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Old 4th May 2021, 10:54 AM   #11
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There have been several threads about this in the Tubelab forum. The usual method for adjustable bias is to put the highest value 5 watt cathode resistor in the board, usually 820 or 750 ohm, then use a switch with a selection of resistors that get put in parallel with the 750 ohm resistor on the board to raise the current.

It is also possible to put a 750 ohm resistor in the board, then wire a pot and series resistor combo across that. Using a pot by itself allows for accidentally setting the resistance to zero or some other value that's low enough to force too much current through the tubes.

Since the switched resistors or pot are always used in parallel with the on board part, the current / power dissipation is shared between them. This allows for use of 3 watt parts if desired.

Simple, but not as easy to use method....

Put the 750 ohm resistor in the board. Make some resistors with alligator clips on the ends in the 2K to 3K range. Connect the clipped resistors in parallel with the 750 on the board as needed.

I used the 750 ohm by itself with an EL34 in triode mode. That gave me about 55 mA with the tubes I had. I clipped on some 2.2K resistors when I used 6550's in UL mode. This combo gave me 560 ohms for about 80 mA.
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