Shared cathode resistor/cap bypass in preamp

[bluemonk]

Hello. I'm checking my diy Hiwatt and I have a doubt about a mod I did.
The original design use one triode for each input channel (norm-brill):
http://www.drtube.com/schematics/hiwatt/hwpre1.gif

My mod use just one triode and a switch to select norm-brill.
In the original design the two triodes share the same cathode resistor/cap (1k5/100uf).
Since my amp has only one triode should I modify the resistor/cap values to 3k/100uf?
Is that correct?
Thanks in advance.

[xtian]

No, the shared resistor is twice the value of a single, so you want 820R. [I stand corrected.] And even with a 25uF bypass cap, it will be fully bypassed.

[pdf64]

Certainly, to maintain the same operating point the cathode resistor value for a single triode would need doubling compared to one shared by two triodes, but also the two 10k B+ droppers and the 2nd stage 100k plate load should also be doubled.
That assumes the rest of the B+ system is the same.

For the cap value, I'm not sure, as the tube's cathode impedance will be in parallel with the cathode resistor.
It's probably immaterial, because as long as it's above a rather lower value (eg 10uF), it could be considered fully bypassed, especially for guitar bandwidth.

[romberg]

Merlin covers this in his book. WIth tubes totally in parallel you will have the same change in grid voltage resulting in double the change in plate/cathode current (relative to a sinle triode). So, to keep the bias point the same you would halve the Rp and Rk.

In your case, the original shared Rk but has seperate Rps. So, a rough back of the napkin guesstimate would be that the original Rk also has twice the current (just like a parallel setup). So, if you want the same bias point using only one triode you would have 1/2 the current and this need twice the Rk.

So, I'd say yes. 3k for Rk in your case.

Mike

[Colossal]

...and double the capacitance for Ck for a triode in parallel.

[bluemonk]

Many thanks to everybody!
Best regards