JMPGuitars wrote: ↑Thu 03/28/24 4:38 pm
I'm only a pseudo-engineer, so I can't give you a proper long-winded answer on this one. I will say that drawing 3 is electrically about the same as drawing 1. So that would be the way to go. I say about the same because physical placement can have an effect on things (heat issues, long wire run issues, etc.).
That is what I figured, electrically equivalent but distance and proximity to other parts, and heat can induce other issues. But the zener would function the same.
I am working on a 2x6V6 vintage amp that has a factory schematic and PT that puts 450VDC on the 6V6 plates with 117VAC primary. With 122VAC at my wall, in this actual amp, its knocking on 480VDC.
Also unusual in that it splits the B+ rail after the first filter cap which feeds the OT, with one resistor going to the power tubes, then another resistor and subsequent string feeding the preamp.
I'm thinking through how to have the power tubes and the first filter cap be able to handle such high voltages, and come up with:
1) Keep PT, reduce B+ (zeners, resistors, etc., whatever method) - and also likely need to series connect two filter caps at first node to increase voltage handling, currently a 475VDC 16uf cap there.
2) Use 6L6 tubes which would require a new OT, or rewiring current OT (I think I can shift the outputs one way or another, like use 8 ohm out for 16 ohm speaker?)
3) Use a different more appropriate PT, and adjust B+ network for appropriate voltages throughout.
Its a Bogen CHA-20A -Here is the schematic:
My amp was slightly different than schematic as it was missing the 68l 2w resistor that bridges the 6V6 screens and cathodes. And my filament CT was just grounded, not wired to 6V6 cathodes.
Lastly, there is another factory mod that others CHA-20a amps have, that mine didn't, which is a 22r 2w resistor between the HV and Filament CT's.
The amp runs fine and sounds pretty good (with variac managing the voltage during testing, currently about 100-110vac) - and only voltage reducing (and noise filtering) step I've taken is a new B+ node A in front of the stock one with a 16uf 475v cap then a 1k 5w resistor then on to the rail per schematic.