Preamp Cathode Bias Resistor value in 5F1

[Gregg Hermetech]

Quick question: My 5F1 clone has 1.5k cathode bias resistors on V1A and V1B, for a 12AX7 (gain factor of 100). I've replaced it with a 12AY7 (gain factor of 45) and much prefer the tone, reduced gain, more clean headroom etc. Is the bias resistor still within an "acceptable range" for a 12AY7, or is it now too hot, or too cold? Is there a way to work out what I should replace it with, if needed? I'd like maximum longevity for the preamp tube, i.e. a safe bias setting at idle (I keep the amp on all day most days, even when not playing through it, and am using an expensive NOS tube).

I'm a relative newbie so please go easy/explain clearly. Thank you very much in advance!

[Gregg Hermetech]

Schematic if helpful:

[ampdan]

It's fine.

[martin manning]

I don't know what plate voltage you have on the preamp power supply node, but if it's typical of 5F1 you are far from the plate dissipation limit on either tube type.

[Gregg Hermetech]

Thanks! Yes, pretty typical 5F1 (changed a few components to reduce flub/add treble), so good to know all is within spec.

[B Ingram]

Quick question: My 5F1 clone has 1.5k cathode bias resistors on V1A and V1B, for a 12AX7 (gain factor of 100). I've replaced it with a 12AY7 (gain factor of 45) and much prefer the tone, reduced gain, more clean headroom etc. Is the bias resistor still within an "acceptable range" for a 12AY7, or is it now too hot, or too cold?

Fender used 1.5kΩ for 12AX7 and also for 12AY7.

That latter schematic shows "820Ω" but it also has 2 triodes sharing the resistor:
820Ω x 2 = 1640Ω, so
"Tube-Current x 2 x 820Ω" roughly equals "Tube Current x 1500Ω"