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In Reply to: RE: The amp is a Futterman type if I recall right. posted by Ralph on January 24, 2014 at 08:29:59
I guess the further questions would be:
Does the bias circuit even permit setting up equal voltages across each cathode resistor, and what would be the correct voltage across each of said resistors?
Follow Ups:
For your answers I have to open the amp.
Given the information that there are single bias and balance controls for each channel, it cannot be possible to adjust voltage across each cathode resistor, especially given that there are 30 output tubes!!! So that question is answered.The remaining question is where to set bias. You might start by looking at the parameters of a 6C19. The total bias voltage (or current) will be the sum of what is an acceptable steady-state plate current for a single 6C19, multiplied by 30. It is probably OK to choose a current that is 50% or 60% of what is shown on the data sheet as "max plate current". (I am assuming that the voltage is measured across a single resistor to result in a certain total current. If you can open the chassis and determine the value of the resistor, then we can estimate total current. Look for a resistor with taps on either side of it that go to the taps where bias V is measured. It works by Ohm's Law, V = current X resistance. Only here rearranging to Current = voltage/resistance.
Ralph, please check my reasoning to be certain that I am not misleading Si. Thx.
You might get a clue by measuring the Bias voltage as it is now set. Probably that puts you in the right ballpark. We can go from there, once you've measured that resistor so we can determine total current.
Edits: 01/24/14
So we are looking at averaging the voltages across the various tubes.
If it were me (and it has been in the past) I would put a current meter in series with the plate circuit. That would allow me to view the overall plate current of all the tube banks, given its a Futterman.
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