Bias circuit help

[Bonsaimaster]

Can someone help me with this bias circuit. I get about 14mV DC with the bias knob completely in one direction and only 5 mV DC in the other direction. I have changed out the 150K resitor to 470k to get this sweep. I need a greater sweep with this circuit. Should I still increase the "150K" resistor to 1M or is there a different approach.

[xtian]

I assume when you say “5mV” you’re talking about the voltage across the bias-sense resistors on your power tube cathodes. Correct?

To heat up the tubes (hotter bias) further, change the 10K tail resistor to 5K.

[jam-mill]

Martin Manning posted these circuits; may it will help?

[martin manning]

Can someone help me with this bias circuit. I get about 14mV DC with the bias knob completely in one direction and only 5 mV DC in the other direction. I have changed out the 150K resitor to 470k to get this sweep. I need a greater sweep with this circuit. Should I still increase the "150K" resistor to 1M or is there a different approach.

I don't see a 150k, but I would adjust the bias voltage range by adjusting the 180k. If that's the resistor you are referring to, 470k is much too big. If your AC voltage input is around 280 VAC as shown in the schematic, 100k should get you a bias voltage range of from about -10 to -50V.

[Bonsaimaster]

Yes sorry its the 180K resistor. and yes the bias voltage across the bias resistor - I replaced the 180K to a 220K and got 6.8mV on one side and 17mV on the other. I cannot get current to equalize on both sides. My understanding is that this is because there is too much negative voltage thus raising the 180K resistor would help. But I am getting different answers. So either I lower or raise the 180K resistor or decrease the 10K resistor? But not sure which one is correct.

[martin manning]

Higher (less negative) bias voltage (at V- on your schematic, and eventually at the grids of the power tubes) will increase the idle current, and increase the voltage you measure across the current sensing resistors (the 6.8 and 17 mV). I would not call that bias voltage, though, it is indicating idle current, where each mV = 1 mA. You can shift the range of available bias voltage (and idle current) by adjusting either the 180k or the 10k, but if you have a bias voltage that is common to both sides (as it appears) that will not affect the balance. That depends on how well the tubes are matched.