Peavey Classic 50 50 Power Amp Bias mod/repair

[Rambo227]

Hey I'm new to the forum so hello everyone!

I recently picked up a Peavey Classic 50/50 power amp and found out after receiving it that one of the output transformers isn't original and curiously enough one of the channels sounds like crap. (muddy lacking in high's and low's)

I was told that it is possible that one of the channels could have the bias off and I was wondering if anyone here would be able to show me how to add some ajustable bias pots into the amp? If this isn't the problem does anyone know what could be causing this problem and some possible fixes?

I have some experience but not a lot soldering and repairing electronics but basic stuff. I am aware there is danger here but it would be a good experience if I could fix this myself. Any input would be appreciated. Thanks.

[Jazz P Bass]

How about we repair the amplifier before modding the bias circuit.
You will need a number of pieces of test equipment.
And a fairly good idea of how tube amps function.

[Amp Kat]

Also think that the bias supply which is probably fixed @ around
-15vdc feeds both channels so if one channel works fine and it's bias is ok then bias is not the other channels problem. Agree with JazzP that there is a problem that may take some test equipment to solve unless you get extremely lucky.

[Rambo227]

Hmm, I was thinking that the bias could have been the actual problem but I don't know enough about it.

I have a electrical meter that measures voltage/amps/ohms. That's about it for my test equipment.

What might be a good starting point to figure out how to fix this?

[loudthud]

Step 1: Swap the tubes between the two channels to see if the bad sound follows the tubes.

[Rambo227]

Well I tried swapping the four el84's from one side with the four on the other side and still the same issue. Channel 1 has a dull flat tone lacking in clarity and high/low end. I put in some new preamp tubes and that didn't make much difference either.

[Rambo227]

Pictures of amp and possible problems

I decided to remove the tubes and take a close look for anything abnormal and I found some darkened spots that might be some kind of burn marks, also the solder on the back almost looks like it's possibly arced over on the other side where the burn marks are.

I'm curious to know what anyone else thinks of these.

One thing I just noticed is that there is felt pen marks in the shape of almost an arrow pointing toward the white retangular thing on the circuit board under the bottom left corner. I don't see any other black felt pen marks anywhere else on the inside of the amp.

[Rambo227]

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[kg]

those two 1k0 resistors that are flying high from the board have most certainly been replaced.

the large white box looks to be a ceramic high power resistor, also not OEM.

[Enzo]

When you replaced the "preamp" tubes, did you just put in new ones of what was already there? Or did you follow the schematic?

The small tubes are one 12AX7 phase inverter and one 12AT7 driver tube for each channel. Looking from the front, left to right, they should be AX - AT - AX - AT. Are yours? Wrong types and wrong positions will make this amp sound crappy.

Just to be technical, this is a powr amp and has no preamp. The small tubes are as stated. One is input buffer and phase inverter, while the other is a driver for the power tubes.

[kg]

One is input buffer and phase inverter, while the other is a driver for the power tubes.

is it basically a williamson circuit, enzo?

[Rambo227]

Forgive my ignorance for mistating the proper names of the tubes. Fortunately the original stickers were still on the amp for the phase inverter and driver tubes so yes the proper tubes were in place 12ax7 - 12at7 - 12ax7 - 12at7.

When I swapped them out I used a few spares of the matching type for each position so we can exclude that as a possible cause for the amps problems.

[Enzo]

kg - I'm not sure just what the topology is, here is the schematic below for reference.


Rambo - I'm not picking on you for terms. Just a tech lesson for anyone interested. AS a repair shop I often get circuits like this with the tubes in the wrong holes. The various SVTs are famous for coming in that way.


From the photos I'd say someone repaired it after a power tube or two burnt up some resistors.

You might determine if the odd transformer is the one for the bad channel or not.

If it is not something basic like the tubes, then I'd get out the voltmeter and start comparing channels. On the bad one, is ther B+ voltage on both plate and screen pins of all four tubes. COmpare to good channel. Is the grid voltage the same on all four. (bias voltage.)

And compare DC voltages on the small tubes. If those check out, then apply a steady signal and check signal voltages (AC) at the various tube elements. If for example one side of the phase inverter signal was being lost, it would sound crappy.

[Rambo227]

Enzo,

Perhaps I should have worded my reply differently because I am glad to have someone take the time to explain so that I learn the correct name and function of the tubes. Thanks for posting the schematic as well.

When I originally looked at the inside of the unit I followed some of the wires and it looked like the newer transformer was channel 2, the one that sounded better.

I am going to have to study over the schematic and try and figure out how to do what you described. Hopefully I can return with some more useful info then.

Thanks for the help

[kg]

ahh, not quite a classic williamson circuit.. that would have eliminated the cap coupling between the input stage and the concertina, and directly coupled them. in fact i'm surprised peavey didn't do that, as it would have eliminated c201/c210, r209/r232, r213/r236.

also surprising they used the at7 as just a follower, and also chose not to direct connect it to the output stage. i guess it's because of the mono/stereo switching. of course i would have direct coupled those suckers, injected the bias at the at7 grids, and ran the el84s into grid current.