Tube ringing

[Nubus]

I have this amp that once in a while starts to sing it's own note. I've tapped on the tubes with an eraser and found that the three 12AX7 seem to ring that particular note. Does anyone know if that is a fair assesment of the problem? I doubt the amp was ever serviced as it was only manufactured in the 90s.

[dagosto]

I had an amp do that. I think that is called a microphonic tube (correct me if I'm wrong here). Only one of the tubes did it in my head. I was using roove tubes so i just replaced that one and the problem went away. My amp also sounded better.

I think this can be caused be not allowing the tubes to warm up on standby and also by regular wear and tear.

[Nubus]

Did your amp sometimes ring constanly even when you weren't playing or plugged in? That's what this is doing. It's like the tube somehow starts to resonate and pick itself up. They're definitely microphonic. Sometimes the ring will just fade away too.

[dagosto]

It would definitely resonate when I played really loud. It was only a 60 watt fender though, so 10 was not that loud and it didn't go up to 11. It could be resonating off of the line noise in the amp. That thing needs to be replaced.

[Whoopysnorp]

That is absolutely a microphonic preamp tube (or three, in your case). You don't want them to do that--replace them ASAP. It's something that happens every once in a while on any 12AX7.

[Nubus]

The guy who works on my amps tells me that the preamp tubes don't go under a lot of stress from turning them on, or not letting the amp warm up. So he thinks that it just goes microphonic over time. He said the stand by feature really is for the protection of the power tubes.

Anyhow I ordered three new tubes and UPS smashed them. UPS showed up with a flat piece of cardboard and no merch.

[Whoopysnorp]

Ouch. Well, that's the shipping business for you. I usually use www.dougstubes.com, and he uses USPS. I've had no problems so far going that route.

[Nubus]

I threw the new set in there last night, it sounds much better. Somehow it's even louder.

[Whoopysnorp]

Yeah, I can easily believe that. There are so many variables with a device as crazy as a tube that volume fluctuations between sets of tubes are inevitable.

[smopo24]

Originally Posted by Whoopysnorp

Ouch. Well, that's the shipping business for you. I usually use www.dougstubes.com, and he uses USPS. I've had no problems so far going that route.

dougstubes is a great source; but i also use thetubstore.com and tubedepot.com; all very reputable (not to mention they take great care to properly package the merchandise).