Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Valiant Audio Mod Help




 
The AM Forum
December 12, 2019, 09:07:58 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Valiant Audio Mod Help  (Read 893 times)
0 Members and 1 Guest are viewing this topic.
wa2fxm
Member

Offline Offline

Posts: 22


« 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 99 times.)
Logged
W3GMS
Contributing
Member
*
Offline Offline

Posts: 2979



« 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
Logged

Simplicity is the Elegance of Design---W3GMS
wa2fxm
Member

Offline Offline

Posts: 22


« 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?
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7355



WWW
« 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.


* ValiantMods.jpg (458.63 KB, 1420x1210 - viewed 77 times.)
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
W3GMS
Contributing
Member
*
Offline Offline

Posts: 2979



« 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. 
 
 
  
Logged

Simplicity is the Elegance of Design---W3GMS
wa2fxm
Member

Offline Offline

Posts: 22


« 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?
Logged
wa2fxm
Member

Offline Offline

Posts: 22


« 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.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« 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
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« 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
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
wa2fxm
Member

Offline Offline

Posts: 22


« Reply #9 on: November 26, 2019, 12:00:08 PM »

250VDC.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« 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
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
wa2fxm
Member

Offline Offline

Posts: 22


« 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
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1374


« 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
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
kb2vxa
Member

Offline Offline

Posts: 141


I modulate, therefore AM


« 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.
Logged

73 de Warren KB2VXA
Station powered by atomic energy, operator powered by natural gas.
Steve - K4HX
Administrator
Member

Offline Offline

Posts: 2569



« Reply #14 on: November 28, 2019, 11:06:21 PM »

Tron mods here.

http://www.amwindow.org/tech/htm/valiantbytron.htm
Logged
kb2vxa
Member

Offline Offline

Posts: 141


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?
Logged

73 de Warren KB2VXA
Station powered by atomic energy, operator powered by natural gas.
Steve - K4HX
Administrator
Member

Offline Offline

Posts: 2569



« 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.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.068 seconds with 18 queries.