Un-sharing Fender cathode cap/resistor

[Paul P]

In Fender amps there are places where two triodes share the
same cathode capacitor and resistor. Is there any problem in
using separate capacitors and resistors instead ? I want to
put different circuits on separate boards and this would be
easier and clearer for me.

Paul P

[cbarrow7625]

Not a problem

Nope, not a problem. I will sound slightly differnet but not too much really.

You need to double the size of the resistor if it is only going to be connected to one triode instead of two. That will keep the same relative amount of current flowing in each individual tube.

If it is an 820 Ohm / 25uF shared, each individual tube should get 1500 Ohm resistor (close enough to double...and the value Fender uses for single tubes) and 25uF cap. The cap size stays the same because the impedance at the cathode does not change appreciably when splitting things up, so the LF roll-off will be almost exactly the same.

Good luck.

[DirtyGeorge]

Paul P, There is no problem with splitting up the cathodes (assuming it is not the PI ). You should double the size of the resistors to maintain the same operating point for the separated triodes. The bypass cap value would then be cut in half to keep the same rolloff frequency.

DG

[cbarrow7625]

George,

Technically halving of the cap value is not necessary. Even though you double the resistor it is still in parallel with the impedance of the tube's cathode. It is the combined impedance that determines the LF roll off, not just the resistor / capacitor calculation.

I suppose, technically, some adjustment would be required to keep ithe LF roll off freq exactly the same but I think in practice it does not shift by a great amount.

I think Kevin O'Connor has a good sest of calculations in one of his books related to LF roll off and cathode impedance. I can't remember which, I read it a long time ago. Anyone know which book that appears in?

[DirtyGeorge]

cbarrow7625, thanks for the correction. I didn't see your original reply before I had posted mine.

What would you estimate the cathode impedance to be in the average 12ax7 circuit?

DG

[cbarrow7625]

Wish I could tell you. I just don't remember exactly. That's why I asked about the book reference. I'll see if I can find the exact reference when I get back to my book shelves this evening.

-Chris

[Earl]

I think the cathode resistance is something like - Rk' = (Rp+ra)/(mu+1). Rp being the plate load resistor, and ra being the tube's plate resistance.

So the parallel combination of that number and the cathode resistor get's you the output resistance at the cathode. Then it's just 1/(2PiRC) for the roll-off

[Paul P]

Thanks for the help. I wouldn't have known to double the
resistor value. I just want to split the two preamps onto
separate boards.

Paul P