Crosstalk between triodes

[Paul P]

Is it possible for what going on in one triode to affect what's
going on in the second triode of a twin triode tube ? More
specifically, can a tube like a 12ax7 be shared by two unrelated
circuits without them affecting each other ?

Paul P

[dai h.]

yes, through proximity (no shield between the sections) and through the power supply. For example, there was somebody way back who eventually traced a noise problem to a 12AX7 sharing a gain stage and something else--maybe a tremolo cct. stage IIRC--then solved it by separating them.

[Steve Conner]

Yes, there are a few pF of capacitance between the two tube plates that will allow signals to crosstalk. So it's not a good idea to have, say, the first and last gain stages of a high-gain preamp sharing the same bottle.

You also get crosstalk between bottles, I've had the plates of my power tubes crosstalk back to the plate of the first preamp tube in a high-gain amp and cause massive ultrasonic squealing. Tube shields cure this, though.

[Enzo]

And there is nothing scientific about this opinion, but I'd have to say in my experience, there is a lot more to worry about from the wiring and components outside the tubes in this regard.

I did have an old Silvertone on the bench once, and to access the volume controls on the panel, I removed a sheet metal shield that sat between the front panel and the very close 6L6s. The amp became unstable at anything over moderate levels. Reinstall the shield and you could crank it.

I think preamp tube shields are a good idea, but I would use them more for reducing hum than for crosstalk.