Cathode resistor value on a single 12AX7 triode

[dehughes]

Quick question again...

If you have a 12AX7 stage with each triode going to a different channel, and then remove one of the channels so only one triode is being used, should you then change the value of the cathode resistor on this tube in order to keep the used triode running in the same manner as before?

Thanks!

[Allynmey]

Only if you have a shared cathode resistor! If you do, double it for one triode use. Most modern pres have separate cathodes for each stage.

Allyn

[dehughes]

Only if you have a shared cathode resistor! If you do, double it for one triode use. Most modern pres have separate cathodes for each stage.

Allyn

Yup....this schematic is based off of one that has a shared cathode resistor for both sides of the 12AX7, so I should double it then. Thanks! That's what I thought!


david

[dehughes]

Even quicker question:

What replacement resistor value would you all recommend, when "doubling" a 1k5 cathode resistor on a newly liberated triode: 2k7, or 3k3? I don't have a 3k resistor, so I can't exactly double.... The plate resistor is a 220k (think AC30ish voltages and circuit), if that affects your decision...this is just the 1st stage after the input jacks, and then off to a volume pot, a cathode follower stage, eq network, PI, etc etc etc....

Thanks!

[dobbhill]

Some folks divide the plate resistor (220k) by 66 to get a starting point (3k3) when using 12AX7's.
HTH
D

[unklmickey]

Even quicker question:....

equally quick answer:

i think you're splitting hairs at that point. either way should work just fine.


more complete answer:

a larger cathode resistor will bias you closer to cutoff.
smaller........closer to saturation.

but since the "cathode sharing" is seen more often, early in the preamp stages, the signal is so small that you likely won't get near either one.

without looking more closely, i'd probably choose to go smaller.

what do you think, Allyn?


cheers,

unk

[dehughes]

Thanks guys! I GREATLY appreciate your help... I'm inclined to go for 3k3 so as to keep the tube I'm using (Tung Sol 12AX7) on the cooler side of biasing, as those tubes are proving to be kinda noisy...gainy, and articulate, and super cool...but kinda noisy.

[dehughes]

Hmm....I changed Rk to 3k3 and think that it sounds a little cold and sterile now...I think I liked that stage better when it was hotter. 1k5 is probably fine (albeit a bit hot...), but I'll try a 2k2 or 2k7 just for curiosity's sake...