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Author Topic: Valiant Audio Mod Help  (Read 5961 times)
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wa2fxm
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« on: November 24, 2019, 09:06:42 AM »

Need some help figuring out the audio mod in a Valiant that I'm trying to get acquainted with. I picked up the Valiant over the summer and while testing it on the bench I made a few contacts and got "good audio" reports. I buttoned it up and hauled it over to the operating rack and, of course, it died. Good carrier no audio. Back on the bench I open it up again and find I'm lost in the audio section.

Here's the mod according to the marked up schematic that came with the rig. But it's not completely accurate and other changes are not included. The Audio Driver plate resistor is actually 470 ohms and R45 at the cathode is actually 270 ohms. The plate coupling capacitor is a big fat 47uF 450V electrolytic(?!?). Back in the 2nd audio section R52 is 4.7k and R26 has been eliminated.

I know how to read a schematic and use a soldering iron but I'm no audio designer. And this configuration doesn't look like any of the half dozen Valiant mods I've seen posted around the web. But apparently the audio coming out of it was ok. From you smart audio wizards, is this audio chain worth repairing? Is there anything inherent in the design that might have caused the failure? Should I dig into this configuration and try and fix it, or should I scrap it and start with something else?

Mark WA2FXM



* ValiantMods.jpg (182.55 KB, 1420x1210 - viewed 396 times.)
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« Reply #1 on: November 24, 2019, 09:22:22 AM »

Hi Mark,
Those are standard audio mods.  Getting rid of the clipper and shunt feeding the driver stage are good things to do.  The stock interstage driver transformer does better when you remove the DC on the primary of the driver transformer. 

If you have a AF signal generator, I would feed that into the input and take a scope and go stage by stage and see what died.  It should not take long to find the issue.  Also, make sure you still have the right static resting plate current on the modulators.  If you don't have a scope, you could take a signal tracer and go stage by stage and see where the problem is.  Also, as a basic test, it never hurts to check the DC voltages, going from stage to stage.

73,
Joe-W3GMS
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wa2fxm
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« Reply #2 on: November 24, 2019, 11:00:54 AM »

Hey Joe,

But a 47uF 450V electrolytic for a coupling capacitor? Maybe that's all they had in the junk box and as long as it's a capacitor and it blocks the DC that's OK? And then 470 for the plate resistor instead of 4.7k. Closer inspection shows that this resistor is cracked open. I'm assuming that happened when the failure occurred. Like I said I don't know tube audio design, but it sounds like as long as the components are orders of magnitude close in value and do the job the design is ok?
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« Reply #3 on: November 24, 2019, 01:21:55 PM »

I suggest moving the 0.1uF capacitor, provided you can use suitably rated resistors. The effect of charging and discharging the cap especially when there are transients or other issues related to time contants can be reduced when has a lower voltage across it.

Also, a small cap to compensate for high frequencies across the resistors may flatten the response. The value has to be experimentally found using frequency sweep at full power (near clipping). A flat response well past the intended voice range will give less distortion, if other parts don't prevent the response.


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« Reply #4 on: November 24, 2019, 08:09:15 PM »

Hey Joe,

But a 47uF 450V electrolytic for a coupling capacitor? Maybe that's all they had in the junk box and as long as it's a capacitor and it blocks the DC that's OK? And then 470 for the plate resistor instead of 4.7k. Closer inspection shows that this resistor is cracked open. I'm assuming that happened when the failure occurred. Like I said I don't know tube audio design, but it sounds like as long as the components are orders of magnitude close in value and do the job the design is ok?

What you have to remember is the lower impedance your driving the larger the capacitor.  I have no idea what the primary impedance of the driver transformer is.  If the cap is not large enough, you low frequencies will go down the drain.  A good rule of thumb for coupling circuits is C X R = 25K.  In the end the cap will be pretty large.  Since your not driving a high impedance, but rather the primary of of driver transformer, the plate resistor has to be much smaller than the normal RC coupled audio stage.  The plate load resistor as shown on your schematic is 4.7K not 470 as you mentioned. 
 
 
  
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wa2fxm
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« Reply #5 on: November 25, 2019, 09:15:36 AM »

What you have to remember is the lower impedance your driving the larger the capacitor.  I have no idea what the primary impedance of the driver transformer is.  If the cap is not large enough, you low frequencies will go down the drain.

So the high value 47uF electrolytic is not unusual in this situation because of the low impedance of the driver transformer. And what I think of as "normal" coupling with lower capacitance values (say .01 uF) is what you see between high impedance tube stages. Maybe I've learned something here?
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wa2fxm
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« Reply #6 on: November 25, 2019, 10:53:15 AM »

Since your not driving a high impedance, but rather the primary of of driver transformer, the plate resistor has to be much smaller than the normal RC coupled audio stage.  The plate load resistor as shown on your schematic is 4.7K not 470 as you mentioned.
That was part of my confusion. The plate resistor actually seemed to be a 470 ohm resistor not 4.7k as was shown in the schematic. (Perhaps the heat scorched the red stripe to brown?) In any case I replaced C99 in case it was leaky and put in a 4k ohm 1W plate resistor which was all I've got in my junk box. On the scope I now see mic audio at the driver plate although there's only 250V there. But I think I'm headed in the right direction.
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« Reply #7 on: November 25, 2019, 10:38:25 PM »

Need some help figuring out the audio mod in a Valiant that I'm trying to get acquainted with. I picked up the Valiant over the summer and while testing it on the bench I made a few contacts and got "good audio" reports. I buttoned it up and hauled it over to the operating rack and, of course, it died. Good carrier no audio. Back on the bench I open it up again and find I'm lost in the audio section.

Here's the mod according to the marked up schematic that came with the rig. But it's not completely accurate and other changes are not included. The Audio Driver plate resistor is actually 470 ohms and R45 at the cathode is actually 270 ohms. The plate coupling capacitor is a big fat 47uF 450V electrolytic(?!?). Back in the 2nd audio section R52 is 4.7k and R26 has been eliminated.

I know how to read a schematic and use a soldering iron but I'm no audio designer. And this configuration doesn't look like any of the half dozen Valiant mods I've seen posted around the web. But apparently the audio coming out of it was ok. From you smart audio wizards, is this audio chain worth repairing? Is there anything inherent in the design that might have caused the failure? Should I dig into this configuration and try and fix it, or should I scrap it and start with something else?

Mark WA2FXM



1) He bypassed the clipper which is good.

2) he used the 4.7k in combination with the 47 uf cap to keep DC off the primary so the driver transformer would not saturate and produce distortion.

3) Feedback from the Mod. transformer's secondary is fed to a voltage divider where the resulting and reduced feedback voltage is applied to the cathode of V14 for further distortion reduction. Due to this arrangement, it most likely changes the bias of V14 somewhat.

If you find you need more low-end audio increase C90 to 0.05 uF.


Phil
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« Reply #8 on: November 25, 2019, 10:50:55 PM »


 
Quote
On the scope I now see mic audio at the driver plate although there's only 250V there. But I think I'm headed in the right direction.

Is that DC or peak to peak audio?


Phil - AC0OB
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wa2fxm
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« Reply #9 on: November 26, 2019, 12:00:08 PM »

250VDC.
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« Reply #10 on: November 26, 2019, 04:48:01 PM »

250VDC.

Sounds about right.

I am curious as to the voltage at Pins 3,8 as that could tell us how much current the 12AU7A tube is carrying. I suspect each triode section is carrying at least 7 mils.

According to EF Johnson's voltage charts for the stock circuit, there is 14 volts of Vk across 820 ohms so that should be 17 mA or 8.5 mA per triode section.

Thanks

Phil
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wa2fxm
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« Reply #11 on: November 27, 2019, 06:08:42 PM »

I am curious as to the voltage at Pins 3,8 as that could tell us how much current the 12AU7A tube is carrying. I suspect each triode section is carrying at least 7 mils.

According to EF Johnson's voltage charts for the stock circuit, there is 14 volts of Vk across 820 ohms so that should be 17 mA or 8.5 mA per triode section.

R45 at the cathode is actually a 270 ohm resistor not 820 ohms. That's what was in there before the audio went south (another part of my overall confusion with this circuit). I measure 7.0 VDC at pins 3,8. The 4K plate resistor I threw in there is dropping 100V, B+ = 350VDC, V15 plate = 250VDC. That's 25mA through a 4k 1W resistor = 2.5W, so it's heating up and bubbling after a minute or so.  Something is still wrong. And a quick look at the driver transformer wiring seems to show the primary still connected to the B+?

I'm going to take a break, stuff myself with turkey at my sisters house tomorrow, come home this weekend and redraw the schematic according to how this thing is really wired. Thanks to all who have helped out here so far. Happy Thanksgiving.

Mark WA2FXM
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« Reply #12 on: November 27, 2019, 10:22:58 PM »

I am curious as to the voltage at Pins 3,8 as that could tell us how much current the 12AU7A tube is carrying. I suspect each triode section is carrying at least 7 mils.

According to EF Johnson's voltage charts for the stock circuit, there is 14 volts of Vk across 820 ohms so that should be 17 mA or 8.5 mA per triode section.

R45 at the cathode is actually a 270 ohm resistor not 820 ohms. That's what was in there before the audio went south (another part of my overall confusion with this circuit). I measure 7.0 VDC at pins 3,8. The 4K plate resistor I threw in there is dropping 100V, B+ = 350VDC, V15 plate = 250VDC. That's 25mA through a 4k 1W resistor = 2.5W, so it's heating up and bubbling after a minute or so.  Something is still wrong. And a quick look at the driver transformer wiring seems to show the primary still connected to the B+?

I'm going to take a break, stuff myself with turkey at my sisters house tomorrow, come home this weekend and redraw the schematic according to how this thing is really wired. Thanks to all who have helped out here so far. Happy Thanksgiving.

Mark WA2FXM

Right, 13 mA per tube is excessive. The 270 ohm cathode resistor is not right either.

Same here, we're expecting about 18 at the dinner table tomorrow and I too am taking a break from my projects.

Bon appetit y'all.

Phil
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kb2vxa
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« Reply #13 on: November 28, 2019, 10:37:27 AM »

Your best bet if you want to use the Valiant's audio chain is get WA1HLR Timtron's mod sheet and rebuild it to his specs. Being a broadcast engineer and AM Gangsta there's nothing he doesn't know about Angel Music with AM Gangsta aka broadcast quality audio. The best part is he eliminates the driver transformer altogether and rewires the 12AU7 as a driver/phase splitter. Here's a hint, the 47K first audio grid resistor is the ideal load for a good broadcast type dynamic mic. If you want to use the classic Astatic D-104, that load puts a presence rise in the frequency response giving voice a bit of "punch". If you want Gangsta audio out of it raise that resistor to 11M.
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« Reply #14 on: November 28, 2019, 11:06:21 PM »

Tron mods here.

http://www.amwindow.org/tech/htm/valiantbytron.htm
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kb2vxa
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I modulate, therefore AM


« Reply #15 on: December 01, 2019, 02:53:51 PM »

Oh goodie, one of Tim's more flaming mod sheets. He makes it clear how much he hates the Valiant and suggests making it into an entirely different transmitter. I modded my share for the CB Gangsta crowd but didn't completely redesign them, just the audio chain, (hang the increased gain, just turn down the mic gain pot) solid state rectum fryers to eliminate fuse blowing 866 flash-overs and didn't worry about the extra 50V from the LV supply. Oh, that of course necessitated changing the drive pot from 4W to 10W and adding a 10K 10W WW resistor to the high end. As long as there's enough juice to give the required grid current just think Alfred E. and what. ME worry?

FYI from the Viking 2 forward Johnson decided to skimp on the power supply, that's what makes the 6146 modulators that should have been left 807s like in the Viking 1 & 2 strain to hit 100%. I found that not to be a serious problem, since the transformers are wound for 110V mains power and solid stating giving an additional boost albeit small, together they give the toobz a bit more headroom. According to the monitor scope I could easily over-modulate considerably in the negative direction with no flattening of positive peaks. Maybe Tim's Valiant wasn't eating its Lucky Charms, they're magically... something. It reached critical mass in the Desk Kilowatt and caused meltdown at full output, at half power they sound just fine. I heard a demonstration, the distortion was awful, a scope would have shown the positive peaks wilted badly.

One last thing, my transmitter modding daze and the CB Gangstas was before the Alpha Hotel invasion that drove us away with all their nonsense. It was a learning experience that led to Amateur Radio for all of the local CB Gangstas, tain fawr?
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« Reply #16 on: December 01, 2019, 11:18:36 PM »

Yea, you have to ask yourself, are all those mods worth it for a 100-150 watt transmitter. I'd recommend wiring the modulator grids to an appropriate transformer and drive with an outboard audio amp. Much less work and probably better audio. Different strokes for different folks.
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« Reply #17 on: January 05, 2020, 08:36:46 AM »

Finally got the Valiant back on the air this weekend. Apparently the 47uF audio output coupling capacitor was breaking down under load (voltage? temperature?) and running up the plate current. I replaced it with a new one, put a 5.6K 2W plate resistor in there, returned the cathode resistor to 820, and grounded the bottom of the driver transformer primary. It all seems to be operating fine now. I'll leave the Timtron mods for the next time I have to open it up.

WA2FXM Mark
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« Reply #18 on: January 05, 2020, 05:11:46 PM »

Hi Mark,

Here are the modifications I implement when a Valiant is in the shop for upgrades/enhancements.

I dispense with the 6AL5, the clipper circuit, and the 6C4 entirely.

R31, and R43 are too low in resistance and don't allow sufficient voltages to build up in the speech amplifier stages. R37's value was mainly for the benefit of L45 to help dampen harmonics.

You won't need that complicated feedback system drawn in Red when the 12AX7A's have been placed into their more linear operating regions.

With the new resistor values you will have a voltage gain of over 1500 in the speech amp chain and more than enough drive for the 12AU7A.


Phi - AC0OB

* Speech Amplifier and Driver Modifications.pdf (125.88 KB - downloaded 101 times.)
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kb2vxa
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I modulate, therefore AM


« Reply #19 on: January 10, 2020, 07:55:43 AM »

High again,

I just remembered the last one I modded for an AM Gangsta who being a broadcast engineer had his AM station made out of AM broadcast gear from mic to antenna. BTW it was all saved from the scrap yard by dumpster diving. (;->) You can use a small hi-fi amp instead of an FM peak limiter he used, but some sort of compressor/limiter is recommended. If you use a solid state amp a 1:1 coupling transformer is needed, SS outpoots aren't fussy about load impedance.

What I did to feed the driver grids with 600 ohm line level audio from the limiter was to use a 600 ohm to 50K transformer with the clipper pot across the secondary and the wiper to the 12AU7 grids. That was needed as a gain control because with constant audio input the mod % varies with final loading. I used your mods between the driver and modulator grids. The result was amazing, the transmitter audio response proofed out fairly flat from 40Hz to 10KHz, not exactly FCC compliant, but you can add a LP filter.
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73 de Warren KB2VXA
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