Peavey Classic 30: Help identifying mods

[clarisso11]

Hi everyone,

I recently bought a used Peavey Classic 30, since I heard good things about the amp.

When I turned it on it hummed quite loudly, so I started troubleshooting by changing all the large electrolytic caps. While I was working on the PCB, I noticed that some parts seemed to have been changed.
I read a bunch about Peavey Classic 30 mods (like the Blue Guitar ones), but the changes on my one seem strange: It's all capacitors that have been replaced with resistors (details attached). The resistors used look like a different colour and make as the other ones on the board.

From what I've read, the circuit never changed on this, is this true? If so, why would someone put resistors in place of those capacitors, and is it safe to just drop the original parts back in?

[mikeydee77]

Do you have a photo of some changed parts?

Just thinking that the modder might have used those caps that look like resistors.

[Chuck H]

Circuit boards and schematics aren't always accurate to what part is installed. Factory changes and updates are sometimes implemented while still using the old boards and such. I don't believe this to be the case here but it should be mentioned. And...

There isn't necessarily a fixed method for mods and not all mod methods can be known. Anyone could have done anything to that amp without the benefit of diagrams or schematics. I don't think I've ever done a mod in my life that was published ahead of time. So there is most likely no record of any mods that may have been done to that amp. And...

Maybe fix the hum issue and see how the amp works and sounds before making other circuit changes. In fact, unless you were able to diagnose the hum issue cause as the "large electrolytic caps" you shouldn't have replaced them. The reason is that human error can never be guaranteed against no matter how knowledgeable or fastidious you are. So shot gunning problems always raises the possibility of complicating further diagnosis with new symptoms. I do hope replacing those capacitors fixed the hum. Or at the very least didn't raise any new issues. But try to avoid effecting repairs without a definitive diagnosis. This absolutely isn't a cut. I'm honestly trying to make the rest of this endeavor easier on you.

If you still have hum there are known troubleshooting methods for isolating the cause. They even usually work.

[Enzo]

The little green "resistors" are caps, ceramic caps. And they likely are original. The parts list is part of the original schematic file, and it specifies ceramic caps in many places. SO please do not yank all those out and replace them, they are original and very rarely fail.

[clarisso11]

Thanks everyone for the quick replies!

Do you have a photo of some changed parts?

Just thinking that the modder might have used those caps that look like resistors.

They are definitely resistors, look like the standard 1/4W type.
Here are photos of some of them, I just marked them so I can find them quicker:

[clarisso11]

Circuit boards and schematics aren't always accurate to what part is installed. Factory changes and updates are sometimes implemented while still using the old boards and such. I don't believe this to be the case here but it should be mentioned. And...

There isn't necessarily a fixed method for mods and not all mod methods can be known. Anyone could have done anything to that amp without the benefit of diagrams or schematics. I don't think I've ever done a mod in my life that was published ahead of time. So there is most likely no record of any mods that may have been done to that amp. And...

Maybe fix the hum issue and see how the amp works and sounds before making other circuit changes. In fact, unless you were able to diagnose the hum issue cause as the "large electrolytic caps" you shouldn't have replaced them. The reason is that human error can never be guaranteed against no matter how knowledgeable or fastidious you are. So shot gunning problems always raises the possibility of complicating further diagnosis with new symptoms. I do hope replacing those capacitors fixed the hum. Or at the very least didn't raise any new issues. But try to avoid effecting repairs without a definitive diagnosis. This absolutely isn't a cut. I'm honestly trying to make the rest of this endeavor easier on you.

If you still have hum there are known troubleshooting methods for isolating the cause. They even usually work.

Some good points here. I decided to replace all of the large filter caps since two of them had high ESR, and the amp is now almost 30 years old.
I will hold off on replacing those resistors with the values specified in the service manual, until I have put in new filter caps and given it a test.

Just seemed puzzling to me, everything else on the board looks exactly as it should (compared to the schematic)...

[g1]

The funny coloured resistors are not resistors, they are capacitors. Try measuring them, you will see.
(body colour often green or pink for caps that look like resistors)

[Chuck H]

Modding those amps has always been very popular. There are dozens of known mods and I'm sure many more that are just random circuit tweaks. How it works and sounds after any initial problems would be as good a place to start as any I think. Identifying everything that may have been changed and restoring it to stock may or may not be worth the effort. There's a reason they were popular mod platforms.

My own experience with a Peavy Classic model (a 2x12 50) was that it was to too bright and snappy in the unpleasant range of HF. Good otherwise but a glassy, sharp edge that persisted on all settings. I changed the speakers in that amp to something unconventional and it honestly sounded fantastic. I never had to change a thing inside the amp.

[clarisso11]

The funny coloured resistors are not resistors, they are capacitors. Try measuring them, you will see.
(body colour often green or pink for caps that look like resistors)

I stand corrected! Just lifted one leg of one of those parts and measured it, and sure enough, no resistance! No capacitance either, but that would be because of the sensitivity of my DMM.

I've worked on circuits (mainly guitar pedals) for a good three years now, this is the first time I've seen a ceramic capacitor like that! It's the 5-band color code that threw me off..

Thanks everyone for pointing that out!

[clarisso11]

Modding those amps has always been very popular. There are dozens of known mods and I'm sure many more that are just random circuit tweaks. How it works and sounds after any initial problems would be as good a place to start as any I think. Identifying everything that may have been changed and restoring it to stock may or may not be worth the effort. There's a reason they were popular mod platforms.

My own experience with a Peavy Classic model (a 2x12 50) was that it was to too bright and snappy in the unpleasant range of HF. Good otherwise but a glassy, sharp edge that persisted on all settings. I changed the speakers in that amp to something unconventional and it honestly sounded fantastic. I never had to change a thing inside the amp.

Thanks again for the advice, I won't alter the original circuit for now (especially knowing now that it likely hasn't been modded at all).

Can't wait to put it together again and give it a try, sure hope the new filter caps will fix the hum!

[Chuck H]

The funny coloured resistors are not resistors, they are capacitors. Try measuring them, you will see.
(body colour often green or pink for caps that look like resistors)

And after looking up images of Classic 30 guts I think they could be stock.

[clarisso11]

And after looking up images of Classic 30 guts I think they could be stock.

Good news, thank you for checking!
I might try some mods once everything is working as it should, but for now it’s good to know that I’m working with the correct schematic.

[Enzo]

The color code on those caps reads like a resistor, except the units are picofarads instead of ohms.