Biasing AB763 6L6GC tubes affect bus voltage?

[RoadShow]

Hello,

As I was trying some pairs of 6L6GC tubes today I noticed something that you all probably know and can explain.

When I set the bias with a pair of tubes the other day my pin 3 & 4 voltages from the bus were 425V and the biasing was about 45mA, so around 65%.

Trying different tube pairs today (different brands, aging, etc.), when I adjusted the biasing up from the initial 25mA to the 45mA range I noticed the pin 3 & 4 bus voltages had dropped about 20V or more, depending on the tube pair.

What would cause this? The transformer has a separate winding for biasing (Hammond 290CX) which by calculations for max current draw for the tubes is running at about 75% or less.  Table used for selection included below.

I've also included the picture from the other day (not updated, the brown grid wires have been switched, all else is the same.)

Thanks.

[sluckey]

That's quite normal. Biasing the tubes hotter means they draw more current from the power supply. IOW, you have increased the load on the power supply. This increased load will cause the PT voltage to sag resulting in decreased B+ voltage.

[RoadShow]

That's quite normal. Biasing the tubes hotter means they draw more current from the power supply. IOW, you have increased the load on the power supply. This increased load will cause the PT voltage to sag resulting in decreased B+ voltage.

Thanks, I was kind of expecting to hear that, so you have to recalculate the power level as you adjust the biasing based on the resulting voltage.

One more question, the tube biasing are about 3mA apart which as I understand is decent.  Would adding a 5k pot before the 220k resistor to get as close to equal as possible worth it or even a reasonable idea?  Or, should I leave well enough alone.

[shooter]

leave well enough alone.

monitor the difference while playing some Black Sabbath with the volume dimed, bet  it's more than 3

[sluckey]

Would adding a 5k pot before the 220k resistor to get as close to equal as possible worth it or even a reasonable idea?

That will do nothing. Let it be.

[Latole]

Do bias at 50%,  amp will sound as good as 65% ( wich are to high IMO) and tubes will have extended life.

[RoadShow]

Do bias at 50%,  amp will sound as good as 65% ( wich are to high IMO) and tubes will have extended life.

I'll give that a whirl before I build the head cabinet...