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In Reply to: RE: Adjusting Plate Voltage on Preamp Output Tubes posted by casouza on June 30, 2009 at 19:15:01
I see your point on plate dissipation, Casouza; it looks like 150V is too much at a grid voltage of 0. Somewhere around 125-130V keeps the dissipation at ~ 70% of max.
Thanks!
Maxamillion, your grid voltage may be zero Volts, however tubes are biased by the difference between grid potential and cathode potential. Usually, the cathode is run at a positive voltage by means of a cathode resistor.
I suggest that you locate the cathode resistor and measure the voltage across it.
That's your actual grid-to-cathode bias voltage.
Divide the voltage by the cathode resistor value in ohms, you will find out the current in miliamperes.
For example on a typical 6SN7 circuit you may find 9 volts across a 1500 ohm cathode resistor:
9V/1500 ohms= 0.006A = 6 miliAmps
I suggest that you read this to clarify my point:
http://www.bonavolta.ch/hobby/en/audio/t_bas9.htm
Yes, that's what I was saying. The grid voltage is 0V, not the bias. I don't see any magic about 0 or any other absolute voltage, it's all relative, so as long as the grid is negative compared to the cathode, and both are negative relative to the plate, what's the difference where the grid is set? Am I off base here?
Tubes are simple analog devices. All curves, etc, are informaion about events from the tube's perspective.
Tubes do not realize anything is grounded.
Measuring relative to ground is only done to try to simplify documentation.
It might ... or might not ;)
Happy Ears!
Al
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