Peavey 5150 Oscillation Problem-Solved!

[CJ]

James Brown, not the soul brother, helped with the design,

we have 6 preamp stages going on here, don't think i have ever seen that many,

nice chassis, we get a power drop and some red plating in the 6L6 tubes, usually one side or the other after thermal runaway happens, yes, even tubes have this problem,

for details see the 5150 xfmr thread,

 

[CJ]

we put a ton of 500 pf caps on all the tubes from plate to cathode, too much tone loss, so we try to isolate the problem,

looks like the phase inverter

 

[RuudNL]

Strange design... V2B has a high gain, but is followed by a 20 dB (or so) voltage divider!
Wouldn't it make more sense to lower the gain of V2B ( resistor is series with C5) and change to voltage divider R87/R91 so that it will cause less loss?
Is the oscillation still there if you lift one leg of C56?

 

[CJ]

looks like i had the tubes mixed up, 

silkscreen is kind of funny,

here is the tube loacation

 

[JohnRoberts]

Yes Jerome (James Brown) designed that one with Eddie looking over his shoulder and heckling him...  :-\

IIRC that amp was all about having too f'n much gain, an Eddie thing.

James has moved on to his own company now http://www.amptweaker.com/

I know it irritates me when people expect customer service for old Peavey products I designed in another lifetime, but James may have some useful observations if you catch him in a good mood. I know he spent many hours with that amp design.

JR

 

[CJ]

here is the inverter circuit

Thanks John!

yes i got an email from James on that 72 Coupe from Kustom which he did,

 

[JohnRoberts]

here is the inverter circuit

Thanks John!

yes i got an email from James on that 72 Coupe from Kustom which he did,

He's good people...

JR

 

[CJ]

ok here are some voltage measurements,

looks like we have a sq wave generator after the first two preamp stages,

notice the 39K cathode resistor,

this is the lead ch with the V dimed>

 

[CJ]

here is the first preamp stage, relay K4 is the crunch switch,

 

[CJ]

intermediate preamps

 

[walter]

I worked on one of those awhile back and I noticed that if I touch the metal brackets by the pre-amp tube pcb, I get a buzz in the output. The brackets that hold the pre-amp tubes pcb is isolated from the chassis with rubber grommets, they are also isolated from ground. They may not be a problem since you wont be touching them typically, but I added a wire to ground. I didn't like the idea of an antenna inside the amp.

 

[CJ]

thanks Walter!  tried grounding that chassis but no luck,

so far we have found a bad cable going to the effects loop which was bypassed, this eliminates about 2 feet of preamp wire,

then we found a mains wire that was threaded thru the terminal lug but not soldered so we fixed that,

then we soldered a 20 pf from V1 grid to cathode, replace the phase inverter and and last preamp tube with 12AT7,
this cures 99 percent of the oscillation, and makes the amp sound better,

then we found a bad jack connecting the OPT to the pwr tube board so we soldered the wires direct,

now the last problem is an intermittant loud burst of noise that happens every hour,

thinking we may have a conductive circuit board holding the pwr tube sockets, a bad transformer, or this clamp circuit here, which is probably used to eliminate switching noise,

 

[Whoops]

now the last problem is an intermittant loud burst of noise that happens every hour,

Had the same problem in a Marshall 6100, spent days over it and was never able to find the culprit.
It was really disappointing and tiring.

I really wish you have more luck

 

[CJ]

well the rewound transformer did the trick, the amp cranks all day without the feedback/volume drop thing,

what a fight this thing put up, now we can use the original12AX7 in the V2 spot, not that you need that much gain, who knows, a squre strat with weak pickups might like it,

 

[Whoops]

Well Done CJ,
Great Job!