Bad valves?

[iep]

I'm mid renovation of a Peavey Classic VT (SS input, 6L6 output valves). All was going well until I finally fitted the new valves and turned it on for the first time. The amp emits a load of noise that sounds like popping/crackling/rustling. At worst it is like a detuned AM radio. Isolate the power stage via the effects loop and the noise remains. So, definitely the power stage.

So, I figure I've broken the amp while replacing the aged filter caps.

However, put the original valves back in (very old) and it works great. In fact, any combination of the old valves and one of the new valves (let's call it new-good) works great. However, any combination of these and the other new valve (lets call it new-bad) has the cracking problem. So, I figure I have a bad valve.

I phone up the shop though and they tell me that amp's bias is probably out and I need to check that before they'll consider a return.

Now, the Peavey Classic VT has fixed bias (no trim pot) so can't be adjusted without modding the PCB. Also, if the biasing was out and one of the new valves works (and they are supposedly matched) why wouldn't the other?

Lastly, would bad biasing really result in loads of noise?

Cheers,

iep

[Chuck H]

It's probably the tubes. Those noises go away sometimes after a bit but new tubes are too often bad nowadays. I can't think of anything about bias condition or something you could have caused by unplugging and plugging in tubes that should cause popping, crackling and rustling, but I've heard plenty of bad tubes do it. One exception would be grimy pin sockets. Try cleaning them. If the noise remains after you've cleaned the pin sockets and the tubes have "burned in" for an hour or so I'd say you have bad tubes. Unfortunately you probably don't have any recourse unless a credible tech sees to and handles the matter. However, it's clear that whoever told you that incorrect bias can cause those problems isn't experienced in these matters or is intentionally lying to you in the hopes that you'll believe the problem is yours instead of his.

[g1]

Also, if the biasing was out and one of the new valves works (and they are supposedly matched) why wouldn't the other?

You are 100% correct with your logic on this. If you can explain your procedure to the vendor, he should be able to understand the problem. If they can't understand that, well...
Maybe tell them you tried them one at a time in a Champ, which is single power tube and self-biased .