Yes, cathode bypass caps often play a role in maintaining a steady
bias, and depending on the circuit, bypassing the cathode resistance
can also increase the gain of the tube by reducing negative feedback,
lower the output impedance, and may prevent oscillation. It charges
and discharges as the grid changes polarity. Depending on the
circuit, if the cap isn't large enough, it can reduce its low
frequency response.
Anyway, I would rather not copy the value another circuit without
knowing what I'm doing (or what the designer of the other circuit was
doing). The use and values of cathode bypass caps vary widely, even
for the same tube. I've found that doing the math, as opposed to
trial and error, often saves me time and money. Increasing a
capacitor increases is physical size, cost, and demand on the power
supply. I'm sure a way to calculate it exists, I just need to find
it.
On 5/18/07,
sasami@... put forth:
>Just use what you see in other circuit diagrams - frequently 220uF. Do you
>understand their purpose? They are there to give what is essentially a rock
>solid above-ground DC reference. The only time the voltage across it should
>change is while the capacitor is first charging. It charges until it is
>sufficiently high above the grid voltage that the tube stops conduction,
>thus biasing the tube.
>
> >Does anyone have any formulae or have any guidelines in determining
> >the size of a cathode bypass cap?? I am interested in both directly
> >and indirectly heated cathodes.
> >
> >Thanks :)