Biasing SE class A, fixed bias

[jbefumo]

I can't believe how fortunate I was to have stumbled onto this thread - I was at that perilous juncture of thinking I knew what I was about, but being totally off base!

[Dave H]

I’m not sure if it’s a good idea to connect the power tube screen to the preamp supply. The increase in screen current will drop the preamp voltage right down when the amp is at full power and it’s only 200V now. Perhaps it would be better to use a separate RC filter from the choke for the screen or a MOSFET series regulator?

[jbefumo]

If I were to use 2 EL34s in SE parallel, what would be the appropriate calculation of OT impedance?

[jazbo8]

It would be half of the Ra as shown in the datasheet, e.g., in the Mullard datasheet, for Vp=Vg2=300V, you would need 3.5k/2=1.75k. What is your current configuration now? Are you using the same PT, with 250R cathode bias?

[Gingertube]

You want Class A Single End, Fixed Bias but Va is a bit high - right?

Perfect scenario for "Back Biasing", with this scheme the bias voltage effectively is subtracted from the B+ so that the tube sees Anode to Cathode Voltage (and screen to cathode) much the same as when cathode biasing.
Could draw up the scheme but why bother when its been already done and explained so nicely.
Back Biasing

Cheers,
Ian

[Dave H]

RA talks about a zener being better than a resistor for back biasing but why not just put the zener in the power tube's cathode then there would be no power supply ripple to filter or time constants to worry about?