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Old 25th September 2005, 06:09 PM   #1
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Join Date: Jan 2004
Location: Minnesota
Default how do i add adjustable bias on amp?

i have a PP 6B4G amp that i just got working.

Problem is all the tubes are running at different points i.e. 61v 55v 45v and 62v across individual 1K cathode resistors.

is the idea to get the same CURRENT through the tubes or the same VOLTAGE across the cathode resistors?

anyways should I just start paralleling resistors across the 1K's till i get same current through all tubes?

or add an adj pot? if so how should i configure it around the 1K cathode resistor?

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Old 25th September 2005, 08:59 PM   #2
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Location: Macedon NY
You want equal currents, which is equal voltages if the resistors are equal - I asume they're 5%.

If your 6.3V supply is grounded, you can get +/-9V from a doubler connected to it - if it has a grounded center tap, you can get only "+/-4V or so - but that's probably enough. I'd filter it with 100 uF/220 Ohms/470 uF. Connect 4 20K pots pots across +/-voltage, and return the four grid resistors to the wipers. You now have combination bias, adjustable for each tube.
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Old 26th September 2005, 02:27 AM   #3
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If you dont mind SS in the amp, try using constant current source to 'force' the tubes to behave.

Use a balance trimpot to divide the current between the tubes.
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