Possible Micro-phonic Tube?

[Old Fart Rocker]

I had my Classic 50 at rehearsal last night. With the gain up on the lead channel, I was occassionally getting this high-pitched whine coming from my amp. I only noticed this when I wasn't playing and it only happened a couple of times. The tubes were replaced 1 1/2 years ago and I gig 3-4 times a month.

Does this sound like a mirco-phonic tube? I've heard of this problem but have never experienced it. If so, is this a sign a tube is going bad? Does this happen with pre-amp tubes, power amp tubes or both? Do I need to change the tubes now or is this something that is a characteristic of the tube?

I usually have my amp set up relatively clean and use pedals to get my dirt, but last night I didn't bother taking my pedalboard to rehearsal and had the gain up on the lead channel to get the dirty tone.

Any advice is greatly appreciated!

[Cirrus]

If it was affected by the preamp gain knob then it has to be a tube that comes before that knob in the circuit - ie one of the preamp tubes.

It's not uncommon and you can test it by gently tapping the preamp tubes with something non-conductive/hard eg a pencil or chopstick.

Annoyingly, going microphonic doesn't always mean the tube is going bad. Two of my best sounding 12ax7s are microphonic. Often microphonic tubes are perfectly useable further down the chain where their noise isn't being amplified as much.

So yeah, turn your amp on, do the tap test, and listen out for one tube which seems to ring out through the speakers much louder/ longer than the others. That's your microphonic tube. Either replace it or swap it into another position like the phase inverter slot and see if that helps.

And just a heads up, this forum is pretty dead now. You might be better off asking this question over on guitarampboard where there's more people to give you answers quickly!

[Miter Gauge]

I agree with Cirrus, it sounds like a microphonic tube to me too.  It doesn't sound like a microphonic pickup but just to rule it out you might want to check the amp with a diferent guitar or the same guitar on a different pickup (like the neck if you were using the bridge pickup at the time).  If you still have the problem it's likely to be a preamp tube.

Start with checking the VI and V2 slots, in my experience that is where I've noticed microphonic tubes the most.  Like Cirrus said, you can still use microphic tubes.  They work well in the phase inverter slot.

Good luck