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THE AM BULLETIN BOARD => Technical Forum => Topic started by: steve_qix on January 30, 2011, 12:53:50 AM



Title: Broadcast Audio from your DX-60
Post by: steve_qix on January 30, 2011, 12:53:50 AM
In the spirit of the QRP AM Net, I dragged out an old DX-60 I had lying around the shack, and fired it up.  It worked great.  Several years ago I had made a number of modifications to the screen modulator, but had never documented these changes.  So, I traced out the circuit, and here it is.

The modulator is capable of SUPERB, pretty much class E quality audio.  The annoying carrier shift problem - legendary with DX-60s, even most modified ones - is elminated, and there is a screen voltage adjustment.  The modulator takes a standard more or less 1V P-P audio source, or you could use the other 12AX7 stage as a mic preamp, but this is not recommended, as there is no compression or frequency shaping.  But, it will work in a pinch.

The audio input is at the 2nd 12AX7 stage, at the audio gain pot.  The modulator features a negative peak limiter, set to approximately 93% negative.

When properly adjusted, the DX-60 with these modifications will modulate more than 150% positive and will give a really fine reproduction of the input signal.  Frequency response is quite flat from 5hZ to over 10kHz.

This is a perfect application for a low power transmitter, for for driving a linear amplifier.  The entire schematic is not shown, only the relevant portions of the modulator.

(http://www.classeradio.com/dx60_screen_modulator.jpg)

Please submit any questions or comments about this circuit.


Title: Re: Broadcast Audio from your DX-60
Post by: Steve - K4HX on January 30, 2011, 03:03:59 AM
Lots of options and mods for the venerable Dixie-Sixty. Mods for the DX-60 and the results by K4TAX several years ago.

http://www.amwindow.org/tech/htm/dx60k4tax/dx60k4tax.htm


Title: Re: Broadcast Audio from your DX-60
Post by: KX5JT on January 30, 2011, 04:44:59 AM
Then there's the circuit that Timtron was discussing with me a couple weeks back that Bob Bealmear, W0CAB used and published in the May, 1989 AM Press Exchange #71 that also switches the modulator over to pure screen grid modulation.

http://amfone.net/AMPX/71_1.gif (http://amfone.net/AMPX/71_1.gif)

(http://amfone.net/AMPX/71_1.gif)


Title: Re: Broadcast Audio from your DX-60
Post by: K4TLJ on January 30, 2011, 08:15:37 AM
And another that retains the controlled carrier.
(http://home.comcast.net/~suptjud/DX60AudioChanges.jpg)


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on January 30, 2011, 09:09:08 AM
Yes, there are other modifications out there.  This one has all of the best features in one circuit, and has a number of advantages:

1) The screen voltage is adjustable (important).

2) The frequency response is outstanding.

3) There is a negative peak limiter.

This one is also shows where to tap in and connect an external audio source, which would (hopefully) take advantage of the low distortion and good frequency response of the [modified] modulator.

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: Steve - K4HX on January 30, 2011, 11:49:47 AM
Thanks for sharing Steve. These mods will get moved or replicated to the Handbook Section soon.

Lots of options. This is also an easy rig to work on and would be a good first project if you wanted to get your hands dirty on a tube radio.

Bob, K4TAX measured the following with his mods.

Quote
Frequency response:  From mike input to about 100% modulation is +/- 2 dB from 35 Hz to 16 KHz*.

Distortion:  At <100% modulation (95% typical): < 1% THD+n at 400 Hz., < 3% THD+n at 40 Hz and 10 KHz.

Carrier noise: Bandwidth limited 50 Hz - 7.5 KHz is some -55 dB below 100% modulation.   This was measured at the cathode of the 6DE7.  Demodulated RF waveform confirmed these values.

* This value seemed to vary noticeably with various 6146A tubes.  Some tubes were good to 14 kHz while others seemed to start a roll-off in the 7 to 10 kHz range.


There are also mods from WC3K (now SK) that have been used successfully by many.

http://www.amwindow.org/tech/htm/wc3kmods.htm


Title: Re: Broadcast Audio from your DX-60
Post by: KM1H on January 30, 2011, 12:00:15 PM
Ive incorporated a mix of those ideas in the T-150A used at the cottage in Maine. Size and weight was the primary requirements along with a bit more power from 2 6146's. Controlled carrier with a bit less swing seems ideal as long as the power xfmr is up to it.

Another option if starting from scratch is Class A bias shift Heising plate modulation.

Carl


Title: Re: Broadcast Audio from your DX-60
Post by: K5WLF on January 30, 2011, 12:19:06 PM
Steve -- QIX,

I've got a DX-60A sitting on the shelf that's waiting until I get the shack re-arranged. In the meantime, your mods sound like a great project. I do have a question -- what's the power rating on the zeners? It's not called out on the skemat.

One more DX-60 noobie question -- Does this mod eliminate the controlled carrier?

Thanks,
ldb


Title: Re: Broadcast Audio from your DX-60
Post by: Steve - K4HX on January 30, 2011, 02:12:19 PM
Quote
Does this mod eliminate the controlled carrier?


Yes!


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on January 30, 2011, 08:41:03 PM
Steve -- QIX,

I've got a DX-60A sitting on the shelf that's waiting until I get the shack re-arranged. In the meantime, your mods sound like a great project. I do have a question -- what's the power rating on the zeners? It's not called out on the skemat.

One more DX-60 noobie question -- Does this mod eliminate the controlled carrier?

Thanks,
ldb

The zeners are low power - 1 watt zeners are fine!  I used the DX-60 today when I was on the New England QRP Net - maybe someone made a good recording.

The mods are relatively easy to do, and the results are definitely worth it!  The only other thing you'll need to have a top quality sound is some type of audio chain with a peak limiter, a reasonable microphone (which doesn't have to be at all expensive) and some type of eq.  All of this can be simple.

And as Steve HX pointed out, the controlled carrier (and all carrier shift) is eliminated.  There are a lot of DX-60 mods out there.  If there is a resistor with a capacitor in parallel with the resistor, and if that combination is in series between the screen and the modulator cathode, there will be carrier shift.  The zeners eliminate this problem.  The reason there will be carrier shift (negative, in this case) is because the screen voltage/current curve is not linear.  The current rises faster than the voltage, resulting in a variable voltage drop across the series resistor/capacitor combination depending on modulation - and this will cause a carrier shift.

I'll be glad to answer any questions that might come up as you proceed with the modification.

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: w3jn on January 30, 2011, 08:45:03 PM
Steve, thanks much for this - could you put all the writeup in one place and we can shoot it to the Handbook section once the discussion settles down.

BTW this should work on the Cheyenne MT-1 xmitter which is pretty much a  DX-60 with a VFO but no internal PS.


Title: Re: Broadcast Audio from your DX-60
Post by: W2PFY on January 30, 2011, 08:51:10 PM
Quote
I'll be glad to answer any questions that might come up as you proceed with the modification.

Hi Steve, I wonder if you would take it a step further by a design for a pair of 4-400 tubes or other tetrodes?   


Title: Re: Broadcast Audio from your DX-60
Post by: K5WLF on January 30, 2011, 09:03:00 PM
Thanks for the clarification and additional info, Steve. I'm looking forward to getting the DX-60A on the air to fill the gap until I can get something that'll heat the shack a bit more. Setting up the audio chain will be no problem. In a previous life, I paid the bills working live sound gigs and recording studios. I think I'll order some parts tomorrow.

Thanks again,
ldb
K5WLF


Title: Re: Broadcast Audio from your DX-60
Post by: W1AEX on January 30, 2011, 09:47:10 PM
...I used the DX-60 today when I was on the New England QRP Net - maybe someone made a good recording...

I did catch some of your DX-60 today Steve as you were chatting with Wayne WA1SSJ. The audio sample starts out with the receive bandwidth at 8kc and at about 15 seconds the bandwidth is increased to 12kc. There's a bit of Wayne's signal at the end for comparison. If I had a DX-60 I wouldn't hesitate to do that modification. It sounds very smooth and it's very asymmetrical.

Rob W1AEX


Title: Re: Broadcast Audio from your DX-60
Post by: Steve - K4HX on January 30, 2011, 09:50:11 PM
The DX-60 sounded quite nice.

Man, Wayne had some low end. I don't think I've ever heard him that "ballsy."


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on January 30, 2011, 10:24:30 PM
...I used the DX-60 today when I was on the New England QRP Net - maybe someone made a good recording...

I did catch some of your DX-60 today Steve as you were chatting with Wayne WA1SSJ. The audio sample starts out with the receive bandwidth at 8kc and at about 15 seconds the bandwidth is increased to 12kc. There's a bit of Wayne's signal at the end for comparison. If I had a DX-60 I wouldn't hesitate to do that modification. It sounds very smooth and it's very asymmetrical.

Rob W1AEX

Thanks for the recording, Rob - that's a good one.  Wayne was on his Class E rig - he REALLY sounds good on that thing!


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on January 31, 2011, 06:27:51 AM
Quote
I'll be glad to answer any questions that might come up as you proceed with the modification.

Hi Steve, I wonder if you would take it a step further by a design for a pair of 4-400 tubes or other tetrodes?   

A similar circuit would work.  The 4-400 would require a higher voltage screen modulator (up to 600 volts).  This might be a good job for a MOSFET - no cathodes to float.  I don't know how far negative the 4-400 screen has to go to achieve 100% negative modulation.

I actually ran 3  4-400s in parallel, screen modulated, back in 1983 or thereabouts.  Sounded great, but inefficient.


Title: Re: Broadcast Audio from your DX-60
Post by: w1vtp on January 31, 2011, 11:26:04 PM
Steve

I think your circuit would fit very well in one of mhy 3 Eico 720s

Thanks  Al


Title: Re: Broadcast Audio from your DX-60
Post by: KX5JT on February 01, 2011, 01:28:28 AM
I bet this idea would be rocking with a 4D32.  I'm envisioning about 25 maybe 30 watts of carrier with extreme hi fi. 

Man I love learning! 


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on February 01, 2011, 08:55:53 AM
I bet this idea would be rocking with a 4D32.  I'm envisioning about 25 maybe 30 watts of carrier with extreme hi fi. 

Man I love learning! 

It would be a good low power situation.

The 4D32 is about twice what a vanilla 6146 is with respect to power.  The 6146B has 30 watts of plate dissipation; the 6146 (no b) has 25 watts of plate dissipation and the 4D32 has 50 watts of plate dissipation.  This is the limiting factor with respect to power.

So, with 50 watts dissipation, and leaving plenty of headroom for positive peaks, you can expect about 20 watts output before you start to exceed the plate dissipation of the tube.  You could run about 75 watts INPUT to get just over 20 watts output.  If you load the tube so it is more efficient, the positive peaks will be severely compromised.

So, not too bad...


Title: Re: Broadcast Audio from your DX-60
Post by: n1ps on February 01, 2011, 07:45:08 PM
Steve I heard you also on Sunday while drilling and blasting the linear amp.  I may look for a DX60 at Deerfield in the spring...then I can give my Valiant to Jay my new son-in-law. Anyway at first I thought you were on the E rig...man it does sound nice. Are you sure you were not on the E rig...did you talk into the correct mic?  8)  Actually the day before someone complained about my audio...I was talking into the wrong side of the B2 mic...it had flipped around and I didn't notice. 

Well I am going to bite at the request for comments on the circuit seeings nobody else has... ;D

Of course I am a little dense at times (well OK MOST of the time) but here goes:
1. the 250K bias set pot - how does it adjust the 6146 bias?  I'm not seeing it....assuming the 12V ref supply remains constant.
2. Is the 36 volt zenor string and the 12volt zenor what composes the neg peak limiter?  Curious how you got it to clamp at 93%...trial and error (the second term is my normal method)?

Good stuff....
~ps


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on February 01, 2011, 11:37:27 PM
Well I am going to bite at the request for comments on the circuit seeings nobody else has... ;D

Of course I am a little dense at times (well OK MOST of the time) but here goes:
1. the 250K bias set pot - how does it adjust the 6146 bias?  I'm not seeing it....assuming the 12V ref supply remains constant.
2. Is the 36 volt zenor string and the 12volt zenor what composes the neg peak limiter?  Curious how you got it to clamp at 93%...trial and error (the second term is my normal method)?

Good stuff....
~ps

Hi Pete, thanks for the comments on the audio!!  On the circuit questions - the 250k pot sets the screen voltage of the 6146 and not the grid bias voltage.  The screen voltage set works by setting the DC grid voltage of the modulator cathode follower tube which in turn will set the 6146 screen voltage.

The zener string is there to provide a controlled voltage drop from the (positive) modulator cathode DC voltage.  The 39K modulator tube cathode resistor will pull the modulator cathode, in theory, to 0VDC.  The 36 volt drop across the 3 zeners in series (bypassed with a sufficient capacitor) will force the screen voltage of the 6146 to fall below 0V (to -36 v, more or less).  This is necessary to achieve 100% negative modulation.

The 12V reference is there mostly for the negative peak limiter although the screen voltage pot is also tied to this point as well.

So, the negative peak limiter is simply the 1n4007 diode tied to the 12V reference, which will prevent the screen from falling any more negative than about 12.7V.  The exact limiter percentage of modulation will probably vary slightly from tube to tube, but it's close enough!  :D

The good thing is that the circuit is relatively simple, and it works very well!!!

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: n1ps on February 02, 2011, 07:36:39 PM
Yep...used "bias" and not "voltage" in my text.  Did mean the screen voltage.  Where no appreciable cathode current flows thru the zenors, I could not see how you could set the screen voltage. 

The explanation of the neg peak limiter is perfect...I get it....very clever.  I wish setting a neg PK limiter for a high level modulator was this easy... ;)

The snow is now getting to be a pain.  I wish to register my complaint to the appropriate authorities. 

~ps


Title: Re: Broadcast Audio from your DX-60
Post by: K5WLF on February 02, 2011, 07:57:02 PM

The snow is now getting to be a pain.  I wish to register my complaint to the appropriate authorities. 

~ps

Ma Nature's complaint department is closed due to inclement weather  ;D  But, I'm not griping. Tomorrow is the third day in a row they've closed the university I work at. Only about 3" of snow, but all the roads are ribbons of ice. Nothing on the scanner the last two days except wrecks and cars in the bar ditch. The temp has been below freezing since Monday afternoon and won't get above it until Friday sometime. Hopefully. We not used to this kind of WX here in TX. It must be caused by global warming.

ldb
K5WLF


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on February 03, 2011, 09:58:40 AM
Yep...used "bias" and not "voltage" in my text.  Did mean the screen voltage.  Where no appreciable cathode current flows thru the zenors, I could not see how you could set the screen voltage. 

The explanation of the neg peak limiter is perfect...I get it....very clever.  I wish setting a neg PK limiter for a high level modulator was this easy... ;)

The snow is now getting to be a pain.  I wish to register my complaint to the appropriate authorities. 

~ps

I just plowed again - the sides of the 2000 foot long driveway are getting HIGH!!!

Anyway, the zeners simply drop the voltage by 36 volts from the modulator cathode voltage.  The screen current (at carrier) is sufficient to to charge the capacitor and have the zeners reach their knee.  There is also a resistor to the negative bias supply, but that is just there for insurance.

So, when the modulator cathode reaches, say, 10 volts, the screen will actually be at negative 26 volts because the capacitor across the zeners will stay charged. 

The screen voltage adjustment sets the modulator operating point, which in turn will set the screen voltage.  It's a reasonable way of setting the screen voltage, and lets the modulator tube handle the current (which it has to anyway) rather than involving another power handling component.

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: Knightt150 on February 03, 2011, 03:00:34 PM
Hello: I have two KNIGHT T150's and have spent a lot of time on trying to get 100% modulation (with out distortion) out of the rigs. I can do almost 90% but after that its down hill. Has anyone every done this or can it be done with the DX 60 mods.

John W9BFO


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on February 03, 2011, 03:39:55 PM
Hello: I have two KNIGHT T150's and have spent a lot of time on trying to get 100% modulation (with out distortion) out of the rigs. I can do almost 90% but after that its down hill. Has anyone every done this or can it be done with the DX 60 mods.

John W9BFO

It's a very similar setup, so I would recommend this circuit.  I don't recall the modulator tube used in the T150 - you may or may not have to change that as well, but if the tube is similar in its characteristics, probably not!

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: N4LTA on February 03, 2011, 05:32:04 PM
Slow day today at work. I layed out a PC board for Steve's circuit. May build it and try it in the next few weeks.


Title: Re: Broadcast Audio from your DX-60
Post by: ve6pg on April 11, 2012, 06:14:29 PM
..steve...do you have a dwg of your mods fer the dx60?

..tim..

..sk..


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on April 11, 2012, 06:21:16 PM
..steve...do you have a dwg of your mods fer the dx60?

..tim..

..sk..

Yes, in the first entry of this post (on the first page of the post, this is page 2).  The schematic should be there, etc.

Let me know if you don't see it, as there could be an error.

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: ve6pg on April 11, 2012, 07:23:26 PM
..thanks steve, got it now...maybe a dial-up error...is this the same type of mod bud did to his drake?..

..thanks agn..
..tim..

..sk..


Title: Re: Broadcast Audio from your DX-60
Post by: KM1H on April 11, 2012, 08:00:18 PM
The zener string bypass is non polarized?

Carl


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on April 12, 2012, 06:48:26 AM
The zener string bypass is non polarized?

Carl

In my own transmitter it is non-polarized, but there is no reason why a polarized capacitor could not be used, with the positive terminal connected to the cathode.


Title: Re: Broadcast Audio from your DX-60
Post by: Rob K2CU on April 12, 2012, 08:28:11 AM
Great circuit, Steve.  The cathode follower driver provides a real low impedance supply for the screen. I would only suggest adding a separate filament transformer for the 6DE7. The spec is a max peak cathode to heater voltage of 200V (plate 275V). And though a tube may start out handling the voltages, it will be stressed and may eventually fail. If you can loose the first half with its cathode essentially at ground, and make up the gain elsewhere, so much the better.

JJ had this issue with his neat dual 6AQ5 series modulated QRP transmitter.


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on April 15, 2012, 08:07:18 PM
Great circuit, Steve.  The cathode follower driver provides a real low impedance supply for the screen. I would only suggest adding a separate filament transformer for the 6DE7. The spec is a max peak cathode to heater voltage of 200V (plate 275V). And though a tube may start out handling the voltages, it will be stressed and may eventually fail. If you can loose the first half with its cathode essentially at ground, and make up the gain elsewhere, so much the better.

JJ had this issue with his neat dual 6AQ5 series modulated QRP transmitter.

The voltages are the same as the original DX-60 circuit.  I don't know the exact DC voltage on the 6146 screen, but I don't think it's all that high.  I did question this also, but figured folks have been using DX-60s for around 50 years, and I haven't heard about a lot of problems with the 6DE7 breaking down - heater to cathode wise, so thought I'd just stick with the exiting voltages  ;)


Title: Re: Broadcast Audio from your DX-60
Post by: k3vyy on February 06, 2013, 08:18:07 AM
Does the 250k pot need to be a 2W version, or will a 1/2 watt suffice. Just started on the mod and cleaning the old components out of the DX-60 I acquired.

Dick, K3VYY


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on February 06, 2013, 09:39:40 AM
Does the 250k pot need to be a 2W version, or will a 1/2 watt suffice. Just started on the mod and cleaning the old components out of the DX-60 I acquired.

Dick, K3VYY


A small pot is fine for that application.

Regards,

Steve


Title: Re: Broadcast Audio from your DX-60
Post by: k3vyy on February 14, 2013, 08:46:47 AM
Thanks Steve, 73


Title: Re: Broadcast Audio from your DX-60
Post by: KD7EDW on May 14, 2013, 06:58:43 PM
How do you drive this?  Do you use a mixer board or the just  pre-amp circuit listed on your class-e website?

Fred


Title: Re: Broadcast Audio from your DX-60
Post by: W4RFM on May 18, 2013, 12:14:58 PM
Hey Steve,
That sample is some excellent audio.  I may hit the next fest and snag me a -60 and try this mod. Very nice.


Title: Re: Broadcast Audio from your DX-60
Post by: steve_qix on May 19, 2013, 08:18:29 AM
I use my shack audio system - same one is used for all rigs.  This is really the only way to go if you have more than one transmitter.  Just build up an audio system with EQ, peak limiting, etc., make it line level out and then you can use it with anything.

That DX-60 modification is really just a generic, low power screen modulator that could be used with anything.  An easy home brew project, particularly if you just want it on a few bands - in case you have trouble finding a DX-60 at a flea market, that is.  There are a number of other screen modulated rigs that were made, and the modification would work just as well there!  :D


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 20, 2015, 12:12:50 PM
I built the circuit as a standalone modulator and it works real well if you add feedback from the screen output to the 2nd stage audio amp cathode with a 1 meg resistor and a .003 uf cap.

I added a d104 input but it also has a line level input.
The negative peak limiter works well at 400 Hz but looks very nasty at 40 Hz.
The waveforms look very good at 40 Hz to about 80% negative  and then get ugly when the NPL works.
Not sure that is a problem in the real world or not.

The circuit worked well into a pair of 4x150's running 100 watts output.


Title: Re: Broadcast Audio from your DX-60
Post by: WD5JKO on January 20, 2015, 01:51:41 PM
I built the circuit as a standalone modulator and it works real well if you add feedback from the screen output to the 2nd stage audio amp cathode with a 1 meg resistor and a .003 uf cap.

The negative peak limiter works well at 400 Hz but looks very nasty at 40 Hz.
The waveforms look very good at 40 Hz to about 80% negative  and then get ugly when the NPL works.
Not sure that is a problem in the real world or not.

The circuit worked well into a pair of 4x150's running 100 watts output.

    Brett,

    As you sweep down in frequency your NFB is going away causing a low frequency gain rise. Also audio that goes through a negative peak limiter (NPL) and is then fed back in a NFB circuit poses a problem since the NPL effect is a distortion that we want to keep. For this reason in some of my gear with a high level NPL, I limit any NFB around that stage to about 3 db (that be 0.707 in volts), and then see if I can add more without making things worse..

   That .003uf capacitor has about 1.3 meg ohms capacitive reactance at 40 hz. Maybe change that to a 0.01 uf or perhaps 0.05 uf and try that test again.

Jim
Wd5JKO


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 20, 2015, 02:03:53 PM
Great idea, I will try that.
The original circuit had NO feedback, at least it was not shown.
That might depend on the tube the circuit is modulating, I was doing a pair of 4x150's not a single 6146.

I only added the feedback to eliminate a lot of distortion when I had the normal amount of grid drive to the final, running the grid drive real light (1ma) had it sound ok, but the feedback made the grid drive not matter much.

I suspect the negative peak limiter just does not work well at low frequencies.
I should test just where it starts to fall apart and take pictures.



Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 20, 2015, 08:29:24 PM
I put a .01 7kv cap in and it still looks unclean.
Sounds great tho.
It starts getting funky looking at about 100 hz, but only when it gets into the limiter, and you can not see anything like that on voice.
This circuit works very well for a little tube circuit.


Title: Re: Broadcast Audio from your DX-60
Post by: flintstone mop on January 20, 2015, 08:33:30 PM
Thanks Steve
A very interesting thread about screen modulation. Seems to be a revival for the ease of obtaining hi-fi audio and plenty of positive peaks without the heavy iron and expense of modulator tubes.

Our own RF genius, Timtron, made some changes at WBCQ and got a military transmitter on the air and achieving out-of-this-world-audio. He is exciting this monster transmitter with a Viking II with about 25 watts. The Viking II was missing its modulator.
In Tim's words, " I decided to reserect it as a continuous duty cathode follower screen modulated transmitter. It is good for a maximum outpoot of 25 watts." He bypassed the 6CL6 and 6146 in the Viking II. The first stage of amplification is a PL-172/8295 tube in the IPA of this transmitter

It's unbelievable what this guy can do. He told me that this arrangement would sound much better than a commercially available PWM rig. Before the filters stop high frequency audio from being transmitted, the 'exciter" was good from 5hz to 15kc.....FLAT!!!
Fred


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 20, 2015, 08:49:26 PM
True.
35 years on AM and I never tried it till now, and boy, it sure works well.
No phase shift and other weird stuff that goes on inside transformers, no digital trash or pulses to be filtered out, just class A audio with no real reason for a restricted frequency response.

I was testing at 20 Hz 100% modulation today and nothing was going to blow up, and the waveform looked very good.
And the peak powers a screen modulated rig can hit is very high, I run my big rig at about 250 to 300 watts carrier and it hits 1500 watts pep very easy at 80% negative modulation.
Mic phase really makes a huge difference, no phase shift.
No transformers from mic to antenna...

I also like that most circuits have a power control, just dial up whatever power output you want.
Its a LOT of fun to play with.
 


Title: Re: Broadcast Audio from your DX-60
Post by: DMOD on January 20, 2015, 09:45:24 PM
There are quite a few approaches to modifying the DX-60 modulator.

N1UVI also has a HIFI screen modulated circuit as well:

http://amfone.net/Amforum/index.php?topic=30451.0


Negative peak clipping and power control can also take the following form:

Phil - AC0OB



Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 21, 2015, 10:14:47 PM
I tried it, it does not sound good at all with a 4D32 for some reason.
It only sounded somewhat clean at very low power levels (5 watts).


I bet this idea would be rocking with a 4D32.  I'm envisioning about 25 maybe 30 watts of carrier with extreme hi fi. 

Man I love learning! 


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 22, 2015, 08:22:32 AM
While the 4D32 seems close to a twin 6146 in one bottle, there are differences.
The 4D32 has much higher bias and screen voltage then the 6146 has in the same class of service.
120 volts on the screen for the 6146 I think, and 300 on the 4D32.
Much less grid bias and current on the 6146 also.

I suspect the 4D32 wants a much stiffer modulator.

This modulator works very well on a pair of 4x150/4cx250b's though.
A very clean 75 watts out at 1200 volts on the plates, likely good for 150 watts carrier at 1800 to 2000 volts.

Anyone ever try it with a pair of 6146 tubes? Knight kit 150 type rig?


Title: Re: Broadcast Audio from your DX-60
Post by: fg5fc on January 22, 2015, 03:02:58 PM
While the 4D32 seems close to a twin 6146 in one bottle, there are differences.
The 4D32 has much higher bias and screen voltage then the 6146 has in the same class of service.
120 volts on the screen for the 6146 I think, and 300 on the 4D32.
Much less grid bias and current on the 6146 also.

I suspect the 4D32 wants a much stiffer modulator.

This modulator works very well on a pair of 4x150/4cx250b's though.
A very clean 75 watts out at 1200 volts on the plates, likely good for 150 watts carrier at 1800 to 2000 volts.

Anyone ever try it with a pair of 6146 tubes? Knight kit 150 type rig?

Hello Brett
I'm a little bit surprised that WA1QIX does not do any comments on your results, you have made a lot of job, a lot of comments in building a nice screen modulation project!



Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 22, 2015, 03:55:33 PM
Its an old thread from 2011.
I just wanted to update it in case anyone else wanted to try the circuit, which works great within its limitations.

And I think some tubes just will not sound clean no matter how you modulate the screen.
I lump the 813 and the 4D32 into that category, the 6146 and the 4x150 seem good.
Maybe its tubes that use lower screen voltages that work best, and with this circuit, ones that do not pull much current on peaks and some tubes likely do not track plate current with screen voltage well.

From what I see, it seems like the output impedance is whatever the cathode resistor is?
In this case, 39k ohms?

In my tests, the 4D32 was flat topping easy unless run at very low power outputs (5 watts).
The solid state modulator worked fine with the 4D32's, even 3 of them at once.

I am sure Steve could design a simple mosfet circuit with a real low impedance that took little drive and was good for all sorts of voltage and current.
There are some amazing devices out there these days.



Title: Re: Broadcast Audio from your DX-60
Post by: WD5JKO on January 22, 2015, 06:25:39 PM

Brett,

    Take a look at the "Power-Drive" circuit at the following web site:

http://tubelab.com/articles/circuits/power-drive/

   The 2SK2700 FET is what the guy uses. Look at the gate of FET, and the battery used for bias control of the output tube. Just turn that battery around to set a screen voltage to the RF amplifier.

Jim
Wd5JKO


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 22, 2015, 08:15:46 PM
Yes, something like that would work, somehow.
The voltages are not excessive on many tubes, 100 to 200 volts plus the swing.
The W2IMX solid state design seems good, low parts count, only the screen power supply needed (no bias) and line level input.




Brett,

    Take a look at the "Power-Drive" circuit at the following web site:

http://tubelab.com/articles/circuits/power-drive/

   The 2SK2700 FET is what the guy uses. Look at the gate of FET, and the battery used for bias control of the output tube. Just turn that battery around to set a screen voltage to the RF amplifier.

Jim
Wd5JKO



Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 22, 2015, 10:24:26 PM
Waveforms from this modulator, 80 watts carrier out of a pair of 4x150's at 1300 volts, off the IF out of the homebrew rx.


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 22, 2015, 10:26:15 PM
the edge of the negative peak limiter at 40 Hz:


Title: Re: Broadcast Audio from your DX-60
Post by: flintstone mop on January 23, 2015, 05:36:41 PM

The Triangle looks almost shweeet Brett,
You gotta winner!!!

Now, my question is, once the hairy adjustments are made to get the screen modulation perfect; what happens when you QSY, and go through the peak the grid, and dip the plate of the finals?? The screen modulation adjustments never change??

Fred


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 23, 2015, 07:52:28 PM
I change bands all the time, all you really need to do is load the rig up to maximum power output, then increase the loading till power falls off quite a bit, redip the plate tuning and away you go.
The amount of loading sets the peak power, you can adjust it for 100% both ways and get a bit more carrier output at lower plate current, or gobs of peak power with a lower resting carrier.

I like to do it the way I do it, instead of tuning up for full power then turning things down as there is less stress on things, and less tuning at full power.
You have to be sure the tank coil is cut to give full power output with plenty of adjustments left (tuning and loading) to adjust things to get good peak power.

Grid drive always stays on the low side, the only trick (initial adjustment) is the resting carrier level.
Once that is figured out, its only plate dip and load, over load till the power falls off about half.
A peak reading watt meter is a must, mostly for looking at those crazy power peaks.


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on January 23, 2015, 08:09:45 PM
I will be running the modulator on 40 tomorrow I hope, about 70 watts out from the pair of 4x150's.


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on September 28, 2015, 11:25:35 AM
Just an update about this great circuit, I bumped up the voltage to the pair of 4x150's (4cx250b) to 1600 volts and run 200 watts carrier out of the pair with this modulator circuit and it sounds and works great.
200 watts carrier and 800 to 900 watts pep.
Some dx60 that is!


Title: Re: Broadcast Audio from your DX-60
Post by: VE3AJM on September 28, 2015, 11:40:59 AM
Was that the DX-60 that you were on yesterday on 40m with Russ WB3FAU? It sounded good.

Russ sounded good too with his Globe King 500, with the terrible? single ended 6L6 driving class B 811A grids through a driver transformer?

Al VE3AJM


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on September 28, 2015, 12:02:25 PM
Yes, I was running this modulator, into a pair of 4x150's at 200 watts carrier output.
Russ always seems to sound good.
Love the WRL power hum in the background of the audio, sounds powerful.




Was that the DX-60 that you were on yesterday on 40m with Russ WB3FAU? It sounded good.

Russ sounded good too with his Globe King 500, with the terrible? single ended 6L6 driving class B 811A grids through a driver transformer?

Al VE3AJM


Title: Re: Broadcast Audio from your DX-60
Post by: Donnie SWL on September 28, 2015, 12:54:49 PM
I guess you could run the plate up to 2000 on the 4x150 in screen modulation??The only tube i've seen eimac rate for screen modulation is the 4cx5000 an they run it at full plate voltage 7500 ....


Title: Re: Broadcast Audio from your DX-60
Post by: VE3AJM on September 28, 2015, 02:03:02 PM
Its interesting to me that the hum wasn't really noticeable listening on two receivers. R-390a at 8kc and Kenwood R-5000 at 6kc.

That being said, I've heard this 120hz hum on some WRL gear before. Including txs like some Globe Champion 300 and 300as, besides some GK500s.  I'm curious If this hum issue is VFO based or something in the power supplies with inadequate filtering, ground loops or something else in the audio.. Is there a consensus on it.

Al VE3AJM


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on September 28, 2015, 04:43:55 PM
Well, the audio chain has to go down to 60 Hz...

Much of it is the steel cabinet vibrating I think, at least in some rigs as Russ has said he improved the power supply filtering and the DC is clean.

I think it sounds cool, along with the big thunk on key down some have.


Title: Re: Broadcast Audio from your DX-60
Post by: VE3AJM on September 28, 2015, 05:00:52 PM
My 390a audio comes off the detector feeding a high quality PP tube amp. So hearing 60hz is no problem.

Russ was 80 db signal  ie. a very strong signal at the time.

Al VE3AJM


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on September 28, 2015, 09:13:01 PM
Maybe he moved the mic away from the transmitter, or was running something else?
He always has enough audio, and it always sounds clean to me.

You could likely run the plate voltage up to 1800 volts on the 4x150's, its all about plate dissipation.
With the 4x150's you can run into problems with reverse screen current.

I just made note of how things are running, ran the rig into the dummy load to prevent the swr from changing the power readings.

1800 volts on the plates,
260 ma cathode current,
160 watts out (carrier),

So:
470 watts in,
160 watts out,
308 watts dissipation out of 500 available.

With a 1.4 to 1 swr on the antenna, the power reads 200 watts carrier...

1800 volts is as high as that power supply goes.



Title: Re: Broadcast Audio from your DX-60
Post by: Steve - K4HX on September 28, 2015, 10:32:50 PM
Many were pretty light on the filter cap. IIRC some of the Kings used 4 uF.


Title: Re: Broadcast Audio from your DX-60
Post by: N2DTS on October 10, 2015, 10:55:02 PM
Worked him again today, the hum is there, quite loud, and shows on the sdr.
I had him turn the mic gain down and the hum was still there.
N2LJO joined in on his Globe King 400 and said his used to hum before he put a huge filter cap in.

There used to be an add in the old QST's about how everyone could tell when the guy was using the globe king.
W4TFA, C.L.Meistroff, said the boys know when the unmodulated carrier is on, the Globe King is on the air.
Not a hint of TVI either.
He said his Globe King really gets out!

I think they sound great stock, the background hum just makes them sound very powerful.
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