The power supply is essentially exactly the same as the Bogen. I figured to get the posted voltages, it would be best not to mess with something that is tried and true for this design.
I added the 1M grid resistor, excellent point on that one.
The 10 ohm resistors to the 7868 screens has been changed as well, but I was wondering if I could get a little more info on that. The original bogen schematic uses 1K, which is almost exactly how the output is designed. Also, any amp I have ever built or looked at has always had 1K or 1.5K. It seems like an odd adjustment to make.
http://makearadio.com/schematics/images ... 100b-6.jpg
2. The plate load resistor for the same stage is unusually large for the amount of bias you are providing -- particularly with the two halves paralleled. If the overall resistor is providing the DC operating point you want, it would likely be much better to add a decoupling cap to the circuit. That way, you could use a 390K dropping resistor from the 370 volt source, decouple, and then use a 47K resistor for the actual plate load. The overall DC conditions would remain basically unchanged, but the AC conditions would make more sense. However, even with this change, with the bias level you have chosen for this stage, I think the dropping resistor will end up likely being no more than about 180-200K to achieve a reasonable DC operating point.
I didn't really have a set bias point for this 12ax7. The only real reason I am putting them in parallel is because I felt bad about leaving an unused half in there. From what I could find, when paralleling 12a?7's the plate resistor should be double what it was for a single triode, and the cathode should be half.
If there is a more appropriate layout for this tube I would love to change it up. It always sat just a little wrong in my head, but I couldn't figure out why.
Thanks.
Cory
updated schem.