Shared cathodes but not plates

[lowell]

hey all,
I'm designing a 12ay7 input stage w/ shared cathode R and C, but not shared Ra. Should I still halve Rk? Both tube are getting the same input signal simultaneously.

[Albert Kreuzer]

If you want the same operating point, yes. Halve Rk and double Ck.
It's like you take two separate triode stages and connect the cathodes.

HTH
Albert

[cbarrow7625]

Actually, you don't double Ck. The cutoff frequency is determined by the impedance of the cathode in parallel with Rk. Halving Rk will result in needing a larger Ck for the same cutoff frequency but only 30-50% larger in most cases, technically. In reality, doubling it is probably OK (or even leaving it alone). Either way it will likely be close enough that you can't notice a great difference.

[Albert Kreuzer]

Learned something. Thanks!

[lowell]

hey guys,
so i've got a 12ax7 w/ parallel cathodes and a 470 Rk. This has grossly changed my plate voltages for the tube, which fyi are on separate 100k Rp's. I'm using the 470 value because it's near half of 820. I tried disconnecting one cathode completely and putting an 820 in there but the plate voltage reads much higher. Vp is 120 on both plates when sharing the 470, and Vp is 170 on one plate with an 820 Rk when the other cathode is completely disconnected. Is this normal when running cathodes in parallel like this? If so it must change my load lines right? At this point, unless I'm missing something, it seems I'd have to run a higher supply voltage at this node to bring things up to where they should be: 170v.

[tubeswell]

820R is the usual fender value for 2 shared 12A_7 pre-amp cathodes. 470R makes for a hot-biased pair of stages

[lowell]

Yes I've seen that, however I've come to learn that splitting the Rk value for two parallel triodes is standard practice when maintaining standard single triode operation. This doesn't seem to be holding true unless I'm missing something.

When a 12ax7 is wired in parallel w/ itself, but w/ separate plate resistors, and the Rk value is 410, shouldn't the voltage drop, current, load line etc... all be identical to a single triode w/ Rk value of 820? Maybe it's not holding water because the triode is not sharing Ra?

[lowell]

hey all any comments on this?