Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on October 11, 2011, 02:22:10 pm
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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
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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.
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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.
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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