Amp noise question

[GloryJones]

I have a Blues Jr. that has been giving me problems for the past 3 years. It began with a rattling noise that turned out to be a bad tube. All the tubes that are currently in place are good. The speaker is rated well above what the amp puts out and should not be blown at this point.

Here is what is happening:

After turning my amp on and letting it warm up for about 5-10 minutes, I get a great, warm, round tone. All my pedals sound great and life is good. However, after about 20-25 minutes of playing (not super loud) it starts to produce a shrill, icy tone. The warmth goes away and it just sounds awful. I don't believe it is my guitar or my rig. Nothing changes between the time I start playing and the time things start going bad.

Any advice?

 

[thegoochie]

Have you looked at the board closely (without touching it) and looked for burnt out connections. Replaced all the tubes? Biased after replacing tubes?

 

[GloryJones]

I have looked at the board, will check again though. Nothing really stuck Out to me that I can remember. Definitely will not touch anything. Tubes are fresh, haven't rebiased tho. Had a tech tell me that I didn't have to with this amp?

 

[V2]

Always suspect tubes to begin with. Double check that the tubes are good or that the problem persists with a different set of tubes.

It sounds like heat may be a problem, so perhaps also open up the back (if possible) and put a fan on the chassis.

The fixed bias on the BJr isn't adjustable, and it is possible that something in the bias circuit is unstable, causing the bias to drift as the amp heats up. I don't think this is your problem, but it isn't out of the realm of possibility.

 

[GloryJones]

Always suspect tubes to begin with. Double check that the tubes are good or that the problem persists with a different set of tubes.

It sounds like heat may be a problem, so perhaps also open up the back (if possible) and put a fan on the chassis.

The fixed bias on the BJr isn't adjustable, and it is possible that something in the bias circuit is unstable, causing the bias to drift as the amp heats up. I don't think this is your problem, but it isn't out of the realm of possibility.

Good to know. Will check the tubes. Thanks for the input!

 

[Silent Sound]

Since it seems to be heat related, I'd eliminate the possibility of a bad tube first by replacing all of them with known good ones. Sometimes even a brand new tube can go bad after a few minutes. It's not likely, but it happens. If that doesn't fix it, I'd take it into a shop.

Now if you know a bit about electronics, like how to solder, how to discharge filter caps, and stuff like that, then I've got a next step for you. Get a can of compressed air and hold it upside down and squirt a bit on the components one at a time while playing the amp to hear if the warmth comes back when you spray that component. I'd probably start with the screen grid resistors. Just a dab will do you. If you hold a can of compressed air upside down and spray, it'll shoot out what ever liquid is in there to compress the air and it'll be super cold. That will instantly cool down anything it touches, and if heat is causing the problem, then you should hear a noticeable difference once the offending component cools. Be careful not to squirt your tubes, as they're glass and hot and probably won't like the sudden cool-off.

That's an old trick I learned from some electronics repair guys. Sometimes a part can measure in spec at room temperature, but then go wildly out of spec once it's warmed up. However, if you're not familiar with high voltage electronics, don't try it. Just take it to a qualified tech and let them deal with it. You don't want to ruin anything and make your problems that much worse and more expensive!

 

[Structo]

On newer PCB amps, particularly the ones soldered with no lead solder, the sockets pins can fracture around the joint causing sporadic symptoms.

You have to look real close to see the fracture which will appear as a line around the pin.

Another problem with those amps is the screen resistors get so hot they can de-solder from the board.
Install new resistors and elevate them above the board a bit to help cooling.

 

[GloryJones]

I'm a little nervous to touch the board at all, so I'm gonna stay away from that. If a complete tube change doesn't solve it then I'm gonna bring it in to a tech. Thanks for all the input. Hope this doesn't destroy my bank account.