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Broadcast Audio from your DX-60




 
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Author Topic: Broadcast Audio from your DX-60  (Read 29328 times)
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steve_qix
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« 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.



Please submit any questions or comments about this circuit.
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Steve - K4HX
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« Reply #1 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
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KX5JT
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« Reply #2 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

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« Reply #3 on: January 30, 2011, 08:15:37 AM »

And another that retains the controlled carrier.
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« Reply #4 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
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Steve - K4HX
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« Reply #5 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
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KM1H
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« Reply #6 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
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« Reply #7 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
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Steve - K4HX
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« Reply #8 on: January 30, 2011, 02:12:19 PM »

Quote
Does this mod eliminate the controlled carrier?


Yes!
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steve_qix
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« Reply #9 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
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« Reply #10 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.
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« Reply #11 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?   
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« Reply #12 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
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« Reply #13 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

* WA1QIX DX-60 SAM 3.880000MHz 1-30-2011 4 29 55 PM.mp3 (616.88 KB - downloaded 670 times.)
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Steve - K4HX
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« Reply #14 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."
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steve_qix
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« Reply #15 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!
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« Reply #16 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.
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« Reply #17 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
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KX5JT
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« Reply #18 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! 
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« Reply #19 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...
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« Reply #20 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?  Cool  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... Grin

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
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« Reply #21 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... Grin

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!  Cheesy

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

Regards,

Steve
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« Reply #22 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... Wink

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

~ps
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« Reply #23 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  Grin  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
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« Reply #24 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... Wink

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
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