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I am building a dc 300b amp using this circuit
IN order to dial in my voltages which resistors should be adjustable (initially)
My guess is the 2 cathode resistors
thanks
Edits: 08/18/15Follow Ups:
Your guess is very good. If all the resistors are well-chosen and the power supply voltages are close, then R3 (the driver cathode resistor) will be the easiest place to trim the voltages without disturbing the operating conditions.
If the high voltage supply is off spec very much (it often happens when the power transformer is lightly loaded or designed for a lower voltage than what is actually available on the wall socket) then you will want to revise the expected voltages and currents throughout the circuit - choosing whether to take the voltage difference in the driver, output, or both sections. That's an entirely different can of worms, but most of that adjustment can be made in R7, the power tube cathode resistance.
Personally, I would make R3 a trimmer for tube swapping or aging tubes, and use a fixed resistor for R7, which may never need adjusting.
I notice the circuit is drawn for a 2.5v filament, not the 300B's 5.0v.
Thanks Paul
Yes I a will be using dc 5v filament supply
What happens to V3 if V2 opens?
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Buy Chinese. Bury freedom.
V3 essentially saturates, with a small positive grid voltage, reduced by the R5/R6 divider. Most of the power supply voltage appears across the cathode resistor R7 so the cathode current is high. The voltage across V3 is small enough that there is no problem with dissipation. The maximum cathode current will probably be exceeded, exhausting the cathode material faster than normal. The voltage across the tube is low enough that the lack of space charge should not result in much cathode stripping.
At least that's how I see it.
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