Coupling cap. values???

[stoo]

Hello everyone!
Trying to get a handle on the coupling cap. values for the Phase inverter to P-P output tubes. I've looked at Marshall schematics and they are any where from .022uf to .47 uf with .022 and .1 being the majority!! What's the science here??
Thaanx in advance!
stew

 

[aletheian]

Here you go:
http://www.aikenamps.com/Equations.htm

All the math you need. The Co in conjunction with the following impedance to ground sets the low freq rolloff. The deeper the bass, the more change of ugly grid blocking (or "farting out" as they call it in the guitar world). So it is a trade off of sufficient bass response and potential blocking.