Variable cathode bias resistor How to ?

[kagliostro]

I've hear that is possible to use a normal pot in parallel with the cathode resistor to obtain a variable cathode resistor

as example

if the normal cathode resistor is 68ohm 10W, and we want to have it with regulation

we can use a 39ohm 10W resistor and in series a [33ohm 10W resistor + 1,000ohm normal pot]

they say it is possible because the most of the current will flow in the resistor instead of in the pot

the reason would be because the resistor has a lower resistance compared to the pot

Is that real ?????

Kagliostro

[John]

I don't know about the pot, but I think someone here posted a schematic showing a switchable cathode resistor. They installed that so they could switch between 6v6 and 6L6 if I remember right.

In fact, I think Geezer had a schem for that, and also switchable cathode capacitor as well.

[jjasilli]

Comments: if 68R @ 10W; then 2 equal resistors in series need only be 5W ea.  The reason is the wattage formula:  W = V X I.  The current flowing through the two resistors remains the same; but @ 1/2 resistance value, the voltage drop across ea resistor is reduced by 1/2.  Hence ea of two series resistors need be only 1/2 the wattage rating of the single resistor that they replace. 

By the Rule of 10's, a fixed resistor or pot set at 330R or more, across a 33R resistor, will have no appreciable affect on its value.  As the pot is turned down to, say, 33R, the parallel combination will be like 1X 17R resistor.  A 17R resistor will presumably dissipate 2.5W in this example -- through 2 parallel resistors = 1.25W ea.  Too much for a pot; you need a rheostat.

WHY?  Cathode bias does not require such exactitude.

[kagliostro]

Thanks for answer

John I know that mod, this was a different affair, in this case the matter was about to have fine regulation for cathode biased amps, not have different power levels from the same Power Amp

Jjasilli thanks, I can understand your explanation and I agree with you

WHY?  Cathode bias does not require such exactitude.

that was a discussion between HiFi guys talking about fine regulation

Kagliostro