. No negative peak limiter yet.  Have a little acoustical feedback from the mod xfmr being out of the cabinet - and occasional arcing from the spark gaps.  

Notice the high end sounds reasonable now.

It's getting closer.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 23, 2014, 12:52:34 AM
Tom,

I have been thinking about the plate blocking caps.  I was never 100% sure (in my mind) that the cap has anything to due with the freq response of the xmtr.  All this discussion about the audio being shorted by the RFC that's on the pie network output doesn't make any sense.  The cap is not really passing an audio signal but a RF signal that varies in amplitude at some audio frequency.  The RF signal is not being shorted by the RFC on the output.

This is not true when it comes to the bypass cap on the plate choke.  This cap is seeing an audio signal (modulated B+) and it has an effect on the audio highs.

Maybe we can get some other thoughts and comments on this point.

What does have an effect on audio bandwidth of the xmtr is the Q of the tank coil.  Too high a Q and the outer limits of the bandwidth are reduced.

Fred

I thought about this some more,  maybe the blocking cap does affect the audio highs,  not really sure.

Back for a third comment,  I guess the blocking cap does affect the highs after all.  The bypass cap and the blocking cap are both to ground only separated by the plate choke which has little resistance at audio frequencies.

It's after 2AM, maybe it's time for me to go to bed.  But, when I wake up later, I'm sure I'll have more brilliant ideas.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on February 23, 2014, 03:03:37 AM
This topic is just about like a tutorial!

You are using 160M? then your blocking cap may be large.. but what you said, that the plate choke may pass the audio right through to a big blocking cap. I have 2500pF there. I only use 80M and up, and have a HF roll-off with high modulator drive needed to compensate and high mod plate current results. Maybe it's the same kind of issue. My audio path through the choke, and then to GND would be a 2500pF blocking cap (five 500pF door knob caps in parallel) and an RF choke for 'dc on the output' safety. When the modulator was tested into a resistive load of 7000 Ohms with the RF side dark, the roll-off was not an issue. When the transmitter operates, it is an issue, despite people say it sounds fine a spectrograph shows it up. Could it also stress the modulation transformer?

Fabio's RF is clean I assume. If you do not mind, please tell about Fabio's plate suppressors so I can make one just like it for my single 4-1000.
I've never totally got rid of a parasitic and its cause may be the original home brew parasitic suppressor I'm using.
It's not a harmonic, it's something in the plate area with resonances at ~100 and ~165 Mhz.
The LPF at the output absorbs it OK but it's visible on the scope as barely visible little "notchies" on the RF waveform, and so it's too much and it drives me crazy because it is not perfect.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: flintstone mop on February 23, 2014, 07:55:39 AM
Tom.......where can we hear all of this beautiful TX audio?? And what times? I don't waste time on 75M and the zoo.
Fred MOP radio


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 23, 2014, 10:30:55 AM
Opcom,

2500 is probably is way too much.  Try removing at least two of the 500s and see if things improve.  Also what cap are you using to bypass the plate choke?  This is a plate modulated rig that you're referring to??

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 23, 2014, 01:13:07 PM
Opcom,

Yes, I agree with Fred. Take some careful audio sweep measurements and then disconnect a few paralleled caps. Do both the plate bypass and coupling caps to see what it does.  I'll bet 500 pF is all you need for both places with one tube.  You will need less C than my rig.  Look at the last pictures I posted for an idea of the parasitic suppressors I used. I just wound 5 turns with 50 ohms and that was it. 500 pF at each screen pin. I loaded each grid with a 5K with a .001 cap in series to ground for more stability. It is neutralized too.


You menti0ned that your modulator is drawing big current to handle the high freqs. That is exactly the symptom I had and it went away when the caps were reduced.  I'll bet you are seeing some waveform tilt and harmonic distortion when you run the modulation up at those high freqs.  It is definitely a strain on the whole modulator system.  It's really putting a big cap across the mod xfmr, isn't it?

Today I'm working with Frank / GFZ and his SS driver. I see a very slight slope in the triangle waveform coming out of the driver itself. He doesn't see it on the simulation so it is in my implementation here. Gonna try some different things.

The bottom line - when striving for pristine audio with a tube rig, it's solving a combination of many small details until they all add up in one positive direction.   As W2NBC told me, "refine, refine, refine."    (Don't tear it down)


Fred/ Mop:  I'm just on 75M for now. Plan to try 160 and 40M once things are running right, Catch ya then.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on February 23, 2014, 08:56:41 PM
With all that energy dumping into bypass caps when do they start smoking?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 23, 2014, 11:33:31 PM
With all that energy dumping into bypass caps when do they start smoking?

I wondered the same thing.  I didn't remember to give 'em a feel after a long carrier at 8Khz.

Is the capacitive power factor the reason?   I'll bet if the capacitive reactance was instead pure resistance, it would have smoked by now...  ;D


Added a Hall effect shutdown circuit to the Dual Quads PDM rig today. Nice - if it goes above 800 ma, it shuts down as fast as  a Monty game on Broadway.


Still thinking about an NPL for Fabio II. I don't know if it's smart to be leaning on an NPL with that mod iron. Might be better to run a fat carrier and do 110% positive and call it a day.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on February 23, 2014, 11:45:12 PM
Time for one of those IR thermometers.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on February 24, 2014, 04:26:30 AM
Attached is a schematic simplified for this audio stuff.

Screen is bypassed at the socket with 2000pF. (two 500pF on each SG pin)
  It's fed DC by a 8H Chicago choke and 2K dropping resistor from a 0-700V supply having a 10uF oil cap at the output.
  I seem to have enough screen modulation voltage. I have not tried to check its phase relative to plate modulated voltage phase.

Plate choke is a Raypar RL-100 (never gets hot)
  (96uH, 18KV, 3KW AM rated, self-res 22.5 to 26.5MHz)

Plate suppressor is 4 pcs. 10R 10W Koolohm NI
  (in parallel with Burnstien Applebee 17B684 suppressor coil)
  So, I could exchange that for the 5T coil with a 50 Ohm resistor across it? I could not really see the resistors there but will look again. I see yellow things, they looked in series.

I forgot about the big caps across the mod transformer.
  There is a .003 across the primary and .001 across the secondary.
  Should one of them go, or both? I thought they are needed to protect the transformer.

Yes the DC blocker for the plate is 2500pF. It's five of those TV-type doorknobs.
  I was reading that they handle RF current poorly and can heat up and fail.


It is effectivly a 6500pF load. 0.0065uF. 
  The load presented to the modulator is 7500-9500 Ohms, depending. I'm using only about 300mA @2500-3000V.
  If I did the math, those caps all add up to a 7000 Ohms reactance at 3500Hz.

OK I never noticed how hot any of them get, I usually stay away from them! An IR thermometer may e hard to use safely because of how it focuses.




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 24, 2014, 08:14:13 AM
I never run caps across the mod transformer, and always removed them from any rig that had them.
2000 pf is really high for a screen bypass cap, there go the highs....

They really wanted that rig narrow I guess.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on February 24, 2014, 09:37:59 AM
For a capacitor or an inductor, the current is 90 degrees out of phase with the voltage*. The average power (averaged over each cycle of AC) flowing into the capacitor or the inductor is zero*. [This is also described in the AC power literature as a situation where the power factor is 0.]

During half of each cycle, power flows into the component, and is stored (not dissipated as heat) in the electric field of the capacitor or the magnetic field of the inductor. During the other half of each cycle, this stored energy is returned back to the source as power flowing out of the component.

*Ignoring dielectric losses in the capacitor, and resistive losses in the wires of the inductor.

Because of dielectric losses in the capacitor, the voltage across the capacitor will produce some in-phase current (modeled as a resistor in parallel with an ideal capacitor)... which will produce heating... possibly enough to destroy the capacitor in a dramatic way. Because of resistance in the wires of an inductor, including the skin effect, the current flowing through the inductor will produce some in-phase voltage drop across the inductor (modeled as a resistor in series with an ideal inductor)... which will produce heating of the inductor.

Stu


With all that energy dumping into bypass caps when do they start smoking?

I wondered the same thing.  I didn't remember to give 'em a feel after a long carrier at 8Khz.

Is the capacitive power factor the reason?   I'll bet if the capacitive reactance was pure resistance it would have smoked by now...  ;D


Added a Hall effect shutdown circuit to the Dual Quads PDM rig today. Nice - if it goes above 800 ma, it shuts down as fast as  a Monty game on Broadway.


Still thinking about an NPL for Fabio II. I don't know if it's smart to be leaning on an NPL with that mod iron. Might be better to run a fat carrier and do 110% positive and call it a day.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 24, 2014, 12:12:45 PM
The rig sounded good to me, but then most sound good to me.
I could not hear the arc's.



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on February 24, 2014, 12:28:43 PM
A perfect capacitor would not have any problems. Real capacitors shunting several hundred watts of steady tone are going to show some distress.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 24, 2014, 01:21:44 PM
Opcom,

Yes, sweep the rig and pull out the mod xfmr caps - and start reducing the others you mentioned.

I've had a VERY interesting conversation with Bruce / W2XR.  He feels that tetrodes do not do well with capacitive loads, while triodes do better.   I might run some THD tests as -is (tetrode connected) and switch to triode-connected (grid tied to screen and driven) for the same tests.

Anyone have an opinion of running 4X1s tetrode or triode config?  I'm hoping I will get more voltage swing to work the NPL and maybe see it sweep above 8KHz when triode-connected.

Check out these excerpts from Bruce. VERY interesting comments and think they can help others make a decision too. He gave his permission to post:

---------------------------------------

Bruce's Comments below:


I have been following this thread with considerable interest.

I think the bulk of the problem you are experiencing here may have to do with the tetrode modulator tubes driving a highly capacitive load impedance. Tetrodes do not swing voltage well across a capacitive load; triodes are much better in this regard. That is one key reason as to why I have only used triodes (833As) in my rig over the past 35 years. I have 1000 pf as the RF plate choke bypass, and 680 pf as the plate blocking cap, and my rig can easily modulate linearly to 12 KHz, and with no excessive modulator plate current, etc. I may well reduce the RF choke bypass to 500 pf, and see how much of an improvement results. I suspect the improvement will be minimal in my case.

Reducing the values of the RF decoupling/bypass and blocking capacitance in the tetrode-modulated HPA should result in a substantial improvement in the high frequency response, and a corresponding reduction in the HF distortion, as the modulator will not to be driven as hard to maintain a given percentage of modulation above the point where the capacitive load starts becoming difficult.

The other advantage I like of using triodes as modulators is the reduction in the possibility of over-voltaging the primary of the mod xfmr in the event of a short-term/transient unloaded condition. This is one reason why most triode-modulated BC rigs did not use protective spark gaps across the mod xfmr, whereas it was almost universal with tetrode or pentode-modulated transmitters.

I am of the opinion that one of the best choices of modulator tube for 1000 watt plate modulated service is the 3-500Z; zero bias, class AB1 operation (no driver xfmr or audio power required), and extremely linear operation. Jeff/NBC is of course building a rig around this tube. The downside is the availability of finding good, serviceable Eimacs nowadays.

Your rig sounds absolutely superb, and the audio quality is the benchmark to which I strive for in my plate-modulated rig. I'd really enjoy trading notes with you sometime about your audio processing, etc. I'm back on the radio, typically Saturday afternoons, on 3705 KHz.

All the Best & 73,

Bruce, W2XR
-----------------------

Interesting on your take of tetrodes and capacitance. I never knew that. It makes me wonder if I shud put the 4X1's into triode. (grid/ screens tied together) I ran it that way before but the Tron said it wud sound better in tetrode, so I did. I personally never liked the sound of tetrodes for any kind of audio amplification application, when compared to low-mu/low plate resistance triodes, such as the 833A, but your rig does spectacular! This may well be an example where the technical solution may not be optimum, at least in my opinion, but never knock success. When you designed this rig around tetrode modulator tubes, you did it right; the screen supply of any tetrode or pentode linear amplifier must absolutely be very tightly electronically regulated, to minimize the distortion inherent in this type of tube. By their very nature, tetrodes and pentodes have a much higher plate resistance when compared to triodes, and this in part accounts for the difficulty they have linearly swinging and developing voltage across a substantially capacitive load.
>
> I know that 813s triode-connected work absolutely superb. No question; they were triode-connected, this does reduce the Rp of the tube in this type of operation. That is the only way I personally would run a tetrode as an AF power amplifier; triode connected. But personally, I still think a true (not triode-connected tetrode or pentode, also referred to as pseudo-triode operation) directly-heated triode (DHT) with a low Rp is the best, most versatile, and linear power amplifying device you could wish for. 810s are not may favorite either; they may be a DHT, but their Rp is a little too high for my taste. The same is true for many of the Eimac triode transmitting tubes, including the 3-500Z.

One other really nice thing about triodes in modulator service is the ease of varying the modulator and HPA B+ to lower the RF output. With tetrodes, you hit a brick wall due to the screen voltage, and if you lower the AF output by virtue of reducing the B+ too much, the distortion increases radically. That is one reason why some tetrode BC rigs resorted to stupid (but necessary) means to reduce the RF output (i.e. 1000 watts to 250 watts, etc.) by putting a high-power audio attenuator network between the modulator tubes and the HPA load. With triodes in the modulator, you simply change the tap on the plate xfmr (or variac the AC input down), and all is well.

Did you ever get GFZ's driver going there?  Not yet, but FC and I are in touch about this. I still have to take the scope to one of the push-pull grids of the 833A modulator deck, and measure the positive voltage swing for Frank. My gut feeling is the existing GFZ design won't provide enough linear voltage swing. I also have the 4x 807 DC-coupled cathode driver unit from a Gates BC-1G I can use too, to remove the existing driver xfmr and massive push-pull 845 audio driver I currently use. The p-p 845 DHT triodes, 6 dB of NFB,  and the 23 lb Peter Dahl replacement for the original Gates BC-1F driver xfmr still sounds extremely good and clean to me. The issue is occasional overshoot with this set-up, and my desire to provide even tighter control of the modulation with my Orban Optimod 9100A.

I'm inspired to give the triode connected 4X1s a try.    If I had a pair of 3-1000Zs I use them in a heartbeat, but I don't.

Now that would be a great tube to try in this application, but I am not sure as to what the Rp of a class AB1 3-1000Z may be.       But the 4X1 is so rugged even *I* have never blown one...:-)


I will run some THD and IMD tests on my SDR receiver as-is. Then configure them to triode and test again.  I've used triode-connected several times in the past and liked them.

I'd really like to see the test results of the two different configurations. The load line of the triode-connected tetrode vs. that of a true triode will be different, and the true triode will prove to be more linear. I have no idea as to what the Rp of the triode-connected 4-1000A will be, and I'm sure that Eimac does not publish this information in the tube data sheet, nor would I know how to calculate this. Perhaps this calculation is available in the Radiotron Designers Handbook. Ideally, you need to know the tube's Rp so you can properly select the correct impedance ratio of the modulation xfmr.

I'd like to start a thread that discusses triodes, triode-connected tetrodes and tetrodes on the BB.  Kinda as a learning experience for those who will eventually build some of these rigs.  Is it OK if I cut and paste only the technical paragraphs of your last two PMs and post within my thread?  I will credit you as the author of course.    It might get a few more guys to contribute their own similar experiences... By all means, and feel free to do this.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 24, 2014, 04:10:38 PM
I have tetrodes and triodes as modulators, and can hear and measure very little difference between them.
But my stuff is not in the same class as most, garbage audio equipment, crappy microphone, etc.


But I remember my push pull 812a rig modulated by 811a's was a smooth operating and sounding rig.

I have a bunch of 813's, maybe a pair set up as triode modulators is in my future.
Is there any info about drive requirements and power output?

I have four 100TH's in a modulator, but they take a lot of drive and want very high voltages (to lower the drive needed).

Something like 900 watts of audio at 3000 volts though...

I think its very odd that despite going through everything mentioned in this thread, my 32V3 has the widest response with voice. All the rigs use the same audio drive setup...
I can get them up to 2750 volts before I run out of bias voltage.





Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: VE3AJM on February 24, 2014, 05:42:28 PM

Check out these excerpts from Bruce. VERY interesting comments and think they can help others make a decision too. He gave his permission to post:

---------------------------------------

Bruce's Comments below:

I am of the opinion that one of the best choices of modulator tube for 1000 watt plate modulated service is the 3-500Z; zero bias, class AB1 operation (no driver xfmr or audio power required), and extremely linear operation. Jeff/NBC is of course building a rig around this tube. The downside is the availability of finding good, serviceable Eimacs nowadays.


I've never seen any tube data for the 3-500z running in class AB1 modulator service. Does anyone have any information on this.

Al VE3AJM








Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on February 24, 2014, 05:57:33 PM
A 3-500Z would put out very little in ab-1.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 24, 2014, 06:05:10 PM
AB2  -   At 3KV with 244 mA grid current a pair  will put out 1400 w of audio:

http://www.g8wrb.org/data/Eimac/3-500Z.pdf


But AB1.... as Dave mentioned, AB1 is a very stingy mode for output power.  That's the reason I started with the 4X1s as modulators thinking in AB1 they would have the headroom.  But even they are close to maximum because the grid current wants to flow early.

Though, I think maybe the 3-500Zs in AB2 might be a good choice cuz they were designed to run in GG linear service (with grid current) and are pretty clean tubes that way.  Though, I've never seen a tube in linear service that didn't degrade in THD/ IMD when grid current started flowing. It's just the nature of the beast.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on February 24, 2014, 11:31:06 PM
I never run caps across the mod transformer, and always removed them from any rig that had them.
2000 pf is really high for a screen bypass cap, there go the highs....

They really wanted that rig narrow I guess.

It was built in the days of SSB promotion where "narrow" was the happy fun goal.
The spectrum received on an SDR set on 16KHz BW is attached. I was only recently aware of this.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on February 25, 2014, 12:04:48 AM

But AB1.... as Dave mentioned, AB1 is a very stingy mode for output power.  


T

I think Al AJM was thinking about tetrodes. AB-1 plays a lot different with those : )


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 25, 2014, 12:16:19 AM
Here's some more information gleaned from a sharp amplifier engineer regarding triodes / tetrodes,  negative feedback and more.  I axed about switching my 4X1 tetrodes over to triode-connected config, etc.   I know some of you will find this info as valuable as I did. I have decided to stay with what I have - AB1 tetrode modulator operation with NFB.

-----------------------

His comments:

[axed about triode-connected compared to my current tetrode config?]
AB2 or B will require a really good low impedance driver and with the same 10 dB of NFB, probably be worse in distortion.   The gain in output power from the rig with AB2 or B is not much practically speaking since you already have 2X in plate dissipation in the modulator as in the final.  
 
[Asked about running bigger audio]  BTW, I have come to the conclusion that asymmetrical audio has no advantage over symmetrical audio and will always sound more distorted with most receivers. 110% positive is fine with negatives going to maybe 95% at the most.  The gain in loudness in going to 150% is under a dB and that's assuming a perfect detector in the receiver.  That's a lot of audio and increased distortion for little return.  I.E.: not worth the effort.  All BC stations these days including just about all SW broadcast now use symmetrical modulation. The gain in loudness and intelligibility comes from multiband audio processing while maintaining low distortion.
 
Run the rig with the maximum carrier capable of just 100% positive peaks with the available audio.  The PEP will be the same as with reduced carrier and 100+% audio.  The big carrier approach will sound cleaner and give you a better "quieting" effect.  Plus it moves the S meter slightly higher.
 
[Talking about source impedance of tetrodes with NFB vs: triodes] Yes, the 4X1's in tetrode mode with NFB will get close to the 3X1's and if you can run enough NFB, maybe pass them!  and yes, does lower the source impedance driving the final but that may not make much of a difference modulating a class C final since it's resistive and constant in impedance.
 
[Why is my mod xfmr / audio highs topping out at 7500 Hz?]  Well, the mod transformer is a big player here.  The leakage inductance will come into play and be your limiting factor.  Do you know what it is in that iron or do you know how to measure it?


[How to measure L leakage?] Easy.  Disconnect all leads going to the transformer. Short the primary and measure the inductance on the secondary.  It will be in the mH range.  That represents the loss due to incomplete coupling and results in an effective series inductance with the primary and secondary windings.  This "stray" inductance acts as a seris L with external reactances and typically limits big mod iron in the 6 to 12 KHz range. It is the main reason for phase shift at the high frequencies that limits the amount of NFB you can wrap around it.
 
Iron designed for triodes and other low impedance tubes will be a bit better, thus the anecdotal knowledge that triodes work better into squirrely loads.  
 
Bottom line is that you are probably extracting the last ounce of performance from that transformer!

Usually 6 to maybe 10 dB of NFB is OK but at some point you get into transient instability which what sounds like is happening to you.  You want to stay at least 6 dB away from that since that will be the beginning of IMD.  BTW, how does IMD look, two tones, say 300 Hz and 1350 or some such non even or odd multiple of two frequencies?  Try 50 Hz and 4 KHz if you are bold and have a spare transformer...{:>)
 
[Notice to stay 6 dB away from the instability point and get 6dB of working NFB means the circuit must be able to handle 12 dB of NFB without taking off - not an easy task]


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on February 25, 2014, 12:20:38 AM
The only warning about the 3-500Z is that when used as zero bias grid driven audio amplifier it has a grid impedance to the driver that varies from about 2000 Ohms to about 500 Ohms over the half cycle. For this reason, a cathode coupled driver or another driver with good regulation might be best.
However, the drive part of it is not in any way causing a frequency response issue and the distortion looks low because the traditional "speech amp" driver has been super-sized and swamped.

This weekend's a Hamfest. Later I can work on the TX with the great cap-removal suggestions given.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 25, 2014, 12:32:55 AM
More great info from Bruce / W2XR regarding NFB:

------------------

Indeed, the judicious application of NFB will definitely reduce the source impedance of the tetrode (or any other tube), but the issue is this: too much feedback can ruin the purity of the audio as well, and not only from the loop bordering on the point of going into oscillation with respect to phase. A very good rule of thumb to consider when using NFB is that true directly heated triodes (not triode-connected tetrodes or pentodes) need only about 6 dB of voltage feedback to substantially improve the bandwidth, amplitude flatness, and to reduce the distortion components. With tetrodes (and even triode-connected tetrodes) you will typically require about 14 dB of NFB to achieve the same results. This is another good reason to consider the use of real triodes in the audio path of the rig, including the high-power modulator stage.

Too much feedback can result in slew-induced (SID) and transient IMD (TIM), both of which are well known to experienced analog audio amplifier designers, but probably little known elsewhere. It can also substantially increase the spectra of the higher-order odd-order distortion products, and this is the most dissonant sounding distortion of all, on par with IMD. I personally prefer as little feedback as possible to do the intended job; for example in my homebrew directly heated WE-300B class A power amplifiers for my all-homebrew stereo system, I use zero NFB, including in the phono-playback preamp. No voltage feedback anywhere! The RIAA EQ is accomplished with a passive split RC network, and not with an RC feedback loop.

For real Hi-Fi audio, (and not amateur radio purist AM transmission, where these things don't apply), feedback also kills the dynamics of the sound (think of an audio compression loop), and to my ears at least, imparts a "lisping" quality to the audio. This "lisping" may be due to time domain issues, as the feedback is always attempting to correct something that has already occurred.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on February 25, 2014, 01:03:36 AM
Never could get the NFB concept,  I always thought the FB was behind the signal or in effect occurs after the signal.  Anyway, thought it just may be something I'm missing.

Very surprised to read the last line in Bruce's comment, "as the feedback is always attempting to correct something that has already occurred".

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WD5JKO on February 25, 2014, 07:06:06 AM

  One thing about NFB seldom discussed is what happens when an amplifier with NFB is driven into overload. If there is 10 db NFB, then during the overload portion of the waveform, the amplifier goes open loop...meaning that the gain is increased 10 db during the overload. This squares off the overload more then if there had been zero NFB, and also..the worst thing is with R-C coupled stages within the NFB loop, there can be a sudden bias shift from grid current. Then after the bias spike, there is an R-C delay based upon the circuit R-C time constant. Imagine those 6550's in a Ranger with no driver transformer. Folks often have .1 uf coupling caps, and a 470K grid resistor...here the time constant is 47ms, and it might take 100ms or more to audibly recover from a single transient overload..this is VERY audible; much worse then the overload itself.

   Most of us running AM do not have precision level controls, and compressers almost always overshoot during the the "Attack Time". We need some sort of limiting following the compressor. When we inevitably have that occasional voice peak that overloads, the best way to limit the carnage that follows is to limit the NFB around the stage overloaded to 3-6 db, and to have a low impedance grid circuit to minimize any DC shift following the transient overload. An interstage transformer usually considered as a "has to go item" has the benefits of supporting class Ab2 operation, and a low time constant should the amplifier have that occasional overload.

Jim
Wd5JKO

 


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 25, 2014, 08:01:26 AM
That looks about 8kc wide, which should be good.

I always thought you could have a great sounding signal without the audio going 20kc wide.



I never run caps across the mod transformer, and always removed them from any rig that had them.
2000 pf is really high for a screen bypass cap, there go the highs....

They really wanted that rig narrow I guess.

It was built in the days of SSB promotion where "narrow" was the happy fun goal.
The spectrum received on an SDR set on 16KHz BW is attached. I was only recently aware of this.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 25, 2014, 08:34:04 AM
Very interesting stuff.
I always thought the cleanest audio amps (tube) ran AB1.

And AB1 can be very clean with some tube types.

With modern stuff, or special (very big) tube drivers, AB2 or class B could be ok.
The easy way is to drive tubes that take grid current with an 8 ohm to say 4000 ohm center tapped Hi FI transformer.

You can also drive AB1 tubes with a transformer (my 4x150 mod deck is set up that way), and there is no feedback.


I also always thought it was a waste of time to run 200% modulation.
Phase the audio right, run it up to 90% negative and something like 120% positive.
Anything else is just distortion, no?

Some people on the air do the big carrier and do not push the modulation.
I remember when FM broadcast started. We had WYSP in Philadelphia, who played great music, and the DJ's spoke softly and at reduced volume. It was wonderful.

I guess its good to want to be very clean and hi fi, but I think its all in the EQ and the voice.
You can have a great sounding signal with some distortion (plenty really) if the EQ is good or the voice is a radio voice.

One of my favorite signals was a guy over in PA, forget the call, but he was running an old Sure S40 mic into a stock 32V3.
Sounded like god.
 




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: VE3AJM on February 26, 2014, 11:26:24 AM
In my previous post in this thread I was asking a question regarding Bruces comments here as quoted by Tom JJ about the 3-500z and then later, about the 3-1000z:

-------------------------------------

Bruce's Comments below:

I am of the opinion that one of the best choices of modulator tube for 1000 watt plate modulated service is the 3-500Z; zero bias, class AB1 operation (no driver xfmr or audio power required), and extremely linear operation.


And later as I read on this:


Now that would be a great tube to try in this application, but I am not sure as to what the Rp of a class AB1 3-1000Z may be.      

I understand that the 3-500z is a zero bias class AB2 tube and the 3-1000z is zero bias class B triode. I have designed and built a class AB2 3-500z modulator for my TMC GPT-750.

Just to clarify things with where I was coming from with the question. :) I have enjoyed reading this thread as it has gone along of course. Thanks.

Al VE3AJM


Title: Good News - Re: What is causing this audio highs attenuation
Post by: K1JJ on February 26, 2014, 01:12:09 PM
Good news...
 
The hard work is paying off.

After a number of tests and changes, the key was to switch the NFB tap on the mod xfmr from the secondary winding, to the  primary. I had wanted to include the mod transformer in the NFB loop, but it was a mistake. (in addition to the mistake of using big bypass and coupling caps)

I ran some new audio sine sweeps. The audio now sweeps cleanly up to 10K at 100%++ modulation !  This is drawing the normal 400 ma – same current as the lower freqs.   It will actually go to 12K, but only hit 80% modulation cleanly before there is excess current and slight crossover distortion begins.  The triangle wave looks even better now at 5Khz.  
 
So, now Fabio II will sweep cleanly from about 20 Hz to 10K at 100%++  mod -      and cleanly from 10K to 12K at 80%.  Not bad.  Still working on refinements for more NFB and this may get better..  I BE HAPPY!!
 
Looks like tapping the NFB off the secondary was causing all kinds of problems with the high end.    Funny how one mistake like this can cause so much effort to find...  ;D
 
I still have only 7dB of NFB and working on increasing that now.   I actually ran out of NFB pot adjustment and need more NFB signal to work with.  My goal is to run it up to 15 dB of NFB and then back it off to about 9 dB, if possible without instability.  With the mod transformer phase shift no longer in the loop, this  may be possible.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 26, 2014, 04:03:33 PM
I do not understand.
How are you doing the feedback now?

What happens if you do not use any feedback?



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 26, 2014, 04:59:57 PM
I do not understand.
How are you doing the feedback now?

What happens if you do not use any feedback?

Hi Brett,

The negative feedback is derived from one 4-1000A modulator plate cap to a 1.3 Megohm  resistor ladder that feeds the first stage of the solid state audio driver, 180 degrees out of phase.

Before, I had the NFB tap on the secondary of the mod xfmr. This added some phase shift problems. Though, if the feedback ladder was compensated for the mod xfmr phase shift, it would probably work FB.

If I disconnect the NFB, the audio is not as clean sounding. I did not do detailed sweep tests with or without NFB yet, but suspect it would show a cleaner output, especially in the highs where I've had trouble before.

The NFB also lowers the tetrode's source impedance approaching a triode, if the NFB is high enough. This may help when modulating a capacitive load, though I was able to reduce this C to about 1/4 of original via the cap reductions mentioned earlier in the thread.

I am presently at a stable 11dB of NFB  -  and just ordered some 47K, 3  watt, 750V resistors to build a new NFB ladder that will be safer and allow more NFB. I'd like to see 15dB, and then backed off to about 9dB, maybe more, depending on stability.  The audio now is as clean as any plate modulated transmitter I have ever built. Very pleased with the crisp highs and triangle tests at key audio freqs.

T  


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WD5JKO on February 26, 2014, 06:52:01 PM

Tom,

   "I am presently at a stable 11dB of NFB"

This is a good thread and an excellent opportunity to overload the modulator on a voice peak to see how things behave as you vary the NFB up and down. This means watching the modulated envelope on the scope, listening to recovered audio, and looking for bias shift in the FET audio driver stages.

Some folks would just say, don't overload the modulator, ever. These are sometimes the same folks that run wide open, with no limiters, or clippers. Most compressors overshoot during the attack time too. A whole lot of us punch the audio such that we overload at least a little bit on every syllable spoken...I contend that a lot of NFB used around the stage being overloaded can lead to bigger problems like transient intermodulation distortion when coming out of the overload. You might be surprised with the results. There should be a happy point where the NFB cleans things up a bit, and still allow the occasional overload on a voice peak without having an ill effect.. In some of my gear, that level of NFB is around 6 db.

Jim
Wd5JKO


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 26, 2014, 07:39:55 PM
I would think there would be problems taking feedback off of only one modulator tube....
That leaves the other one out of the loop, no?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 26, 2014, 07:57:22 PM
Hi Jim,

Thanks for the heads up.  You may have saved me from blowing the mod xfmr eventually. I am testing at only 2KV to minimize errors, but still.

Think I will leave things as-is other than building a better NFB ladder. I think I have plenty of headroom right now to back the NFB off 6 dB and end up with 6 to 8dB actual.  I will run some tests to find the sweet spot you mentioned from overload.

BTW, a good friend of mine saw your post and sent me an email that agrees wholeheartedly with you.
Thanks again, OM.



My friend's comments on excessive NFB:

Jim, WD5JKO is right about the NFB amount.  6 - 8 dB max before you get into TIM issues which will blow up mod iron pretty quickly!  This is a transient response due to the NFB overshooting it's correction when the waveform abruptly changes shape and frequency as happens with voice or especially music.  Looking into an inductor like a transformer makes it worse.
 
I've been able to wrap 12+ dB of NFB around a transformer, tapping off the primary as you are doing now and have had severe transient problems even though sine waves and frequency response were excellent.  Like all things in this world, there is a trade off and the one here is between steady state response when you run sine waves vs. speech which has lots of opportunity for transient generation with NFB.  
 
I'd stick with the 6 dB and be done.  Much safer for reliable performance and it cleans up 95% of the audio.  No one will hear that last little bit and in fact too much NFB will sound awful!  If the mod transformer survives!  BC rigs rarely used NFB and dealt with the resulting 3 - 5 % distortion.





Brett, one tap off the plate cap is all we need for full NFB due to the auto-transformer effect working against the center tap.  It was explained to me by a friend:


"You do get a complete signal from each plate on the primary of the mod transformer, one is out of phase with the other but totally complete.  To prove this to yourself, just look at the divided sample from the modulator plate on your Oscope.  If you have a dual trace scope, take the same divided down sample from the other plate and put it on the other channel and you will see the same complete audio waveform exactly out of phase.  If there is some small distortion on one half of a sine wave, for example, you will see it on both traces from each plate so you are losing NOTHING when coupling from one plate."


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 26, 2014, 08:08:06 PM
   Most of us running AM do not have precision level controls, and compressers almost always overshoot during the the "Attack Time". We need some sort of limiting following the compressor. When we inevitably have that occasional voice peak that overloads, the best way to limit the carnage that follows is to limit the NFB around the stage overloaded to 3-6 db, and to have a low impedance grid circuit to minimize any DC shift following the transient overload. An interstage transformer usually considered as a "has to go item" has the benefits of supporting class Ab2 operation, and a low time constant should the amplifier have that occasional overload.
Jim
Wd5JKO

Jim, I just got a chance to really absorb your post back at the top of the page.

I can see now how dangerous for the mod iron it is to run > 6dB NFB and not to have a limiter after the compressor, etc.

I'm moving on to building up a small negative peak limiter using an op-amp and diode. This works well for me in the past.  But what about the limiter for the positive peaks?  I've been axing around for opinions and they vary widely.  I already have a Berhinger 6-band processor and it's in the cellar... don't want all the bells and whistle and menus.

Right now I'm using just an RE-20 mic > 528E voice processor  > 31 band EQ.  Before I search around the web, is there a simple circuit, like the op-amp NPL, that will limit positive peaks effectively?   I don't want a multi-band processor nor a Volumax with transformers...

Any ideas?

BTW, you mentioned the benefits of having a low impedance grid circuit... do you think the GFZ SS driver circuit qualifies in this area or do I need to take some more precautions with the grids?   I can post the schematic if you need.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Tom WA3KLR on February 26, 2014, 08:26:50 PM
Unfortunately I have no time to spend to write this better,  My two cents, written while the previous two posts were being written:

The two modulator plate signals on the primary should basically contain the same signal, one is 180 degrees from the other, can’t use both at the same time.  BUT the modulation transformer combines the output of both tubes into one output signal, essentially averaging the transfer function of the low end of the tube curve and the high end of the tube curve overlaid together, creating lower distortion, without feedback.  This is why push-pull sounds better than single-ended, if properly implemented. 

The bad thing about the modulation transformer secondary is the leakage inductance working against the sum total of the r.f bypasses and pi-network capacitances to ground.  The phase shift from this L-C is the killer in trying to push the frequency response out on the roll-off point when doing global negative feedback, can’t fight Mother Nature. 

Hi–fi amps that have good performance at 15 kHz is due to the basic frequency response of the amp is out at least 100 kHz. so feedback/phase shift at the audio frequency is handled well.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 26, 2014, 08:47:05 PM
Just bought a 1982 vintage CRL PMC-300A modulation limiter on eBay.   Jeff/ W2NBC has one and thinks highly of it.  It's a simple straight forward circuit according to the manual below.  

It has an adjustable negative peak limiter, positive limiting, density and a few knobs that makes Jeff always sound like a million bux.  That's good enough for me... :-)


T

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CCYQFjAA&url=ftp%3A%2F%2Fftp.orban.com%2FCRL%2FPMC300A%2520Manual.pdf&ei=0YEOU9bcIebMsQSus4Ao&usg=AFQjCNEaQQHlRVZs7f7Zj41q7vGPe_uaTA&bvm=bv.61965928,d.cWc



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Steve - K4HX on February 26, 2014, 08:48:17 PM
Been using a 300 limiter since the 1990s. It is FB.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WD5JKO on February 26, 2014, 10:03:44 PM

Tom,

  It sounds like you have a good handle on things. I'm a novice when it comes to audio processing. For the DIY guy though this article is very informative, and inspiring:

http://tonnesoftware.com/appnotes/speech/speechamp.html

James Tonne is an OM now, and has been refining the concepts of speech processing for a long time. He originally had a QST article on this in 1956:

"Compression and Clipping", James L. Tonne, W5SUC, QST, September, 1956

I'd love to see that old tube type compressor. Surely by now the QST Copyrights have expired.

   On a whim I bought the K7DYY processor for my amplified D-104 UG-8 base. I had high hopes, and expectations. These were all met, but I had to do some serious work to it before I was ready to put it on the air. In my case, the MC-320 element was too hot, and the processor chip clipped on audio peaks when close talking, and also during the attack times. So I had to do my own research, make some changes, and in the end, I love my K7DYY setup. To be fair, it seems that many that use this board are using the Ebay MC-320 replacement elements, and these might not be as hot as a good old Astatic MC-320.  If anyone is interested in what I did, please PM me.

Jim
Wd5JKO


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 26, 2014, 10:54:39 PM
Been using a 300 limiter since the 1990s. It is FB.

All these years and you never told me your best secret!  (or I didn't pay attention is more like it)


Jim, I just ran some overload tests.   I did try the NFB at various settings, like 1 to 11dB and could not see any signs of overload transients under heavy voice modulation.  The SS driver stayed stable at the grids too.

Could you describe in more detail what you are looking for and how to make it happen?  Do I need to actually slam the waveform into a severe flat top condition and look for some kind of poor recovery?  I don't have an NPL on there yet, so maybe it's not a good idea to be that aggressive until the PMC-300 arrives.... ;)

In the meantime, I set the NFB at 8 dB and will leave it there for now.


Tnx for the processing link above.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 27, 2014, 08:16:43 AM
I ordered one also!
My first piece of real audio gear!

I have a vx2000, but the compressor in it has a lot of overshoot.



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 27, 2014, 01:18:40 PM
I ordered one also!
My first piece of real audio gear!
I have a vx2000, but the compressor in it has a lot of overshoot.

Cool.  Did you buy the second remaining one from the engineer estate sale?  I got the first one. They looked like NOS, never used.

I read the manual last night and see there is a lot of calibration and adjustment time. Maybe a lot of that can be bypassed.  Jeff / W2NBC says he will guide me through the initial set up and get right to the meat of it.  
He says the NPL works well and the asymmetry positive peaks control will let us dial in say, -95 negative and 150% positive - or anything in between.  (Assuming the transmitter will do it in the first place, of course)

My goal is to run a fat carrier and adjust the audio for about -95% negative and let the positive peaks go naturally where they will....maybe 110% - 120%. Run the modulators as cleanly as possible in AB1.  This limiter is exactly what I need here to keep the mod transformer and other parts safe from negative peaks and positive peak spikes. (and hopefully reduce NFB transients as discussed)


Here's a few comments to get you excited:

" I will say that the CRL is virtually transparent with the amount of negative peak limiting in direct relationship to the amount of drive applied and the recessed "output" pot . The RATIO between positive peaks and negative clipping is adjusted with the "asymmetry" recessed pot.. there is no actual positive CLIPPING .. its a all pretty seamless.. I can , for instance get as much as 150% positive and 90 % negative....   I run it using mostly natural asymmetry and about 130 % with 95 neg... you can crank it all up but does NOT swell the background or get noisy at all..

The cool thing about this "processor" is that it is a superb NPL but with the ability with the "asymmetry" control to limit positive peaks as well. There is also a built-in low pass filter and with simple resistor changes limits at 6, 8, or 10k .. No other frequency limitations or phase rotation, etc. They are pretty cheap on Ebay, and not a bad box to have around.."


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 27, 2014, 02:52:45 PM
I do not remember where I got it from (info at home), but there were a lot for sale.
I did not get the one on ebay because they did not say they tested it.
$240.00 for the one I got.

I just want to be able to limit the negative modulation to about 90% without overshoot.
The passband filter will be nice also, I think I will use 8Kc.

I read the manual, the only odd thing was the bit about using the mod monitor positive mod alarm light to set things up.
Lucky I have a mod monitor with the alarm lights....



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 27, 2014, 03:52:39 PM
They also talk about "old" AM transmitters that use plate modulation.  They have a "phase correction" for both the highs or lows.  I've seen this before and wonder if it is any value if the rig is already running well.

Did you see their examples of what a desired RF modulated waveform looks like? You're supposed to adjust the phase controls to mimic it. Problem is it looks like a flat topped / cross over distortion wonder.


Tip of the day  

For those of us using RF drivers that have tube finals:  (riceboxes or anything now in linear mode to drive your class C final)  I found my FT-102 did not like to key a carrier for more than a minute or so, otherwise it started to act funky from the heat.  I went in and adjusted the PA bias so that the final now idles below cut-off. BIG difference!  Now the tubes run cool and can generate a carrier indefinitely.  I am looking into adding a grid leak resistor to the bias to put it even farther into class C.  I use this FT-102 just as a driver for the class C Fabio II and the PDM Dual Quads rig, so it can stay heavily biased.  As long as there is enough internal drive in the FT-102 to overcome the additional class C bias, it will work FB as a dead carrier driver.  (It is no longer linear and not suited for ssb when in class C)

Update: In my FT-102, by padding R-109, a range resistor for the  fixed bias trimmer adjustment with a 15K resistor, I was able to get the bias to adjust far into class C.  The trimmer still has the range to pull it back into class AB2 if desired.  So your rig (whatever it is) may not require grid leak - rather just a little more adjustment range via fixed bias to become class C efficient as a driver.
T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 27, 2014, 04:25:09 PM
Yes, I read that, and figure maybe with thing adjusted for calibration like they say it will look like that?
Sure would not want to SOUND like that!


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 27, 2014, 10:01:04 PM
I did some tests on 3 rigs tonight.
The 2x 4x150a rig did -30 db 2nd harmonic audio tone at 99% modulation, as did the 3x 4D32 rig.
811a modulators, common supply, has some carrier shift under modulation.

The 2x 813 rig modulated by 4x150a's did 40 db.

All the transmitters did better with more grid drive then I normally use, and the 813's did better with less screen current.

The 4x150 and 4D32 rigs started rolling off at 40 Hz, went 10% modulation less at 4000 Hz, but stayed flat to 18,500 Hz (high as I went) at that 10% down. Why it goes down 10% at 4000 Hz is a question.

The frequency response tests I did at 60% modulation, so at about 4000 Hz, the modulation dropped to 50% from a 1004 Hz reference level.

I think the 813/4x150a rig did well because of the separate power supplies and there being no carrier shift at all.
The modulator is also AB1.



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 27, 2014, 10:11:34 PM
-30 to -40 2nd HD is pretty good.


Try loading the final heavier and see if your THD improves. Decrease the C2 ant loading cap, which will mean a bigger carrier and require more audio. (more audio may erase the THD gains, however)   

I have found that less C2 (heavier loading) helped my THD get better as a result. There is a sweet spot.  A couple other guys have told me the same. You wud think it was the opposite, but heavier loading seems to help clean up a class C plate modulated final.  It helps with linear amps too, of course.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on February 27, 2014, 10:21:04 PM
What about loading it up more but with less plate voltage?
I have the ability to adjust all the voltages, and I adjusted the modulator plate voltage without any change.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on February 27, 2014, 10:41:10 PM
Generally, I like to run as much plate voltage as practical.  Most of my rigs really come to life once they hit a certain voltage level. This assumes the biasing, loading, etc., is right.  In general, higher voltage with recommended current seems a good  plan.

But the best way to find out is to run your various tone tests and try every change you can...  tank Q, voltages, loading, drive, etc. You will find the sweet spots that can sometimes eek out another -10dB of cleaner THD.

T


Title: NEW TEST RESULTS Re: What is causing this audio highs attenuation?
Post by: K1JJ on February 28, 2014, 06:07:39 PM
Finally got to the point of running THD tests with Fabio II. (4-1000A plate modulated rig)  The rig is near to completion, though will get some more fine-tuning attention soon.

Using a one volt, low distortion sine audio waveform into the rig and using my HPSDR receiver to view RF spectrum, here's what I found:


1)  Loading the class C final heavily definitely helped to make THD better by as much as -10 dB. The groups of higher order products up the band became "better behaved" by making a nice orderly decline.  When loaded lightly there is more chaos in the groupings and at a higher level.  Just tuning C2 loading cap could actually find the sweet spot.

2) I found that running a triangle wave is a very interesting frequency response test as viewed on the SDR screen.

3)  Using a pure, low distortion sine wave, I found that at ~100% modulation, the THD generally ran about -40dB 3rd and -50 dB 5th order levels. The 7th order was always lower than the 5th, but was dependent on loading and tuning.  I am very happy with these numbers.

4) I ran some THD tests up at 8 - 10Khz and saw that the THD numbers were not as pristine, but still close to -35 dB 3rd order.   At 6-7 KHz I was seeing about the same THD as the lower frequencies, which is good.  At 12 KHz I did not see any unusual crud up the band and tells me that it is stable.

5)  I tried the THD tests with and without 8 dB of NFB.  With it on, I saw 3rd and 5th IMD generally better - and the farther out crud was better behaved and grouped well with an orderly decline.  Without NFB the higher order crud was higher and somewhat random looking. I would say the NFB helped to improve things maybe -4 to -5 dB at most.  This is using a sine wave, so TIM (transient Inter-mod)  would not show up anyway, only with voice or music programming.

6)  The deep lows swept very cleanly at about 20Hz minimum, which is FB for a rig with a mod transformer.

7) I ran some temperature tests with the laser temp gun and see that the 4X1 plate caps are running about 90 degrees F.  I have max air flowing for tests and this temp will increase once I back the Variacs down for on-air use.

I tried some voice modulation with plenty of S's  and CH's and the spectrum was smooth and contained.  No more spitting and highs distortion or splatter could not be seen.  I plan to add the CRL PMC-300A modulation limiter next week which will limit negative peaks to -95% and we'll have to see how far the positive peaks run in AB1.  My goal is to run the modulators as cleanly as possible, so no grid current is permitted. This will be the bottleneck in performance.

When the CRL limiter arrives, I also will try some aggressive voice audio tests to see if the NFB comes unglued. I want to run as much NFB as I can but still be stable.  (Probably limited to -8 dB)

All in all, I couldn't be more pleased with the test results and it looks like this may be the cleanest plate modulated rig I've built to date....  ;D ;D

Thanks again to all who have helped!

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: w3jn on March 01, 2014, 07:38:15 AM
Tom, not sure about your test.  Are you measuring THD by using a triangle wave input?  If so, all you're measuring is the audio response of the transmitter.

Remember that a triangle wave consists of many odd harmonics of a pure sine wave.  If you're simply measuring the amplitude of those odd harmonics with your SDR you're just measuring how the transmitter is affecting its response to a particlular component of the triangle wave.

A triangle wave is useful to see, in time domain, transmitter distortion as you can easily see bending of the wave's slope, rounding of the edges, etc.

In the frequency domain, however, you're best served by feeding the transmitter a pure, low-distortion sine wave and seeing what harmonics of that sine wave you get.

As you increase the modulation index of the transmitter, a more useful measurement might be IMD.  IMD is much more irritating to listen to, and while you can relatively easily see IMD in the frequency domain given an undistorted input signal, a THD test (generally) will not tell you a thing about IMD.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on March 01, 2014, 11:42:38 AM
  IMD is much more irritating to listen to,

Is he another one of those guys who has a few too many drinks and gets on 75 looking for a fight?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Steve - K4HX on March 01, 2014, 12:16:35 PM
 ;D ;D ;D ;D


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 01, 2014, 01:28:21 PM
In the frequency domain, however, you're best served by feeding the transmitter a pure, low-distortion sine wave and seeing what harmonics of that sine wave you get.


Hi John,

Yes, I've been using a low distortion sine wave for all of my AM tests now and in the past.  But just for the first time I looked at a triangle wave on the SDR and thought it might have some additional THD value, but I guess not.

One thing that cornfuses me...  On AM, the single sine tone gives information about THD.  Do we need a standard two-tone on AM to show the IMD just like on ssb?   When I ran a two-tone, it looked about the same as a single tone but with more peaks, as expected.   If the rig is very clean, can I expect a big difference in general peak levels between the two tests?

The AM carrier seems to supply a tone to beat against on AM, whereas a ssb signal does not. That's what cornfuses me.  It seems like the carrier acts like a supplied note for the two tests. Understand what I mean?

* I edited my post above to remove my comment about a triangle and THD -  to keep it accurate.


A friend commented:

"The reason for using two tones is in a nonlinear condition they mix to generate third order responses. Using 1 tone you would just generate harmonics of the tone. I bet a single tone is cleaner to 6 db above a two tone. The two tones not phase related gives you the most information because the vector addition generates modulation from 0 to 6dB above 1 of the tones.  I suppose if you have a clean system the results will be similar."


Think I will try some 2-tone tests (on the AM rig ) and see if the IMD tells me more.  When I tried it a year ago, it looked exactly like the single tone, but with more peaks filled into the same slope.  So I didn’t bother anymore with it for AM, just for ssb tests..


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 01, 2014, 04:50:29 PM
Just ran a 2-tone IMD test on Fabio II using an Electcraft  2-T two-tone board.  The tones were 700 Hz and 1900 Hz.  This board has good specs and a clean waveform.

At medium power, the 3rd order IMD was down about -35 dB and the 5th order was down about -55 dB at 100% modulation.  Above + 5KHZ up the band all peaks dropped off below -55 dB... quite well behaved.  I would need to generate audio frequencies much higher than 700/1900 Hz to test the extreme highs IMD, but this is a good start.  

I did notice the same THD pattern where heavier loading helped improve the IMD.   The improvement was slightly less than the effect it had on THD, but still showed the same trend.

For a class C, plate modulated final, I am pleased, since these IMD results are slightly better than an average ham ricebox  or ham linear amplifier.

Thanks for pointing out the THD/ IMD  information, Johnny.

T

 


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AMLOVER on March 02, 2014, 10:03:27 PM
Tom,

what is your modified heissing configuration at this time?
Do you return the modulator's cold end to the HV+ or you ground it through the cap?

Stefano


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 03, 2014, 12:32:11 AM
Tom,

what is your modified heissing configuration at this time?
Do you return the modulator's cold end to the HV+ or you ground it through the cap?

Stefano


Hi Stefano -

I have the Heising cap connected to the cold end of the mod xfmr and the other end of the cap connected to the cold end of the Heising reactor.  The "cold" end of the Heising reactor goes to HV+.

BTW, your idea about using a HV relay and power resistor across the mod iron and screen choke to quench sparks is still working FB. No arcing problems since. That was you who recommended this last year, right?

T



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: flintstone mop on March 03, 2014, 06:40:52 AM


I also always thought it was a waste of time to run 200% modulation.
Phase the audio right, run it up to 90% negative and something like 120% positive.
Anything else is just distortion, no?

Some people on the air do the big carrier and do not push the modulation.
I remember when FM broadcast started. We had WYSP in Philadelphia, who played great music, and the DJ's spoke softly and at reduced volume. It was wonderful.

I guess its good to want to be very clean and hi fi, but I think its all in the EQ and the voice.
You can have a great sounding signal with some distortion (plenty really) if the EQ is good or the voice is a radio voice.

One of my favorite signals was a guy over in PA, forget the call, but he was running an old Sure S40 mic into a stock 32V3.
Sounded like god.
 



That's about it for me too. It's always nice to have a lot of headroom, and that makes the TX audio a lot nicer. Tom wants a lot of operating space for the modulator/transmitter so there are no bottlenecks for clean RF and clean sound on the air.
Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on March 03, 2014, 07:56:35 AM
I know he has fun fine tuning his rigs for clean and stable operation.

Too many others do no tuning or checking at all!


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3RSW on March 03, 2014, 09:09:28 AM
KF1IMD again? w2VW already whacked his chops.  ;D


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 03, 2014, 12:36:15 PM

I also always thought it was a waste of time to run 200% modulation.
Phase the audio right, run it up to 90% negative and something like 120% positive.
Anything else is just distortion, no?


Agreed.  There's no doubt in my mind that for the majority of receivers on the air (mostly diode detectors) the cleanest modulation level  is -95 negative and whatever your voice naturally does for positive peaks. This may be 105%, 110% or 120% positive, etc.   This level will satisfy the majority of receivers without excessive distortion.

Chuck said it well in an email... to paraphrase:  We usually have a certain limited amount of CLEAN audio power to work with.  (or pep RF power)  Why not run the carrier up so that the maximum audio power makes 110% positive?  This creates a better quieting effect and S/N ratio when using a bigger carrier.   In contrast, dropping the carrier way back and running the same amount of audio power (at 200% mod, etc) doesn't make sense, since the background S/N and S meter become lower.  Selective fading also becomes a big issue with heavy modulation for diode detectors - unless we are using sync detectors.

I plan to run a fat carrier and use limiting to keep at -95% negative  and protect the mod iron from rouge positive peaks above 130%.


Fred, yes, correct.  I need headroom so that I can run the modulators in AB1 - I cannot draw grid current, thus power output is lower. This is the bottleneck. So I need big tubes that can do their job cleanly.  I COULD draw grid current (AB2) but the THD would suffer based on my tests.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on March 03, 2014, 07:28:52 PM
For me, its all about the carrier.
You can get ssb to sound good frequency response and distortion wise, but its got no carrier to quiet the frequency.

I remember my early days into AM, when I would hear someone with a strong signal key up and you could hear the clock in the background, or the guy next door with the lawn mower.
I do not seem to hear that much anymore, its mostly blowers or nothing at all.



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on March 04, 2014, 12:26:28 AM

I remember my early days into AM, when I would hear someone with a strong signal key up and you could hear the clock in the background,


W2KTU 160 meters.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Chuck...K1KW on March 04, 2014, 11:35:12 AM

 "you could hear the clock in the background"


Like the original W1ZE on 75M in the 50's and 60's?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KB2WIG on March 04, 2014, 12:36:55 PM
And then there are the crickets and dishes being washed.


klc


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: w1vtp on March 04, 2014, 05:35:41 PM

 "you could hear the clock in the background"


Like the original W1ZE on 75M in the 50's and 60's?

Yup, Chuck, remember that well.  Fond memories.  Met Irving while he was visiting my aunt, W1UET's camp one summer.  I was a snotty nosed kid at the time and was impressed with his gentleness and good humor.

Al

Attached his QSL coming up on the 60th anniv of our QSO.  Note the address: how simple life was back in those days


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 06, 2014, 12:33:04 AM
Fabio II is coming along very well.  Just added  the PMC-300A and getting it to work. Should have it on the air and will post some last results soon.

* I do some Kayak fishing in the CT River, but not like this guy.  It's amazing the size fish some of these guys land in their little Kayaks from the ocean.

[I couldn't resist]

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on March 06, 2014, 10:10:23 PM
My pmc 300 arrived today, and was a bust.
Loud hum, very low output, when the gain is adjusted, it trys to blow my mod transformer with really loud low frequency garbage.

The thing is loaded with cold solder joints and bad connectors...



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 06, 2014, 10:38:39 PM
My pmc 300 arrived today, and was a bust.
Loud hum, very low output, when the gain is adjusted, it trys to blow my mod transformer with really loud low frequency garbage.
The thing is loaded with cold solder joints and bad connectors...

Aw, that's too bad. Sorry to hear.  Hopefully you can get a refund.

It might be worth giving it another try.   Jeff / W2NBC gave me quick start instructions to get mine running and I'm very pleased with it!  It's limiting the negative peaks to ~95% and I put a limit on the positives to about 130%.  I can scream into the mic and it stays within parameters.  The rig sounds wonderful in the monitor.  This was the crowning touch and Fabio II is now the best plate modulated rig I've ever had.

It raised my background blower noise very slightly simply cuz the audio level is higher since I can lean on the NPL now.  It seems quite transparent and just what I needed.  I played with the 4X1 modulator negative feedback and brought it up to -12 dB without problems. Tmw I will do some THD tests to see what setting is best.  Maybe -8 dB or so.  

So in summary, the PMC-300A is a worthwhile box if you can find a clean working one.

And again, my thanks to Jeff for his email assistance!  


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on March 07, 2014, 12:54:17 AM
Is this a decent summary  about the frequency response, distortion, and feedback ?

It was the broken ss driver, for the actual repair that brought this into the open when the transmitter was tested afterwards.

The negative feedback around the modulator (taken from the modulation transformer secondary), switched in and out, accounted for 10dB gain.

Changing the .002 coupling and .002 plate bypass caps to 500pf each to remedy need for a lot of extra modulator drive and a slightly higher THD above 6500Hz, and reduced the modulator current from 700-800mA to 400mA @7KHz.

Changing the 1000pF screen bypass caps on the RF stage to 500pF to reduce phase shifting on the self-modulated screen.

Large caps like 0.001 or 0.003 are not to be used across the modulation transformer.

Transient instability in the modulator (coming from too much NFB and happening when the modulator is overloaded/clipped) can blow the modulation transformer. the maximum feedback should be more than 6dB less than the amount at which transient instability is seen. 3-6 dB of feedback may be enough around the modulator stage, 8dB max.

NFB signals should be taken from the modulation transformer primary because of phase shifts and leakage inductance in the transformer.

NFB can be taken from just one modulator plate. Or both. The effect should be the same.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 07, 2014, 01:59:58 AM
Yes, I would say that's a good summary, Pat!

The only modification  listed that I don't think made a big difference is changing the screen caps from 1000 pF to 500 pF.  The screen was modulating well either way. 

The biggest improvements were repairing the SS driver followed by reducing the .002s to 500 pF in the plate coupling and choke bypass.


I have yet to see the effects of the NFB going unstable due to a transient, but with the new PMC-300A limiter and keeping the NFB under -8 dB, maybe I won't have to experience it.

T



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: AB2EZ on March 07, 2014, 09:33:43 PM
I'm trying to understand this issue:

The gain of the amplifier, with feedback, is: A/(1 - AxB), where A is the open loop gain and B is the fraction of the output that is fed back to the input.

If B were to go to zero, then the gain would jump up. But B is typically determined by passive components.

If the open loop gain saturates (I.e. the slope of the output v. the input gets smaller)... but does not introduce phase shift as it saturates... the closed loop gain will also saturate. I.e. the change in closed loop output corresponding to a change in input will get smaller.

For example:

If A=-100 and B=.05, then the closed loop gain is: -100/6 = -16.7

If the slope of the open loop gain decreases from -100 to -25, then the slope of the closed loop gain will decrease from -16.7 to -25/2.25 = -11.1

If the slope of the open loop gain decreases to -10, the the slope of the closed loop gain decreases to -10/1.5 = - 6.67

More generally:

The closed loop gain = A/(1 - AxB) = -1/[(-1/A) + B)]; which decreases as |A| decreases.

Therefore, it appears (to me) that a problem would occur only if:

A) The saturation of the open loop gain is accompanied by a significant increase in phase shift.

or

B) There is an amplifier in the feedback path whose gain drops significantly for some reason.

Stu



I have yet to see the effects of the NFB going unstable due to a transient, but with the new PMC-300A limiter and keeping the NFB under -8 dB, maybe I won't have to experience it.

T




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Steve - K4HX on March 07, 2014, 11:51:14 PM
I've never heard of an amp become less stable with the application of negative feedback.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 08, 2014, 12:29:49 AM
I appreciate the the math, Stu.  It appears from what you are saying there should not be a problem if there was a lot of NFB (-15dB for example) and the amplifier went into an overload (flat top) situation.  I don't know enough about this nor have seen it occur so far.

Talking with an amplifer engineer, he says that the Dynaco series 70 amplifier has -23 dB of NFB.... and there are a few BC transmitters (Gates BC-1T through the BC-1H)  that use =14dB around the 833As.  

I could make a mod to let me dial in some more NFB and may play around with it once I am sure the rig is very stable as is.   There is always the risk of having the audio system take off due to too much phase shift. That has happened before when I tapped off the mod xfmr sec. But tapped at the plate cap, I've not seen the limit yet.

Today I had the rig on 75M for an hour running about -12dB of NFB. Got some excellent audio reports. I didn't see any signs of NFB instability so far.  The extreme highs were very clean.  Looking at the SDR spec analyzer, when I hit an "S", the spectrum was contained to my pre-set limits. When I set the high cut to 6Khz bandwidth, it WAS 6 KHz and rolled off sharply.  In the past, I could hear tearing and would see the noise floor rise up way above 6Khz. That is the advantage of having clean extreme highs.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 09, 2014, 01:24:01 PM
Was on 75M for a coupla hours last night. The rig ran FB and then had an arcing type crap-out.   It was the old problem of the grid input tuning. I was using thin miniductor for the coil and the plastic form melted and shorted the turns. The last time the input bandswitch melted and shorted the 4X1 grid to ground - fireworks!

I underestimated the current in the coil. The current is Q *  circuit current. If the grids were drawing 80ma, then a Q of 20 = 1.6A circulating thru the coil. It takes about 80 watts to drive the rig. That very thin miniductor is no match for 1.6A of RF.

 Today I'll wind my own coil with #16 wire and be done with it.   We all tend to underestimate the input tuning requirements and focus on the plate tank, I guess..

Other than that, Fabio II did pretty well last night. At least I haven't gotten frustrated, given up and torn it down to build another  4X1 PDM rig.  That is my normal MO .... :-)

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on March 09, 2014, 01:28:20 PM
I wind my input coils on ceramic forms with double cotton wire.  I then varnish the winding,  seems to work FB.

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 09, 2014, 11:31:00 PM
Here's a pic of the new grid tank coil. I took out the bad mini-ductor and wound up a real maaan's coil.  This one uses #12 enamelled wire with Teflon spaghetti wound to separate the turns.  It's about 20 uH and covers 160M too.

I ran some full operating rig tests into the dummy load and it's rock solid.

However, there was actually a second problem:  The FT-102 exciter drive was moving up and down - unstable. I found a bad cap in the driver and fixed it.  Now Fabio II can drop a carrier for 10 minutes without a sweat.

It seems that whenever I get on the air, there is some new bug that gets weeded out. Eventually all the bugs will be found and it will be bullet-proof.  (I hope)    I think the grid tuning cap has some spacing problems when fully meshed, so its arcing may show up on 160M.... [sigh]



T


Covers 160 - 40M.   Underneath the sub-chassis uses all Teflon wire, of course. (of course)    That sub-chassis has had a hard life - it's housed about four rigs in the past. A coupla of Rico-Suaves and Fabio I and II.
 


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on March 10, 2014, 08:12:05 AM
Looks tight in there.
Are those crimped connections???


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on March 10, 2014, 09:24:39 AM
They are crimp connectors.  Hopefully he soldered them.

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 10, 2014, 12:46:26 PM
Crimped connections?  Crimped connections?   ;D   That's when my problems would really begin.

Yes,, but I pour solder down each and every hole.   One thing about my work is it is solid and rarely fails from a bad connection. But because I change things around a lot, over time they may not look as pretty as when first built. I don't usually spend the time making thangs real pretty cuz they may be recycled at any time.  So you see the warts and all...


But I think Fabio II is a keeper.

T

The beginning is the prettiest:



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on March 10, 2014, 02:47:17 PM
Tom

OK FB,  glad you solder the crimped connectors.  Very often I remove the plastic, crimp, solder then use shrink tubing.

Nice soldered connections really adds to the longevity of the project, which, in your case can be as much as a month or two. ;D

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on March 11, 2014, 02:45:13 AM
I also crimp them solder. I even do this when getting a car stereo installed, usually I "help" by soldering what they have crimped. Oh well it is good for someone else to crawl under and into things, and I know the shop so they only charge me about $50 more if I supervise, because it takes more time. Crimps are what they are and some last 40 years but how the industry accepts it as a 'solid' connection is beyond me. 8-)

and I see your grid coil won't be shorting out any time soon.. Learned the same lesson on an MB-40 in the grid of a 4-1000. The link wire is very inefficient above 20M and will melt the insulation, making a smell that is not easy to find.


Title: Finally found crap-out problem -Re: What's causing this audio highs attenuation?
Post by: K1JJ on March 12, 2014, 11:47:42 PM
Finally.   After many spectacular arcing crap-outs over the month, I finally found the problem.

Fabio II  ran fine out of the cabinet up to full power. But once in the cabinet, at medium power there was always fireworks in the RF deck and the HV fuses would blow. I looked many times to see any signs of carbon or arcing. I thought I found it many times, but it kept happening.

Tonight while testing into the dummy load with the cabinet on, there was a sustained flashing, five in succession - the worst arcing ever. I just knew there had to be some signs inside the cabinet. Using a magnifying glass, I found none.  An inspection of the HV RF deck showed nothing.... until I spied a copper connection on the vacuum variable with a single tiny black pit mark.  It had a melted plasma look to it, but almost invisible.  However, the cabinet did not have a matching mark. Where was the path?

Turns out that the copper had a sharp edge and the flame spread out onto the full cabinet aluminium side panel, dissipating the heat, leaving no secondary mark.  I machined the connection with a smooth edge and moved it farther away from the cabinet.  It was originally about 2" away, now 4".  RF can jump amazing distances. The 1/2" = 10KV is for DC.  Under modulation, the HV at the final plate can become X2 to X3 depending on the modulation level.  

Fixed.    With the cabinet on I fired Fabio II up at full power and really beat it hard.  No arcs.  Tried it on the air for a few Yallos. No arcing.   I expect some more problems, but this one was a toughie and may be a breakthrough to a more reliable rig.   Troubleshooting HV RF problems has always been an unnerving  and difficult job for me. Just gotta stick with it.

T


Title: Spark Gaps - Re: What is causing this audio highs attenuation?
Post by: K1JJ on March 14, 2014, 07:14:37 PM
SPARK GAPS:

Today I set all four spark gaps.   [xfmr pri (2), sec (1)  Heising reactor (1) ]   Since the rig is in the cabinet when testing, I can't see the gaps arc. I put a small 1/2" piece of Scotch tape on the each gap. When it arcs, it is easy to later see which one did.  So, fire up the rig, sock some Yallos into the mic, pull the cabinet off and close the gaps of the ones with no melted tape and open the ones with melted tape.

I found that the Heising reactor gap was about twice the width as the gaps across the primary and CT. This makes sense since we are seeing 1/2 the voltage across the CT to ends of the primary.

In the end, after about eight trials, I can JUST get the gaps to arc when really hammering the audio at full power.

I know the transformer can take it cuz the gaps, when set wider, used to arc like crazy before I did spark suppression using vac relays and power resistors across the reactors.  The mod xfmr and Heising reactor are on Easy Street now.

I ran Fabio II today at full power with the cabinet on with no problems. The grid circuit and drive is very stable now. We are almost there.

Hope to be on with it this weekend for the AM rally.
  
T
 
 


Title: Acoustic FeedBack Solved - Re: What is causing this audio highs attenuation?
Post by: K1JJ on March 27, 2014, 04:01:47 PM
Those of you who have heard Fabio II over the last few weeks probably heard the 3,500 Hz tone acoustic feedback problem I had.  If I turned the audio gain up to modulate heavily, this feedback would become very loud.

I first thought it was the mod iron talking back, but it was not.

Yesterday I covered the plate coupling cap with a towel and the feedback went away.  I replaced it with a bigger cap, but the problem remained. Hint: The towel also covered the plate tuning cap during this test.

Today I set the rig up to do a constant feedback tone and went in with a long 5' flexible plastic pipe against my ear, looking for the source.  It took about 60 seconds to find the tone was coming from the plate tuning cap C1, a nice 15KV 500 pF vacuum variable. If I touched the end cap with the hose, the noise disappeared.  The end cap is normally brazed? to the lead coming out of the glass and had loosened up. I added some reinforcing copper strap and the noise was gone.

I can now turn the audio gain up to full with just the hint of acoustical feedback.  OK for now.

The moral of the story is to suspect EVERY part, even if it is a FB vacuum variable. That was the last thing I wud have thought of. The plastic tube to the ear is a very precise and effective method to zero into the exact spot of acoustic feedback generation.  

** Be sure to test the listening tube for HV breakdown and electrical conduction before using it. For example, I tried to add some foam rubber to the end of a fiberglass rod for poking around. The foam rubber drew huge arcs off the RF areas. It acted like a carbon pencil when touched to an antenna tuner coil.   Pure fiberglass is OK as well as PVC pipes. I used a black, 3/4" PVC plastic flexible water pipe for the listening tests.

Be sure the pipe does not have any metal or carbon fiber ribbing. Testing it first is mandatory.

Tom, K1JJ



Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on March 27, 2014, 04:21:49 PM
Weird stuff must be contagious. I zorched through a ceramic standoff toward the end of last night's QSO. Only acted up on modulation peaks!

Symptom was reduced peak power and modulator clip lamp lighting some.

 >:(


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: w3jn on March 27, 2014, 07:25:48 PM
The vacuuuuuum variable in my HB K1JJ tooner talks back also, especially on 160 where it's tooning the 75M dipole.

The tooner sits atop a tall rack and isn't that far from a drop ceiling, whose tiles are scarred with a multitude of scorch marks which are mute evidence of something busted in the antenna or feedline.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KB2WIG on March 28, 2014, 11:25:34 AM
" The moral of the story is to suspect EVERY part, "

So, you are saying to check everything out, before you throw in the towel?


klc


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 28, 2014, 12:28:32 PM
Weird stuff must be contagious. I zorched through a ceramic standoff toward the end of last night's QSO. Only acted up on modulation peaks!

Symptom was reduced peak power and modulator clip lamp lighting some.

 >:(

Your audio peaks and BC sound were outstanding.  Rigs and race cars always blow when they are running at their best - most stress.   No wonder... ;D


Johnny: Did you ever try touching the end cap of the vac variable with an insulated rod when the feedback is happening?  I wonder how common this end cap loosening problem is.


KLC:  You and 51 watt Fred get the coveted Corny Comedy Writing award on this BB....   :D


Update:  Fabio II made it thru a very lively three hour 75M and 160M session last night with no mishaps.  I can't believe it's not butter.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on March 28, 2014, 12:59:11 PM

Your audio peaks and BC sound were outstanding.


T


Thanks! Engineering credit goes to Stu AB2EZ. It's the modulator he used on the GS-35B rig.

All I did was make a change to the ratio to match my xmitter.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 28, 2014, 01:24:58 PM
Thanks! Engineering credit goes to Stu AB2EZ. It's the modulator he used on the GS-35B rig.
All I did was make a change to the ratio to match my xmitter.

Yo -

Please describe your lash-up in mo detail.  I'm axing for mo details, chump.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on March 28, 2014, 01:44:48 PM


KLC:  You and 51 watt Fred get the coveted Corny Comedy Writing award on this BB....    ;D


T


Thanks Tom,

I needed an award for something, anything.  I'm practicing for my stage comedy act when I come back in my next life.

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2NBC on March 28, 2014, 02:10:15 PM
Regarding the GS-35B modulator:

http://amfone.net/Amforum/index.php?topic=13307.50

indeed  ;)


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: w3jn on March 29, 2014, 06:35:40 AM


Johnny: Did you ever try touching the end cap of the vac variable with an insulated rod when the feedback is happening?  I wonder how common this end cap loosening problem is.




T


No sir, never did.  Never got feedback but you could certainly hear midrangey audio from the tooner on voice peaks.


Title: On Air Recording - Re: What is causing this audio highs attenuation?
Post by: K1JJ on March 30, 2014, 12:11:10 AM
A big group on 75M tonight.

I think Fabio II is starting to get dialed in.   Jeff/ W2NBC made this recording from NJ using his R-390 receiver. About a 150 mile path. Running medium power tap.  Audio at -97% negative and 140% positive peaks.

The first station is W1LLY and the second station is Fabio II.

I like what I hear.

T

Listen:


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3GMS on March 30, 2014, 01:24:31 PM
Tom,

I heard you last night on 75M and the rig sounded excellent.  All of that work certainly has paid off.  When I first tuned in it took me a second or so to know it was you!  All your rigs over the years have sounded very good but this one was different.  So congrats and I have to ask, whats the next one going to be?

Joe, GMS


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on March 30, 2014, 03:39:51 PM
Hi Joe,

Thanks much for the comments.  That is the last test - hearing reports from  Bros on the air who have good ears. I'm really happy with the latest results.

The difference with this rig is I went to the ends of the Earth to optimize every detail the best I could. I also got some good email advice from Jeff/ W2NBC, Frank / GFZ, Chuck / K1KW, Bruce / W2XR , and some on-air help from others like Tron.  

This time I didn't give up when something blew up or the arcing continued for months. The high frequency sweep top end was stalled out at 4-5 KHz for weeks until the break-thrus came using the smaller bypass / coupling caps.  In addition, using the GFZ solid state audio driver was key for transparency - as well as testing and tweaking exhaustively the THD and IMD  -  using NFB, regulated supplies and any other trick I could find to make it clean.

In the past I never really had a negative and positive peak limiter like the CRL - PMC - 300A. This is a nice box. And I think using two tubes in the final, but backing it off to 1/2 power, running oversized modulators in AB1, etc., all added up.  

Jeff/ NBC gave good advice when he said not to give up, but continue to refine - refine - refine.  That is where it's at when it comes to ironing out the finer details.

Of course there is more work to do and it will probably never end. I'm now looking at a faster attack compressor to control overshoot of the highs.  I also had some breath pops recently that I think were corrected by using a double wind screen and staying away from the mike by 2".  

Building and getting a big plate modulated rig running pristine, in my opinion, is a very difficult venture.  As Bruce /XR said, this is archaic technology.  There are  many parts that can create problems, phase shifts, mismatches, etc.  The testing and optimization is possibly the most difficult part of the project.

Hope to catch ya on and hear your FB rig soon,  Joe!

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3GMS on March 30, 2014, 04:18:39 PM
Tom,
Just listening to it, I think you will have a hard time improving on the on the air sound.  The way you have it EQ'ed it sounds exactly like you do in person.  Its a very transparent sound.  I was listening on an 8 kHz bandwidth with the audio pickup off the detector driving a 100W amp into a studio monitor. 

I use an Inovonics 222 look ahead type of negative peak limiter.  I like it, but I am not sure I like the sound as much as the CRL which your using.  I have that CRL unit and its always worked very well but has some crazy  intermittency that I have not spent the time to find.  So I called Jim up at Inovonics and got the 222 but am not convinced that its as transparent as the CRL.  For average gain riding compression, I like the Symetrix 528E.  It won't do high speed limiting, but its good to increase the modulation density a bit.   

Joe, GMS


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on March 30, 2014, 04:37:05 PM
Sounds like you have the bugs worked out.

I think the 300a is a fantastic box. Thanks for pointing out you could get one for a good price.
Someone sent me a recording of me on 40 meters today, and I think I sound good, not very smart, but the audio sounds good.
Same deal, pair of 4x150a's running very light at 300 watts out ,380 watts in, 40 watts plate dis per tube, and modulated by a pair of 4x150a's at reduced voltage and power in AB1.

Tends to be a kind of large transmitter for the power, like broadcast rigs.

I suppose its fun to go the other way as well, say a 300 watt transmitter made up of a pair of water cooled 6aq5's modulated by a pair at 3000 volts.





In the past I never really had a negative and positive peak limiter like the CRL - PMC - 300A. This is a nice box. And I think using two tubes in the final, but backing it off to 1/2 power, running oversized modulators in AB1, etc., all added up.  


T


Title: Listing of features Re: What is causing this audio highs attenuation?
Post by: K1JJ on April 01, 2014, 01:30:11 AM
Joe,

Yes, you are right about the 528E not being a fast limiter. I can go "Cluck - tock" with my tongue and see a severe flat top / over modulation from that fast rise time.  I needs a Dominator box... :-)


Well, I decided to finish up this thread with a summary listing of Fabio II features.  Kinda like the new car models:


List of Features in Fabio II, a 4-1000A plate modulated by 4-1000As:

1)   Pair of 4-1000As,  in final class C,  backed off to ½ or 1/4 power for cleanliness.

2)   Pair of tetrode-connected, push-pull  4-1000A modulators in AB1 run conservatively.

3)   Solid state WA1GFZ Mosfet audio driver. (Mod xfmr is the only audio transformer in rig)

4)   DDS VFO, Mosfet RF driver = 80 watts RF drive, all SS.   Driver has 600 watt capability with variable air flow.

5)   Hall effect failsafe shutdown over-current protection for RF final plate, screen and grid.  Also modulator plate, screen, and grid protected with separate sensors.

6)   Neutralized RF final

7)   Vacuum variables and dial counters for plate and loading

8)   Nine meters for full metering

9)   Viewing windows

10)   Six step sequencer (key-up, key-down) which allows variable delay for each step and also forward and backward order can be changed.   (W2NBC design)

11)   RF deck on pedestal for easy turning 360 degrees for servicing

12)   Aluminun cabinets are easy off in seconds for servicing.

13)   1 KW RCA 144 pound broadscash mod transformer

14)   150 H Heising modulation reactor    -   20 H screen self-modulating choke

15)   All squirrel  cage blowers have new bearings and run quiet on Variacs

16)   Modulator screen voltage electronically regulated. Grid bias uses diodes in cathode with selectable switching.

17)   Unique key-up and key-down arc quenching system using vacuum relays and large 200 watt power resistors across the mod iron and screen choke.   

18)   15KV 30A LARGE vacuum DPDT relay used for RF antenna switching. Good isolation for receiver accidents.

19)   CRL PMC-300A with separate negative and positive peak limiters.

20)   RE-20 mike

21)   Variacs on both screen supplies

22)   Silver plated edge-wound tank coil and 8” heavy duty ceramic bandswitch

23)   Standard  white panels, black lettering, IBM blue cabinets

24)   Modulator on casters for easy servicing

25)   HV supply easily switched / tapped for 2200, 3200 or 3800V.

26)   140 uF  @ 10 KV   total HV supply capacitance. (Choke switched in or out as needed)   Regulation excellent

27)   Modulators run as tetrodes, 590 volts screen voltage. Using 13dB of negative feedback around the modulators back to the low level SS driver.

28) THD (harmonic distortion) better than -35dB 2nd harmonic, -55 dB 3rd harmonic...   IMD (inter-modulation distortion) better than -35 dB 3rd order, -55 dB 5th as measured on HPSDR spectrum analyser.

29) Audio frequency sine wave response flat within 1 dB from 15 Hz to 10 Khz at 100% modulation. 10 Khz to 12 Khz  
80% modulation with clean waveforms. Triangle waveform looks perfect at 200 / 1000 Hz.  

30)   DDS VFO and RF output spurious down at least -80 dB from carrier.

31)   Side channel splatter well behaved and down at least =45dB when out of selected audio bandpass. No secondary side humps as seen on the SDR spec analyzer.

32)  ALL balanced audio cables thru-out the audio chain.  Used to run unbalanced for years.

33)   The babes just love Fabio.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on April 01, 2014, 02:56:36 AM
Lot of extras in this rig

What's the sticker price with the shipping??

Tax and title are extra.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3GMS on April 01, 2014, 07:28:15 AM
Kudos Tom! 

It looks like its ready to head down runway 29er put the maul down and get it airborne! 

As you know, its a lot of work to get everything right, but feels so good when you have reached that point.   Unfortunately when the going gets tough, lots of folks just quit and throw in the towel on the project.  You worked through the issues and learned a lot in the process, so whats not to like! 

Joe, GMS
 


Title: Latest TEST Recording - Re: What is causing this audio highs attenuation?
Post by: K1JJ on April 07, 2014, 05:37:22 PM
Jeff / W2NBC has been nice enough to spend time with me tailoring my AM audio chain.  He has been a tremendous source of audio information.

This is an on-air recording of Fabio II - Jeff made this afternoon on 75M, using a flat SDR at  + -  12 Khz  bandwidth.   I must say I am very pleased with the sound -  considering my average voice. My transmitted audio intentionally rolls off sharply after 7 KHz in this recording.

Notice the lows, sss's, ch's and ffff's.   Getting cleaner and the hard work is paying off.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on April 07, 2014, 05:59:06 PM
Tom,

I was listening a few nights back when you were on with Timtron, Tim was comparing different mikes.  Your audio sounded excellent, almost as good as mine. ;D

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on April 07, 2014, 06:07:41 PM
Tom,

Your audio sounded excellent, almost as good as mine. ;D
Fred


Thanks, Fred.   Yes, it would be a daunting task to challenge your audiophile throne...  ;)


Well, one of the last thangs I need to add here is a reduction in background blower noise. It ain't bad now, but I'd like try to reduce it another 4-5 dB.  I have an older DBX 904  noise gate on its way.  Normally noise gates are unnerving on AM, but I'm thinking of using a slow decay and keep some of the noise always there. (down -4 to 5 dB) This may give the illusion of a quieter background. We will have to experiment and see.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on April 08, 2014, 08:09:03 AM
The ideal thing is to have no blowers.
A silent rig is nice, and not just for the guys on the far end.
I built a few rigs that had no fans or blowers at all, and often use the 3x4d32 rig that has no blowers/fans.
The big rig has blowers but they are small and slow, and do not make much noise.

I use the noise gate in the vx2000, and it seems good if you do not over use it.
Its not a gate, its more of a reverse compressor, it reduces the gain when not speaking, kind of slowly, and does not cut anything off like a gate, just reduces the level some.


I hear guys on the air using amps that have blowers, loud and anoying even for me, it would drive me crazy to have that noise level in the shack.

So how big can you go without fans?
My pair of 813's needs no air, and 4 would do 1200 watts carrier easy.
Four 100TH's as modulators would work, with power to spare, and be quiet.

Can you run 4-1000's without air?
You could run 4-125's and 4-400's without air.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W2VW on April 08, 2014, 09:08:09 AM
After 20 years of living with air handlers in the shack I have again remoted mine.

Now the acoustic transformer hum can be heard in its full glory.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on April 08, 2014, 11:39:21 AM
Yes, 833As X 833As would make a nice, silent rig. Blow a little muffin fan air on the seals and they wud be FB.

I suppose a 4X1 in class C could survive without chimney air, maybe some seal air -  but a pair in AB1 modulator service that idled at a few hundred mils might not do so well.  Eimac does spec a chimney and forced air.

Well, at least my quad 6146 PDM rig is silent and will be get used a lot during the summer.

Another thang to think about is the HEAT generated in the summer. This is another reason for remote control out in the garage.   Of course, a class E rig solves all these problems.  But Rico Suave is long gone to another master.
I remember Rico (24 pill class E rig) running in  Jul / Aug with not much temperature rise in the shack. Amazing.

An expander with slow attack and decay might work here on AM. I'll know in a few days.

Some acoustic transformer hum is part of the scene here too.   We all need to find an old telephone booth. Stick it in the shack and operate inside.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WD5JKO on April 08, 2014, 12:02:01 PM
I suppose a 4X1 in class C could survive without chimney air, maybe some seal air -  but a pair in AB1 modulator service that idled at a few hundred mils might not do so well.  Eimac does spec a chimney and forced air.

   I recall a high voltage supply using the pulse version of a 4-1000 (4PR1000?) that was immersed in a tank of insulating transformer oil. It was silent, and no fans blowing. Just immerse the rig into an oil tank, and re-position with the front panel up facing the ceiling. This will also solve any arcing problem.  ;D

Jim
Wd5JKO


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KA2DZT on April 08, 2014, 12:03:03 PM
Tom,

If you put cotton in your ears you probably won't hear the fan noise.  I listened to your signal and did not hear any fan noise.

Fred


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3GMS on April 08, 2014, 12:44:16 PM
Yes, 833As X 833As would make a nice, silent rig. Blow a little muffin fan air on the seals and they wud be FB.
An expander with slow attack and decay might work here on AM. I'll know in a few days.
Some acoustic transformer hum is part of the scene here too.   We all need to find an old telephone booth. Stick it in the shack and operate inside.
T

Tom,
Did you ever think about building a wall with a sloped double pane window in the shack.  You would then have the transmitter room and a studio.  You could still see all the visuals of the transmitter through the large window in the wall separating the two rooms.   Set up a second Mic in the transmitter side of the room and mix in just the right amount of ambiance if you like a little bit of transmitter noise!   

Years ago when I was around 15 and my Mom had enough Ham Radio in the house Dad and I built an outside radio building.  I put a wall in that building separating the shack into two rooms.   I could easily see the two 6' racks housing the final and modulators through the large window.  No background noise and it made it very comfortable operating. 

When Dad passed away and Mom went into a retirement establishment I moved that building up to our home.  Looking forward to operating from that space about 125' from the house. 

Joe - W3GMS           


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on April 08, 2014, 12:45:46 PM
Jim, yes, it wud be impractical, but I wonder how glass (4-1000A) would handle being immersed in oil?   Maybe if it slowly heated up and down it would be OK.  It's just when we pour a cool liquid on hot glass does it crack.  Interesting but difficult to do...  ;D    What a job to change a tube or troubleshoot the rig.  You probably remember some of the old time hams using metal tubes upside down in water.


Fred, the reason for noise reduction efforts is I'd like to increase the density of my audio using the multi-band processor, but this brings up background noise.  If you listen to the audio.wav I posted a few back, you will hear the white noise in the background.  It would probably come up at least 4-6 dB if I do.    At least it's white noise and not bearing noise. Last week I brought all the blowers in for new bearings.  Made quite a difference.

T




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on April 08, 2014, 12:58:15 PM
Hi Joe,

A few guys have suggested your glass separation idea too.  It would work FB, but I just don't have the room in this shack to do it unless I broke down a wall and added the adjoining room.  My homebrew SSB linear is also a noise problem cuz I like to run a little processing for DXing.

I once looked into sound barrier curtains that cost upwards of $1K as well as acoustic tiles, carpets, etc.  But they would hold in the heat... I'd need outside ventilation, etc. I guess nothing beats the glass window cuz it keeps the rig close by, viewable and dead silent.

For now I may add some carpets on the hard tile floors and be satisfied with running lower density audio. After a certain threshold, blower noise is REALLY distracting on the air and should be quenched.  I find that noise shud be down -30dB or more on ssb from peak signal is important.   On AM, at least -40dB down is acceptable.  For a quick ssb reading, just tune in your own signal and look at the s-meter at full output and then key up with just blower noise without voice modulation for the difference in dB.  


Sounds like a nice set up that you and your dad built.

T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KB2WIG on April 08, 2014, 01:04:45 PM
T,

Ya could just put it in the garage, or build a small room. And hook up yer mon ittor. Is Fobio II shy?


klc




Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KL7OF on April 08, 2014, 01:41:43 PM
Tom that rig sounds very nice...I was playing the clip on the computer and Jan says "that guy sounds very good..smooth"...I told her it was FABIO......Nice Job  !
  I remoted the fan on my FRT 39...Put it outside on the shack wall with a weather cover...ducted the air inside...very quiet now...I can keep the exhaust air in the shack in the winter and run it outside for summer time ....Steve


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on April 08, 2014, 02:19:49 PM
Needs the cones of silence!


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3GMS on April 08, 2014, 02:20:09 PM
Hi Joe,

A few guys have suggested your glass separation idea too.  It would work FB, but I just don't have the room in this shack to do it unless I broke down a wall and added the adjoining room.  My homebrew SSB linear is also a noise problem cuz I like to run a little processing for DXing.

I once looked into sound barrier curtains that cost upwards of $1K as well as acoustic tiles, carpets, etc.  But they would hold in the heat... I'd need outside ventilation, etc. I guess nothing beats the glass window cuz it keeps the rig close by, viewable and dead silent.

For now I may add some carpets on the hard tile floors and be satisfied with running lower density audio. After a certain threshold, blower noise is REALLY distracting on the air and should be quenched.  I find that noise shud be down -30dB or more on ssb from peak signal is important.   On AM, at least -40dB down is acceptable.  For a quick ssb reading, just tune in your own signal and look at the s-meter at full output and then key up without audio for the difference in dB.  


Sounds like a nice set up that you and your dad built.

T


For some reason I find blower noise much more annoying on SSB than on AM.  For that reason, I always use the downward expander on my Symetrix 528E when I get on SSB.  Nothing is worse than hearing a SSB signal that say is 20 over and when he stops talking the S meter barely moves!  

Agree that to much blower noise on AM is not good either.  I think Bill, DUQ has the balance pretty close to perfection.  Just enough to add a bit of flare, but not so much that you would say, "listen to the blower noise"!      

Yep, sounds like it time to bump out some walls or add an addition on!

Joe, GMS


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N2DTS on April 08, 2014, 02:24:50 PM
http://www.youtube.com/watch?v=vsNR9FnxOdY


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on April 08, 2014, 02:46:14 PM
For that reason, I always use the downward expander on my Symetrix 528E when I get on SSB.  Nothing is worse than hearing a SSB signal that say is 20 over and when he stops talking the S meter barely moves!  
Joe, GMS

Thanks for the report from the XYL, Steve.  Most of them think we are weirdos, so that's a nice change... ;D


Joe, I also have a 528E here that I use as a "mic conditioner."  I use it as a preamp and a slight touch of compression before it hits the EQ and limiting.  I just tried the 528's expander and as you know, the technique shows promise for AM - HOWEVER, it is abrupt and unnerving. Probably be FB on ssb, but on AM we need a slower attack and decay as well as a variable atten level to work with.



*  WARNING:  This particular unit (card) requires an edge connector and has no case around it. You must also supply the +- 24VDC and +- 15V.    There are complete rack units available for more $.

Here's an older $35 DBX 904  I bought on eBay the other day. Shud be here soon. There are a coupla left if anyone wants one. This has all the knobs to make a seamless transition.  (I think)   It will let us keep some residual noise in there for masking the effect:


http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=161255261744&ssPageName=ADME:L:OC:US:3160

Here's the manual. Looks like 1980s technology, but that's OK.

http://adn.harmanpro.com/product_documents/documents/498_1323992363/904%20Owners%20Manual_original.pdf


The ad says:

"The 904 is an expander gate using the dbx® OverEasy® action for a smooth onset of gating. Attenuation limit, attack and release rates and threshold are all adjustable. This popular gate also features Programmed Latch Mode which mutes a channel until an above threshold signal is present. It then latches open until reset. The 904 is a very fast voltage controlled below threshold downward expander. It senses the level of an input or keying signal and determines whether the level is below threshold. If it is, the signal gets attenuated; if not it passes at unity (0 dB) gain. The amount of signal attenuation is a function of its own level and the 904's Attenuation, Limit, Ratio, and Threshold settings."

Keep it close to the noise source. (mic)   The 528E has  "preamp out / compressor in"   jacks on the back to allow this box to be inserted in an early position.


T


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WD5JKO on April 08, 2014, 07:58:38 PM
Jim, yes, it wud be impractical, but I wonder how glass (4-1000A) would handle being immersed in oil?   Maybe if it slowly heated up and down it would be OK.  It's just when we pour a cool liquid on hot glass does it crack.  Interesting but difficult to do...  ;D    What a job to change a tube or troubleshoot the rig.  You probably remember some of the old time hams using metal tubes upside down in water.

  The big tube is pretty buoyant, so the plate cap was down. This kept the tube in the socket when immersed in oil. Getting the tube in there is a blind operation, and often your arm goes all the way in such that the armpit is at the top edge of the tank and the shirt sleeve is wicking up the oil. Many of these oil tanks had high voltage supplies composed of voltage multiplier stacks. The caps were often about 5000pf @ 40KV, and the size of a hockey puck. If one of the diodes blew, then the cap next in the string remains charged...just waiting for an arm to get nearby. Too often there would be a jolting reaction followed by about 5 gallons of oil spraying around the room.

  We used to charge one of the caps up, and leave it on somebody's desk. They would hold that charge for years...someone sits down at your desk, and starts examining the paper weights. Its amazing how long it takes. Happened to me, and I threw it through a wall in an instant.

  Having a 4-1 in oil should not be a problem as long as the tube is heated up after being immersed.

Jim
Wd5JKO


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: W3GMS on April 09, 2014, 08:44:07 AM
Joe, I also have a 528E here that I use as a "mic conditioner."  I use it as a preamp and a slight touch of compression before it hits the EQ and limiting.  I just tried the 528's expander and as you know, the technique shows promise for AM - HOWEVER, it is abrupt and unnerving. Probably be FB on ssb, but on AM we need a slower attack and decay as well as a variable atten level to work with.
Here's an older $35 DBX 904  I bought on eBay the other day. Shud be here soon. There are a coupla left if anyone wants one. This has all the knobs to make a seamless transition.  (I think)   It will let us keep some residual noise in there for masking the effect:
[/quote]

Tom,
The DBX unit should, based on what I am reading, be a lot more controllable than the very simple downward expander in the 528E.  Yes, the downward expander on the 528E works very well on SSB but I  have never liked the sound of it on AM.  It tends to "flutter" at or near threshold and just creates an annoyance at least to my ears.  All the other functions on the 528E work extremely well and I really like the box.

Looks like the DBX unit you purchased is designed to plug into some sort of power module.  Although any well filtered +/- 24V DC supply should do the job.  I would likely use some +/- 24V 3 terminal linear regulators for that task. 

It will be interesting to see, with the control it provides, how transparent it will be.   

Joe, GMS 


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on September 16, 2015, 05:35:21 PM
http://www.youtube.com/watch?v=vsNR9FnxOdY

yeah I know 60+ days, but are those BC-610 plate tank coils in that video, on the top of the domes?


BTW Tom, did you ever have time to make the schematic for this set esp. the modulator with the feedback and tweaks? The MP3 file sounds so good..

I notice you said it is a class C RF stage. Is this a mod applied to the GG amp, Rico II, or a totally new set?
The reason I ask that question is because I was thinking about how a GG stage could be plate modulated. The stability of a GG stage, plus the ability to plate modulate it, would allow it to be used in two modes: As a GG linear amp, or as more of the AM transmitter - biased heavily for Class C, still with cathode drive and plate modulated. Does that make any sense?


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on September 17, 2015, 12:10:32 PM
Hi Pat,

Fabio II is the pair of 4-1000As in class C  plate modulated by a pair of tetrode-connected 4-1000As. The grounded grid amp is an 8877 and another completely different rig. That's Fabio I, thus the confusion... :-)  Rico Suave was the 24 pill class E rig which has been disassembled -  and another ham presently has and will someday resurrect.

Here is the GFZ audio driver I use for the 4-1000A plated modulated rig.  (Look at the third post with the file for the schematic)

http://amfone.net/Amforum/index.php?topic=23632.0

I never did document the final fine-tuned audio feedback circuit except for what is in the schematic. I think every rig will be a little different based on the mod transformer and Heising reactor used, as well as layout. So start with the basic feedback circuit as shown and tweak from there using an audio generator, scope and spec analyzer.

The changes I made also involved stability... by adding some small value bypass caps that tamed the audio feedback down but did not affect the highs too much. I know I had it right when I was able to dial in over 12 dB of audio negative feedback from the 4-1000A plate cap back to the low level 1 volt stage of the SS driver without oscillation. It made a big difference in the rig's ability to pass lows below 100 Hz  cleanly. As you can read in the various threads, I eventually got the transmitter flat and clean, 140% modulated,  from 15 Hz to about 10 KHz.  (less % but still clean from 10-12 KHz) This is not easy when using a mod transformer. The fact that this mod transformer is the only transformer in the complete audio system certainly helps.

At first I tapped the NFB off the secondary winding output of the mod transformer thinking I could correct for some of its phase shift. But in the end, the overall sweep results were better just tapping off one of the 4-1000A modulator tube plate caps and letting the mod iron have its way..

BTW, look for Stu's work here using plate modulated grounded grid amplifiers. I have never tried it but from his tests it seems to be quite do-able.

T


Title: Isn't she pretty? 4-1000A UPDATE
Post by: K1JJ on January 08, 2020, 11:05:28 PM
After seeing some of the AM rig pictures on this website with BIG tube viewing windows, I realized I've been missing out. For the 4-1000As  modulated by 4-1000As I had been using a tiny 3" window. Today I got out the saw and opened it up.

I can't believe the effect and improvement in viewing ambience.  I plan to get some real glass cut to size with brass bezels to cover the windows.   It's a thrill to watch the modulators change color as I speak. (AB1)  The finals are in a line so I see mostly the front one, but they stay almost gray running class C.


I'd recommend anyone planning a big tube project to make that window as large as they can. It's more work later to open it up because of the aluminum filings, so do it early on.

Fabio is now on air regularly and I finally gots it running close to where it was 3 years ago, but now the highs are even cleaner.  


T

The MODULATORS:   (click on file and should open straight up and down)




Title: Isn't she pretty? 4-1000A UPDATE
Post by: K1JJ on January 08, 2020, 11:25:41 PM
RF FINALS:


Are these coming out right-side up when clicked?  They do for me, but how about others?


And, 2 1/2 year old Yaz 4, English Springer Spaniel.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: WA2TTP Steve on January 09, 2020, 01:27:24 AM
They are right side up on my iPad.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: Opcom on January 09, 2020, 02:19:26 AM
good on windows/firefox!


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on January 09, 2020, 02:19:58 AM
Thanks, Steve and Pat...

I still can't get over the difference such a simple mod can make.  I still need to get the file out and clean it up - or maybe the new bezel and glass will cover up my cutting job... :-)


Old modulator vs: new version:


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: KK4YY on January 09, 2020, 01:29:12 PM
Some Car Door Edge Trim Molding would clean up the look.
https://www.ebay.com/itm/Car-Door-Edge-Guard-Trim-Molding-Rubber-Seal-Strip-Scratch-Protector-16FT-5M/123855665411?hash=item1cd65f7903:g:x3MAAOSwvW9dnDoF
Maybe sandwich in a thin sheet of lexan.


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: N4LTA on January 09, 2020, 05:37:43 PM

Looks Great! Sounds Great!

Pat
NLTA


Title: Re: What is causing this audio highs attenuation/ distortion problem?
Post by: K1JJ on January 14, 2020, 08:07:31 PM
To keep continuity of this thread, here are the finished products after adding in glass windows to the 4-1000A final and modulators.


T
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands