shared V1 cathode?

[daz]

Someone elsewhere was asking about some amp that has shared cathode on V1. I was under the assumption the tube was NOT run in parallel but 2 seperate stages yet with the cathodes sharing the same R/C. I don't think i have seen that before except on a tube where it's run parallel. Can someone tell me what the tonal difference would be and whether you would double or 1/2 the cap and resistor values to have the same values as you did when running the cathodes desperately. IE: if each cathode is running a 1.5k and a .68uf when run separately, would you go to 3.0k and 1.3uf or a 750R and .33uf?

[soundguruman]

There are some Fender amps that share the same RC for two cathodes. Fender used the same values as if the cathodes were separate. No, they did not double the value...
The tone difference was not a lot different, however these type of designs tend to oscillate and run in an unstable fashion. Many were changed to separate RC after the factory made the original RC combined.
It is probably is better to separate them, just from a stability standpoint.

[JoeM]

....Fender used the same values as if the cathodes were separate. No, they did not double the value...
.

???? Are you referring to the bypass cap value or the cathode resistor value? The cap value Fender used for the shared cathodes was the same (25uf), but the resistor value was (approximately) 1/2 - 820 rather than 1.5k.

[Chuck H]

Right. And the 25uf value was probably left alone because the LF pass frequency, even at the lower impedance, was low enough to let it slide. Where the resistor sets DC bias point, which is more critical to actual function.

To be clear... You would half the resistor and double the capacitor for similar function and tone. This as it applies to typical gain stages. Now...

Daz also asked about series vs. parallel triodes. Sharing the cathode on parallel triodes is typical, as we all know. It is rarely done with series triodes. The reason is that the two stages are out of phase and working on the same AC signal. Not much of a problem if you use a fully bypassed cathode circuit. For guitar amps that would mean a bypass cap with a -3db knee below 40Hz or so (remember that some guys tune low, and they make 7 string guitars now). If you were to use a smaller cap there would be some phase cancellation below the knee frequency and a gain increase above the knee. This could probably used as a voicing design tool except for another caveate.... Electrolytic caps have comparably poor HF performance. So considering the low impedance of the cathode circuit there probably would be some interaction between the triodes at higher frequencies. Also, things can get dicey WRT phase when you consider tone stacks, signal padding and power supply decoupling impedances can all have an affect. Phase stability can be problematic enough with guitar amps. Why complicate it further.

That's why you rarely see series triodes with a shared cathode circuit. And IMHO it should never be done without a large enough cap for full bypass. If at all.