I got the microphonic blues!

[Stankfut]

A whle back, I rebuilt a Kay 703C. Replaced all the electrolytics, added some extra filtering. Quite as a church mouse. Problem is, when I crank it up, there is a slight ringing sound in the overdrive. I figured the preamp tube was microphonic. I lived with it for a while, but I finally bought a new 18GD6A. Same problem. Is it the preamp tube, or could it be something else?

[HotBluePlates]

It could be anything.

A tube could be microphonic (any of them). Slight tapping with a pencil may be heard through the speaker as a "thunk" even in a good tube; if you get a ping that quickly turns into a howl, then you know you have a microphonic tube. I have had some bad enough before that no playing or tapping was needed: if you turned the volume above 4, the amp started howling.

Sometimes people recommend brushing the glass with a toothbrush to find the tube that's truly howling. Tapping/banging a tube could send vibrations through the chassis and shake the truly-microphonic part that you're not directly tapping.

Caps (and resistors, but less often) can be microphonic. More often when they seem microphonic, they have cracked leads or marginal/bad solder joints which respond to tapping.

The amp could be oscillating, or your guitar's pickup could be ringing as well. I'd try to rule out the guitar first, then microphonic preamp tubes (maybe try holding them still with an oven mitt?) before trying to chase down a wiring gremlin.

[Stankfut]

Okay, maybe not a microphonic tube? The issue I have is a faint rattle when cranked. it never turns into a howl. Ever shake an incandescent bulb with a blown filament next to your ear? That's the sound. Amp works great oterwise. Clean is very nice, and I like the overdrive (with the exception of the rattle) really raw sounding.

[HotBluePlates]

Sometimes microphonic rectifier (or output) tubes manifest as a rattling sound.

Of course, I guess we can't rule out mechanical rattles.

Feedback/oscillation can manifest as all kinds of weird noises or effects. Unfortunately, you might need the help of an o'scope to pin that down, if oscillation is the cause. For the most part, too-long grid wires, or high-level output stages with wiring passing by low-level input stages, cause these kinds of problems.

A picture would probably suggest whether there is a lead-dress issue, while only your poking/trying different things will likely figure out a microphonic or mechanical rattle.