Davedaveweyer wrote: ↑Tue Jan 09, 2018 2:27 pmSomething to keep in mind; the lowered screen voltage means the screens will not draw nearly as much current during full power situations, therefore the resistor on the screens has much less current limiting effect. Bottom line is that any size will work, and will lower the voltage at the screen grid only slightly at full output power. It is handy to have a resistor there to act as a fuse should a tube short, and even though the tendencies for oscillations are drastically lowered with the reduced screen voltage, it is probably wise to have a stopper there. Also remember that the voltage developed as AC across the screen resistor is current feedback, and lowers the gain of the tube accordingly. You could probably get by with 100 ohm resistors.
In situations where the plate voltage was only 580, I usually derived the preamp supply from the HV--this is a personal taste thing, but higher preamp supply voltages give a different distortion characteristic downstream.
I'll be sharing the split supply Marshall Major mod I just completed here on the forum in just a few days, maybe there will be more West Coast ideas that you can play with, possibly even some I haven't mentioned yet.
Cheers to the mods!
In that mode of operation not only screen grids draw less current but power tubes plates to ,which indirectly results in lower cathodes emission and cathodes wear, that Significantly Extend power tubes life ,
my praxis is also to add 10 ohm /1W from each power tube cathode to the ground , except that those two small values resistors improve dynamic mutual current sharing ,they in the same time makes very good test points for biasing control ,
btw , I collect parts for one Marshall clone with four 6L6GC ( RCA- NOS ) and three ECC83( EI-NIS -NOS) , OPS will work with 500V/250V , and B+ supply for preamp stage will be derived from 500V line .
Best Regards