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In Reply to: RE: Adjusting Plate Voltage on Preamp Output Tubes posted by Maxamillion on June 30, 2009 at 18:23:59
Maxamillion, I have no experience with 2C22 tubes, however the RCA data sheet recommended operating point is -10.5V grid (or + 10.5 V cathode bias) at 300V plate voltage and 11 mA plate current.
You will find it here:
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/2/2C22.pdf
Take a look at the -10.5 line on the last graph.
Most 6SN7s run around 9V cathode bias and 180-250 V plate voltage, so you are correct in thinking that a different op point may be better, however, it is dangerous to look for the best op point around 0 Volts grid. The proper way to "ballpark" a good op point is to look for equally spaced grid lines around a SAFE op point. Equal spacing shows linearity, meaning that for a given grid input signal, the plate signal will swing equally in the positive and negative portion of the waveform.
By SAFE I mean: most triodes will go into thermal runoff (lack of control, maximum plate current, red plates, kaput) near 0 Volts, so a good starting point is the manufacturer's recommended op point.
You may work up or down from there, but anything lower than 3 V bias is asking for trouble. You want to stay away from 0 volts even at maximum volume with a highly modulated CD (2V RMS).
Also do not exceed the maximum power of 3.3 Watts.
300V * 11 mA = 3.3 Watts, so you may want to reduce the plate voltage or plate current by 20% to be on the safe side.
I suggest that you post the power supply (B+) voltage, a simple schematic and ask for favorite operating points from those that have experimented with 2C22 tubes.
A change in plate resistor will certainly require a change in cathode resistor to bring the tube to your chosen operating point.
Good luck
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