Plate cathode capacitance

[sand]

My Ceriatone 2204 clone has a 100pF capacitor between plate and cathode on V1a. What is it for? Doesn't it enhance the Miller capacitance? Isn't that something to avoid? The Marshalls don't have that. Thank you.

[mooreamps]

Something as a "must have". It refers to Plate Neutralization or Anode Neutralization.. It keeps the tube from breaking into parasitic oscillations.. I use them as well, but slightly higher values.. See the attached link for more details.... Also, in the V1 position, it dumps the gain of stage at IF frequencies, thus preventing the tube from picking up and amplifying AM radio broadcasts...

http://www.cpii.com/docs/related/22/C&F5Web.pdf


-g

[sand]

Thanks. Great read!

[redelephant]

I'd say it's sort of a 'must have'. The high value grid stoppers typically used in git-amps (the 68k in front of most 12AX7s) do pretty much the same job.
That's not to say I don't use them, but for tonal reasons, usually 1nF-5nF values depending on my mood.