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In Reply to: RE: Adjustable cathode bias posted by Frihed89 on November 02, 2015 at 13:49:13
I have done this in one of my SE amps so I can use a 6v6 or 6l6 tube. I am also using a LM317t as a voltage regulator/ CCS for setting the cathode bias. I switch out the voltage setting resistor which is only 1/2 watt. I suppose you could install a pot in its location if you want to adjust the bias continuously but that means installing a test point and measuring the voltage at the cathode. cheers, Dak
Follow Ups:
Not knowing too much about electronics, I only know that variable resistors can get quite hot. An SPDT switch, I guess, wouldn't have that problem. I could add a 200-215 ohm, 3W resistor in parallel with the 500 ohm 3W one and use the switch to cut off the smaller resistor, right?
If you are going to use just the large resistors then you need to wire the cathode to the center pole of the switch and run one resistor from the outside pole to ground and again the second resistor on the other outside pole to ground. And don't forget your bypass cap. cheers, Dak
My suggestion was to leave the 500 ohm resistor in place and use a switch to place a 220 ohm resistor in parallel with that 500 ohm resistor when 150 ohms are wanted and out of circuit when the 500 ohms are wanted.The 220 ohm resistor could be permanently connect to ground and the other end could be switched to connect to the cathode or disconnected from the cathode.
He uses one shared cathode resistor for both output tubes so it would only take one single pole single throw switch to get the job done for his stereo amplifier.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 11/03/15
nt
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