OK, I have this amp in hand. It needs a bunch of work in short order, as I need to get it installed this week for a Saturday recording session.
I'm decades-rusty in tube design calculations, so I need a bit more help figuring out how to get rid of the need for the field coil.
1. With four new Electro-Harmonix 6V6GT tubes and changing the output cathode resistor to 150 ohms, what, if anything do I need to do on the plate side? The output transformer is 4KCT impedance with a DC resistance of 75 ohms per half. (PRR: you were referring to a straight cathode resistor to ground instead of the fixed bias, right?)
2. What should I hang on the screen grids? Assuming a voltage divider, but what kind of current?
3. What effects would unsagging the B+ make on the preamp* section and detract from any tone-desirable distortion characteristics? What changes would be advised to mitigate that?
*I am considering wiring in a 9-pin miniature socket in parallel with the 6SN7's octal socket to provide optional substitution of a 12A*7A