Peavey C30: moan from the speaker

[pulsar705]

Today i turned on the amp and for a few seconds i heard a strange noise, like a moan from the speaker. Which cause,bad cap,tube microphonic ? I was not playing!

thank you,jon

[6267]

That's called an oscillation. They can sound like lots of stuff, but any sort of tone that the amp makes when you're not playing is 99% oscillation (there was some amp with a built-in tuner...). It can show up when the amp is turning on and shutting down, other times it's constant, other times it can be started by playing certain notes etc. I just tested an amp I'm designing which uses an interesting output transformer setup...I connected the feedback wrong, and it was making starwars/willywonka noises at full power through a concert stack (this was a nightmare sound, and I'm sure my roommates enjoyed it...oops).

Mostly, it means that some part of the output signal is being sent back into the amp to be amplified again. It can result from bad circuit design, connecting the feedback backwards (my problem), faulty tubes, filter caps, crossed wires, etc. It's definitely one of the 900-pound gremlins of all tube amps, and they can be hard to trace. Start by swapping preamp tubes. If that doesn't fix it, try pulling tubes (starting with V1 and proceeding until the power tubes)...if the problem goes away, it's happening in that tube or before it. It won't hurt your amp to have preamp tubes "missing" when it's turned on. Does it happen all the time or only sometimes?

[pulsar705]

Does it happen all the time or only sometimes?

Ever happen,yesterday only a few seconds.I tried to replace V2

[tubeswell]

Try replacing all the pre-amp tubes - one at a time. (The first thing to check is tubes). A microphonic pre-amp tube can make all sorts of unsolicited noises - any this can be intermittent to start with - if the tubes is starting to go bad.

Another check for microphonic tubes is to hold them up to your ear with one hand firmly grasping the tube by the base, whilst gently flicking the side of the bottle with one of your other fingernails. If the tube makes a muffled 'dink' sound it'll be okay - if it jangles like a jingle bell, it has haddit.

The other thing is output tubes can also induce weird oscillations if they are starting to go bad. Those EL84s really need replacing every 12 months or so depending on how much you play the amp. Get a new quad and put them in. You're going to need a new quad someday anyway right?

If it still makes funny noises once you have tried replacing all the tubes, then report back.

[pulsar705]

if the noise was always present I could do some tests to identify the cause and fix it, but it happened only once yesterday for a very short time and I felt it only because I was not playing.
However, after the event I replaced the pre-tubes (the quartet of EL84 is new and well selected). I hope it will not happen again.

thank you,jon