Bass pot broken after modding the tone stack on Peavey C30

[rookey101]

Hi everyone,

I wonder if you could help me out with a small problem I've got?

I've just completed Steve Ahola's tone stack mods on my Classic 30, notably replacing the bass mid and treble caps, and notice a HUGE improvement in tone control and overall sound quality (I can't believe those tiny yellow blobs I took out are capacitors?! The metallised poly one's I replaced them with are massive).

However, on testing, I find the bass tone control doesn't do anything any more. It seems to have stopped working. At first I thought maybe my soldering or the components were are fault, but after swapping over the bass and mid caps, the problem still persists.

Does this sound like the pot is broken? How can I tell if it is? I have noticed that the bass knob does feel different to the others when I turn it - it feels lighter, like it's got less resistance to turning.

Should I need to replace the pot, short of getting in touch with the spares department at Peavey, could I replace it with standard linear pot? Any idea what the value of it is (1meg perhaps?)?

Thanks,

Matt

[Chuck H]

It's a 1M audio pot. I've never been in one of those amps so I can't tell you if it's a standard pot size and spacing. But you can because you can inspect it. You would need to remove the pot from the circuit for testing, sorta. You could test it to see if its adjusting at all by doing an ohm read while turning the pot. Also, check for shorts at either end of C18. One end should be about 2.5M from ground and the other about 470k. If either end shows no continuity there is an open circuit and this is why your bass pot isn't working.

Chuck

[Enzo]

It is a spider pot - sits up off the board on long legs.

Just call PV and order one. It isn't worth the time and trouble to engineer in some other pot that won;t have the long legs, won't fit the hole, won;t have the right shaft, and won't fit the knob, all to save a dollar.

First make sure you broke none of the small bare wire jumpers that connect the boards together.

[Chuck H]

Thanks Enzo. I do try to help, but the practical experience of doing repairs and having seen the innards of so many amps is soo much more valuable.

Chuck

P.S. What I posted still applies.

[tubeswell]

Of course it could be that the pot still works and that you merely just haven't soldered something in properly (I would guess maybe C18?). Sometimes if you are soldering stuff onto pc boards, and you don't put enough solder on, the connection isn't made. Go back and check your soldering - and possibly re-flow the solder on both sides of C18. (You might have to suck some of the existing crap off with de-soldering braid first.)

[Chuck H]

Thanks Tubes, Thats why I mentioned C18 in my previous post. PC soldering on mods is ALWAYS suspect.

Chuck

[tubeswell]

Thanks Tubes, Thats why I mentioned C18 in my previous post. PC soldering on mods is ALWAYS suspect.

Chuck

Right you are Chuck - sorry I didn't read your post thoroughly, I just jumped to the logical conclusion same as you.

[rookey101]

Thanks guys for the expert advice. I will get in touch with Peavey and see about getting a replacement pot.

In the meantime I'll also check the values at C18 and the pot, and recheck the soldering and jumpers.

Thanks again!

[rookey101]

Ok, I think I found the route of the problem. Me.
Doofus here has gone and installed 0.22mf caps, not 0.022mf...

So here's another question: I do have a couple of 0.022 orange drops spare, but they're rated to 100v. Would these be suitable for use in C18/19 or would they need to have a higher voltage rating?

Cheers,

Matt

[Chuck H]

You'll need a higher rating. Go with 400V.

Chuck

[tubeswell]

Actually you could easily use 100V for any of those tone stack caps in the C30, because C6 is the actual coupling cap from the previous stage, therefore all the DC has been blocked before it enters the tone stack.