New 4X1 X 4X1's Rig - Problem with audio spikes - Questions - Pictures

[K1JJ]

The reactance at 10Kc in a 500pf cap is >30K. I don't think it's a problem.

(See recording below of 4X1 rig by Al/ VTP)


Interesting.  I did the C calculator before putting them in there, so figgered they wud be transparent for audio. The fact that the modulators are on the same chassis made me do it.

I just got a recording from Al/VTP.  For a first shot I thought it sounded FB.  Though, Tron said earlier he heard some slight distortion. He's got such good ears for this stuff maybe he's hearing the effects of that driver transformer, dunno.

OK on the info, Bruce and Stu. Good points and I'll keep them in mind as I continue here.

Hope to get back on 75 in a while while the skip is still short.

[WA1GFZ]

run that time constant with the output Z of the driver transformer

[N2DTS]

I never used any capacitance at the grids of the mod tubes, no parasitic chokes, just a resistor across each secondary winding of the driver transformer to give it a load to look at.

I was home for lunch and heard you on Tom, with others, very strapping signal into South Jersey, I did not notice the audio (a good sign) as I was doing antenna work, getting the swr down a little and running some rg214 to the roof.

I got the swr down to 1.3-1 at 3880....

From what I heard, it sounded good, but I am no judge at all.

It seems like you are getting to the nit picking phaze, which if you let it, can go on forever....


Brett

[N2DTS]

I was not compairing the caps to oxygen free cables.
Dont all audio amps look into a dc short?
Is that not what a voice coil is in a speaker?
Much lower resistance than a transformer I would think.

I suspect really odd things to happen if you used half a winding, or tried to run audio through 2 windings in paralell, or even if you ran it into a much higher impedance than it was designed for.

Brett

I think you just need some oxygen free cables between the audio amp and the transformer.
Brett

  Brett,

   A direct coupled SS amp looking into a DC short is a bad thing. In order for that to work, the AMP must have zero DC offset, and the audio waveform must have ZERO asymetry. Those two things are tough to achieve. I don't put the series cap idea in the catagory of oxygen free cables. Sorry.

Jim

[W2XR]

run that time constant with the output Z of the driver transformer

That's exactly the way I look at it as well.

73,

Bruce

[K1JJ]

OK, I'll take out the 500 pf caps and see if it helps the high end.  Tnx.

I'll replace them with some 20K resistors to ground.

Had a ball on 75M tonight with the new rig.  Plate modulation has a soul- the only way to go... 

T


BTW, Here's a recording from Jeff/W2NBC.  I'm generally pleased with what I hear so far! Hope to fine tune it some more.  Tnx, Jeff.

[N2DTS]

Sounds good to me!

Brett

[Gito]

Hi

Reading N3DRB report ,receiving  your Transmitter,He wrote that the audio  sound is flat,I have the conclusion that Your Transmitter is Okay,but lags "power" in it(audio),So since You all ready used SS Audio Amplifier,why don't You used a preamplifier with tone control, or compressor or some kind of audio prossesor  to Shape Your Audio,in front of Your SS amplifier.To boost the high tone or the Mid tones or the low tones,suitable for Your "taste"
Just a Suggestion .


Gito

[N3DRB The Derb]

oh it sounds good now.    I mean, like : really good. 

no joking, that transmitter is smoking. It's got personality now, Thomas. Good show.

[K1JJ]

Thanks, Derb!  It was a nice surprise to hear ya break in tonight.  You were vely loud here with the Gonset.  I will have to slide down the band and join you guys this weekend for the WFD Net.

Gito:  Yes, we have used what you suggested for many years. I use an Electro-Voice RE-20 mike into a 32-band equalizer, into a 6-band Beringer audio processor. I think the problem yesterday was I had the wrong taps on the driver xfmr and was killing the highs according to my audio sweep test.

It's sounding better now and I have plans to improve some things further. It's a long haul to get a plate modulated rig just right.  I never have an easy time with it.

Now to pull out those 500pf grid bypass caps and see what happens.

T

[WD5JKO]

I was not compairing the caps to oxygen free cables.
Dont all audio amps look into a dc short?
Is that not what a voice coil is in a speaker?
Much lower resistance than a transformer I would think.

   Brett, Sorry I snapped a little there. Im on a business trip that has went awry, and I'm out of clean clothes, and too many 12 hour days. 

Here is a post of spec's on a 4 ohm impedance 800 watt RMS 15" Woofer:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-420

This dude has 3.8 ohms DC resistance in series with 2.5mh, and a rating impedance of 4 ohms. No worry here driving a big amp into it. A big transformer on the other hand on the low - Z primary can be much less than 1 ohm DC resistance even though the impedance rating could be 4, 8, or 16 ohms.

Tom, That rig sounds fantastic. Congrats!

Jim
WD5JKO

[K1JJ]

Thanks for the link, Jim - I'll check it out.

Well, Frank and Bruce - you guys were right!  I took out the 500 pf bypass caps in the modulator grids and the high end now goes well up into the 8-9kc range without distortion!  Those caps were definately affecting the transformer. 

I still notice the mod iron doesn't want to modulate heavily up above 6kc or so, but at least it's clean. The highs start to roll off at about 6kc but the modulator current stays the same and the mod tubes start to show color. Looks like typical iron capacitive shunting. It's an RCA BC 1KW mod xfmr.

I ran into another problem tonight that took me about 4 hours to find. Seems the modulators started idling at about 600ma out of nowhere. What a bear it was to find, but turns out one of my terminal strips that connect the grids went intermitant and the grids were floating on one of the tubes at times.

During the task, I accidently left the Jesus stick on the HV and turned it on. It burned the arms off my HV keying relay. It looks like an amputee.  So screw it.. I may not be keying the HV anymore and just leave it on the particular rig.

All in all, the audio sweeps pretty nicely now in the high end. That's probably what the Tron was hearing for distortion b4 - I'll see what he says tmw.  I can sweep from about 25 hz all the way up to 7 kc and see little variation in the sine wave - looks quite nice... only that it starts rolling off above 6kc. I plan to roll off the audio above 5kc anyway, so that's not a bad thang.

Later -

T

[Gito]

Hi Tom

I  also used also used  Behringer  Compressor for a small Bc Station in my town.
Actually its  6 band  compressor ,limiter ,gating ,which can be set for each band channel individually  ( the frequency band is divided into six band.)
The purpose is that if  a frequency say low frequency is compressed it doesn't compress the high Frequency band,and vise versa  if the high frequency is compressed .it doesn't affect the Low frequency band.

It is as a leveling amplitude of the frequency band,reducing  the dynamic range an make a more solid sound .

 I think the equalizer must be placed after the Compressor :  Compressor- Equalizer- Amplifier.
If you put a signal in the the Equalizer and boost it for instance 1 v and the compressor treshold  is .5 v than you always get a .5 v out put,You boost it to 2V You always get a .5 v output.
So how you try to boost your high frequency band and the end it is limited by the compressor.

So say if you use a pink noise/white noise injected in your Compressor,and after the compressor  You have a flat compressed Frequency response and drive with it Your Modulator/transmitter and at the end You has a lost in the High frequency notes.

It can be caused by the interstage trafo ,You used a 80 ohm impedance trafo as an 4 ohm input,even if the impedance ratio is okay (primair to secunder) a 80 ohm impedance must have much turn winding than a 4 ohm impedance,so the inductance is much higher than needed by a 4 ohm design trafo. the effect can boost the low frequency .and some looses at high frequency.
And maybe caused by some other reason.

So after  having a flat response  audio  after the compressor and using  an equalizer after it,you can change this flat frequency to a frequency with a frequency band with a boosted high Frequency end by the Equalizer,and hoping it can compensate the high frequency lost in the Modulator.

And since You used it after the compressor, the highest peak output of it is still the same,even how hard  You drive  your amplifier( first limited by the compressor than shape by the Equalizer).
Again I can be wrong.

Gito

[steve_qix]

Definitely time to get RID of those transformers in the driver chain 

I have an audio driver project in the works (for push-pull tubes).  When it's complete, it would be very instructive for me to bring it to your site and hook it up to your modulator.  Then we can actually hear and measure the results!

At that point, there will be a practical, proper implementation of something that will potentially be of a benefit to many.

Regards,

Steve

[K1JJ]

Gito:

Thanks for your info. Yes, the 6-band processor is a very nice device for bringing up the average density independently of freq.

I use my compressor last in the chain ... mic > preamp > EQ > 6-Band processor.   I always thought the low end could be manipulated better that way and was always told to put the compressor /limiter last in the chain.  Does anyone else have an opinion on that?  There was a time when I swapped them back and forth and eventually settled on this config.  I'm always open to new ideas.


T

[W2XR]

Definitely time to get RID of those transformers in the driver chain  

I have an audio driver project in the works (for push-pull tubes).  When it's complete, it would be very instructive for me to bring it to your site and hook it up to your modulator.  Then we can actually hear and measure the results!

At that point, there will be a practical, proper implementation of something that will potentially be of a benefit to many.

Regards,

Steve

Hi Steve,

I would be very interested in this circuit as well.

I could replace the audio input xfrmr that splits the phase at the input to my audio driver, and the driver xfrmr that couples the audio from the p-p 845s to the 833A grids.

I still would need to couple the existing negative feedback loops (one for each 833A) in my rig into this audio driver of yours. I assume your design makes provision for this.

Can you briefly describe the topology of this circuit for us? Are you doing just the engineering and design, or are you also providing an unstuffed PCB with a parts list and schematic, etc.?

73,

Bruce

[KC2IFR]

Tom,
Heard your new transmitter yesterday......all I can say is it sounded GREAT....nice job.
I recorded u but so did other people so I wont waste your time sending u the recording.
Hope to work ya soon.

Bill

[WA1GFZ]

Yes a pair of 11N90s as source followers driving the grids would motivate the modulators well into HF if done correctly.

[K1JJ]

Tom,
Heard your new transmitter yesterday......all I can say is it sounded GREAT....nice job.
I recorded u but so did other people so I wont waste your time sending u the recording.
Hope to work ya soon.

Bill

Thanks, Bill. Yes, I received four recordings so far - that's plenty...

Last night I improved the high end some more and think I've licked the grunge Tron mentioned. So I could use another recording the next tme you hear me.


Steve/QIX - I sent you an email about working with you on the MOSFET driver project.  I'm in no rush to put together a new driver now that this SS amp and xfmr are acceptable for now.

Bruce (and Steve) - For Audio Negative feedback, consider that KLR cathode circuit I posted earlier in this thread. It has a NFB stage that will mix with the regular low level audio. Bottom line is I have a universal stand-alone NFB board that will feed ANY audio driver -   ready to go.

T

[KD6VXI]

The reactance at 10Kc in a 500pf cap is >30K. I don't think it's a problem.

Series resonance?

--Shane

[K1JJ]

I'm in the process of reconfiguring the 4X1 modulators from triode connected to conventional AB2 tetrode connections. I built a 400V regulated screen supply and bypassed the heck outa the modulator screens. I will drive it the same way with the grid driven SS amp and xfmr. It will be able to draw grid current for power. The screen will fluctuate between 0-95ma.

The Tron said his old 4X1  X 4X1's triode connected had some linearity issues and suggested trying them as tetrodes. He said the screens neeeded  a higher voltage above the grid to function better.  (As usual) We'll soon see.   If anything, they will be much easier to drive and the SS amp / xfmr will have a much lighter load.

I'll post a pic later of the rewired underside of the three 4X1's.

T

[WA1GFZ]

Tom,
Try to get away with low screen voltage to avoid distortion when the plate swings below the screen. I noticed something in the low end when you were hitting it hard last night. Not sure if it was over modulation because didn't have the spectrum display on. Maybe you could drive the screens and ground the control grids as an option.

[K1JJ]

Here's some pics I took of the rewired 4X1's underside. You will see the RF tube at the left end and the modulators at the right. Notice the new screen bypassing in the modulators - setting up for regulated 400V of screen voltage as tetrode modulators. That 600 uf bypass cap shud do it for the lows.  The LC tuned circuit for the final is about 4" away in a sub-chassis. I wanted the area clear for air as much as possible.  The RF deck is very stable with neutralization and a 14K resistor network as swamping. Easy to drive with about 10w.

Also shown is the RF deck with the tubes removed. Terry, there's a view of the aluminum angle/ sheet construction you wanted to see.


Notice the tiny vac variable neutralizing cap.  The C1, plate tuning cap is not visible cuz it is directly behind the 2000 pf loading vac var.

See the weatherstripping that provides a tight seal for the chassis for air flow. I use four large ground straps on the chassis corners to RF ground the subchassis since the weatherstripping is insulated.  The tank coil looks crowded in the pic, but actually has a diameter of space around it.

TEFLON wire used, of course...

OK on the report, Frank. Yes, there are still some minor issues that I need to work out. I think this tetrode mod and an audio driver w/o a transformer will do it.

Later -

T

[W1AEX]

I've been listening around the past couple of days hoping to hear your new rig on the air. I figure those flashes in the sky off to the southeast are probably occurring when you key and unkey. You do nice work Tom! How come my stuff looks like the picture below?

73,

Rob

[K1JJ]

heheheh... Rob you bagger - I've seen some of your work and it looks quite anally inspired..  Meaning you dress it up nicely.

You will find with my work that I don't make any real effort to make it pretty. It is strong, fuctions well, I try to keep things square and at right angles, (sometimes) but that's about it.   There's lots of work out there that will put my work to shame.   I tend to recycle and tear down things a lot, so it's more of a time thing too. But you can drop my stuff from a few feet and it will stay together and it will work well when I'm done - that's really my only requirements...

T