What is wrong with my Classic 30?!

[Kyle vs. Guitar]

Since about October or so, my amp has been making hideous roaring noises when I turn it on, which go away after about ten or fifteen minutes. This happens when I have the amp and guitar volume turned all the way down, on the clean channel, so it's not like I'm failing to let my amp warm up or something. When I play, it usually seems to make this noise when I hit the bass strings, but like I said, it eventually (mostly) goes away.

I thought the problem might be due to tubes, so I replaced all four power tubes with new ones back in November, I think. It didn't make noises for about two weeks or so, then it started doing it again. I bought the same type of tubes as were originally in the amp, so I don't know why this is happening. Any ideas on why my amp is acting up, and how I can get it fixed?

[StompBoxBlues]

Others hopefully will join in but a couple of things...

I think it sounds like it IS in the power section. Do you still have the old power tubes? If you do, and I were you, I would pop them back in and see if the same problem occurs. BEFORE doing that though, I would look (in a fairly non-lit room) at the power tubes to see if any of them glow red, or seem to be different than the others.

Did you buy a matched set?

In any case, that would be the place to start I think. I could be wrong, but think that if the noise is happening with guitar unplugged, (or off) amp volume off, then I think that points to the output section.

And because it acts as it does...it seems like it might be heat related. Usually heat problems go the other way (sounds okay...after good warm up begins to cause problems) but there are heat problems I have seen go this way too...problems when cold, as heats up something expands, makes better contact, etc. and runs okay. For one...the tube heater, maybe?

[Kyle vs. Guitar]

Unfortunately threw out the old tubes after I got new ones. The replacement tubes are JJ EL34s, bought from The Tube Store website. They're supposed to be matched. I checked the tubes while the amp is on, and they all glow pretty much the same red color, like I would expect. None of them are brighter than the others.

[StompBoxBlues]

Originally Posted by Kyle vs. Guitar

Unfortunately threw out the old tubes after I got new ones. The replacement tubes are JJ EL34s, bought from The Tube Store website. They're supposed to be matched. I checked the tubes while the amp is on, and they all glow pretty much the same red color, like I would expect. None of them are brighter than the others.

It's always good to hold on to them at least for a little while but no problem. Have you watched the tubes from when they are COLD ? Just to see what happens?

Listen, if you know you are going to want/need backup tubes at some point as backup, for next tradout...and if you have enough cash, you ought to buy a new set.

The idea I am having is this...for one, watch the tubes from when the amp is cold til it starts happening after you turn it on, til it starts sounding normal. See if any changes during the 10-15 minutes.

Here is the thought though...if there is a heater connection that is bad, it could be that it is taking the time for the OTHER surrounding tubes to actually heat (by convection) the bad tub plate so it starts working?

I had a heater circuit problem on an amp, and it did that...Something similar anyway. I can't remember if it was the preamp or output, but there were tubes that weren't lit up (I believe I saw the red glow not be there on one tube) and just to be clear the tubes should not glow red (but up at the top is a little red glow) but blue if anything I believe.

[meterman]

I see you are near Atlanta, there are two very good techs here, Jeff Bakos from Bakos Amp Works and Jeff Andrews from Andrews Amp Lab. I'd start with a full new set of pre- and power tubes from Bob at Eurotubes, the JJs sound good in the C30 and Bob tests his tubes before sending them out. It's possible to have new tubes go bad immediately...a new set would be your cheapest bet at about $65 or so, and keep the old ones for backup/spares. If new pre and power tubes doesn't fix it I'd take it to one of the techs I mentioned and get a diagnosis, usually for a $40 bench fee that is applicable to any necessary repairs. Trouble is, the C30 uses relatively cheap PC boards and ribbon cables making them difficult to work on. They also use cheap components, it could be that something has just failed and needs to be replaced....

One thing you can do, is lightly tap each tube with a pencil or chopstick while the amp is running, to see if you can narrow it down to one that might be acting up, sometimes you can recreate noise or microphonics this way....

BTW you want EL84s not EL34s, just a typo I'm sure but....

[Kyle vs. Guitar]

Yeah I just typed it wrong, I have EL84s in it. I'm tempted to just take it in somewhere for repairs since I've already tried messing with the tubes and that didn't fix anything.

[StompBoxBlues]

Originally Posted by Kyle vs. Guitar

Yeah I just typed it wrong, I have EL84s in it. I'm tempted to just take it in somewhere for repairs since I've already tried messing with the tubes and that didn't fix anything.

In looking at the schematic, if you have an extra 12AX7A (if not you could trade V3 with V1) I would also try replacing V3 preamp tube. It's safe to do (with amp off of course) and may or may not make a difference.

IF you do try this and even if it still isn't sounding right if it changes that will tell you something. In fact, if you are changing the power tubes, it is a good time to change preamp tubes too maybe? In any case, I would try that before taking it in.