Tube DIY Asylum

RE: Adjusting Plate Voltage on Preamp Output Tubes

189.122.116.24

In Reply to: RE: Adjusting Plate Voltage on Preamp Output Tubes posted by Maxamillion on June 30, 2009 at 20:41:23

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

Biasing tubes (Open in New Window)

Follow Ups:

RE: Adjusting Plate Voltage on Preamp Output Tubes - Maxamillion 05:44:34 07/01/09 (1)
- All voltages are relative only to the tube - tubegarden@aol.com 13:34:48 07/01/09 (0)

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