Modifying a power supply, slightly

[Even Bigger D]

My current project is building a power amp based on a soldano SLO power section - basically a SM-100R clone, but I don't have the schematics for that so I'm modifying the SLO ones I do have. The problem I've run into is in the power section, trying to retain the same voltages for various stages. I think I understand how to solve for the voltage drop over each resistor in the chain using ohm's law, but the problem is that I'm not sure how much current each triode will pull. If I backsolve on the schematic I have, it says all the triode stages together pull just over 11mA. There are 10 stages in the full SLO, and I'll be keeping 4 of them (PI, and a doubled input stage probably) so I could just guess 4-5mA and go from there.

But is there a better way? How do I look at various stages (common cathode, cathode follower, PI) and figure out how much current they will pull?

[Steve Conner]

I usually just guess to start with and then adjust those dropper resistors by trial and error.

Each triode section of a 12AX7 usually runs about 1 to 1.5mA, because that's the recommended operating current in the tube datasheet. If you see a 2.2k cathode resistor instead of the usual 1.5k, you know it's going to be lower, about 0.7mA.

You sometimes see 12AT7s running more, particularly in the reverb driver stage of old Fenders.

SLOs have a "cold biased" stage with an extremely large cathode resistor, and I expect that would run a couple of hundred microamps.

If the schematic shows the rail voltage and the plate voltage for each tube, and the value of the plate resistor, then you can work out the plate currents exactly with Ohm's law. Ditto for a cathode follower, but you're looking at the cathode resistor because there isn't a plate resistor.

[J M Fahey]

Also remember that if some triode gets, say 350V per schematic and now has 375V, that's no difference in practical terms.
You can also add, say, a 100K 2W resistor to ground at the end of the supply chain to simulate the missing triodes.
It will "eat" 3.5mA, equivalent to 2 or 3 triodes; add as many as you want to compensate what you pull.
It's the most easily reversible mod, just pull the resistors, plug in the missing tubes, and you are back to original amp

[Even Bigger D]

Thanks guys - what you're saying matches what I'm seeing on the schematics. The resistor values I've got are close enough for government work I think.