Voltage dividers - calculating resistor values for tube substitution?

[Alex_SF]

I've been kicking around plans for this project for a while now: building a clone of an old Gibsonette GA-8 (parallel single-ended 6V6 design), but using Russian 6P1P tubes, of which I acquired a significant quantity at a bargain price. The 6P1P is a 9-pin tube like an EL84, with characteristics roughly equivalent to a 6V6 except that it uses lower plate and grid voltages (+250v on each, similar to a 6AQ5).

My donor chassis is a Hammond AO-39 power amp from an A100 organ, with sockets for 2-EL84s, a 12AX7, and a 5U4GB rectifier. If I use this for the Gibsonette clone project, I'll gut it, put in a new SE output transformer, and build the new circuit from scratch. The power transformer on the AO-39 puts out 350v, about the same as that on the Gibsonette schematic.

I'm a noob at this, but I think i understand that I can get the plate and grid voltages down from the specs on the Gibsonette schematic (+320v on the plate; grid is not labeled) by substituting higher dropping resistor values in the power supply voltage divider (R13 and R12 for plate and grid, respectively, on the attached annotated schematic) for a larger voltage drop at each of those stages. Correct so far?

If my limited understanding of voltage dividers is correct, I'll also need to use lower dropping resistor values after the ones for the plate and grid, so there is less voltage drop across each of those, in order to keep the downstream voltages the same as spec. That would mean R9, R7, and R5 on the attached schematic, correct?

And that is about the extent of my understanding. I'm now at a loss as to how to calculate what values of resistors I would need to substitute to get lower voltages at those two stages of the divider network while keeping the further downstream voltages the same. Any ideas, or links to a good reference source that will help me make the calculation?

Thanks!

[booj]

Instead of using series resistors from the screen supply to feed the voltages to the preamp, tie the pre to the 320 volt supply through an appropriate resistor to drop the voltage to spec.