PV Triumph low gain

[drewl]

odd problem, I have low gain on the crunch and ultra channels, clean channel fine.

Swapped tubes, they're good.

look at V2a- pin 1 has nice gain, about 100v ac, but after the 100k resistor R23 to R97 and C19 coupling cap, it knocks the gain way down there.
C19 was changed, and jumpering another 100k across R23 or R97 doesn't help.

All preamp tube voltages are good, what am I missing?

[nashvillebill]

This is a difficult amp to troubleshoot, IMHO, the way they stack the boards.

I'd check to see if you are getting signal from the tone control board back and forth to the preamp board, as well as through the relays S2 and S4 properly.

The noise gate diodes CR1 and CR2: were they jumpered or did somebody replace those diodes with something goofy, or maybe cut one?

When I think of this amp, I think "what if"...What if Peavey had stuck it in a full-size chassis like their older classic twins, with only one board instead of four, and without all those jumpers and insulation-displacement connections...they would've had a reliable amp that kicked butt.

[drewl]

heh, I read your posts on repairing one of these.

The relays are good, the signal is getting attenuated after V2a, before it goes down to the relay board, that signal is going through the Ultra and Crunch pots and back up to the top board.

I'm going to replace the coupling caps and plate leads in the crunch and ultra stages and see how it is then.

The signal is getting past the diodes okay, slightly attenuated or "shaped: as it will, but the main problem seems to be V2a.
The way they have the signal going through the two 100k resistors before the coupling cap or it's tone network seems odd.
Unless it's supposed to be a much larger signal by then.

[nashvillebill]

What's your heater voltage on V2?

[drewl]

Like 5.8v dc.

[nashvillebill]

Mine was about the same, just a little lower than I'd like.

Have you changed the cathode caps on those three gain stages? C14, C15, and C18

[drewl]

nah, all DC voltages on those stages are normal, each stage is amplifying. they are cheesy electrolytics, I'm sure changing them would make it sound better.

[Gtr_tech]

I'd get rid of those "gating" diodes and replace one with a jumper. Those have a negative effect on tone....'specially if you like to run the gain lower. I did this on my Ultra head that resides in a friend's jam room and it made quite a noticeable difference.

[drewl]

AAGGHHHH!!!!

This is pissing me off.......I rebuilt the gain stages replacing the coupling caps, cathode caps with no improvement.

Jumpering across the diodes doesn't change anything.
The only odd thing is there is -24v dc on V2 pin 2 and -12v dc on V2 pin 7.
I thought maybe the diodes were causing this, but the negative voltage is still on those inputs with the diodes jumpered.

I removed R97, but the gain is still doen on the other side of R23, V2a plate.
Unless there is some stray capacitance or resistance around that stage causing the problem.

Otherwise I may set V2a up like a regular stage removing the 10k R22, and putting a coupling cap right on the plate.

Any other ideas?

[nashvillebill]

Dunno, mine had the stock factory values for all the components and it had gain out the wazoo (just the intermittent crap in my original thread).

My personal philosophy is that if the original design values worked OK when it was new, it should work now without redesigning anything; if it doesn't work right, something's broke...

What's B+ and how much ripple is on it? And have you reflowed all the solder joints on V2 and re-tensioned the socket?

[drewl]

Hmmm......it may be the stupidity is strong with this one...(me).

For the heck of it I checked the manual, and the preamp out voltage is 100mv nominal. The clean channel just puts out more.
FFFUUUUU......I have 100mv on both the Ultra and Crunch channels, so for the heck of it I swapped V1 12AX7 into V4 12AT7 and now it's pushing close to 60w.
I'm sure with correct replacements it will be fine.
That's the problem with playing with stuff here at work, I don't have any tubes around.....guess I'll bring some in.

[nashvillebill]

So it appears to be OK with a tube swap? Great!

Still might want to recheck the traces around V2 as well as the solder joints there, maybe seating the new tube temporarily made a dodgy connection get better. Tube sockets on PCB's really isn't the best design idea in the world, IMHO, though I understand the need to do that from a cost perspective.

[drewl]

Nope, it's not okay....but at least I'm not an idiot!

I can get 100mv at the effects send on Ultra and Crunch channels, but the clean signal is over 1v!

The only way I can get V2a stage to put out is to jumper a cap from pin one to the other side of R97, otherwise the signal on the other side of plate lead R23 is severely attenuated....

[nashvillebill]

The FX send is nominally 100mV at 47K load for both Ultra/Crunch as well as Clean, according to the owners manual.

Aha! Light bulb just went off. Ultra/Crunch go through the Thick tone relay, which bypasses the tone stack. Clean goes back through the Tone stack and bypasses the Thick relay. With tone controls dimed, maybe Clean will put out a full 1V.

There is a mod to bypass the Thick relay (leaving tone on all the time), I forget which jumper is cut.

[drewl]

Hmm, no "thick" control...is it on the footswitch?
This footswitch doesn't have it....Relay S1 pins 3,5,7 ?