Drop B+ and Idle Bias

[PDC]

Am I correct that - all factors being equal - as you lower the B+ on an amp the idle bias will read higher?

On my 50 watt QR, the tubes settle in right at 40 even with the bias control at the lowest end of its adjustment. All of Scott’s 50 watt variants are Drop B+ by default. I am used to setting bias for EL34s anywhere from 32-37. The amp sounds incredible. The tubes get pretty warm to the touch after playing loud for a good while, but they stay that nice orange color, they are not drifting hot white.

I guess my options are 1) leave it alone and keep playing; 2) find a tech to swap the Bias Resistor; 3) find a different pair of tubes that will bais a little cooler.

If I wanted to go with option 3 for now, how would you go about finding tubes that would bias in at a lower value? What value or number am I looking for with regard to the tube that indicates where it will bias?

Thank you all.

 

[fusedbrain]

You need to know what the plate voltage is.
But, for example, if the lower b+ was 430v, 40ma would be right around 70%.
If the plate voltage is lower than 430v, than plate dissipation will be lower as well.
This is for 25w el34's.
So basically, as long as plate voltage is less than 430v, you're fine.

 

[PDC]

“You need to know what the plate voltage is.”

You are so right - I really do need that part of the equation. I’ve got a Bias Master, which does a great job of reading all the tubes at once to confirm how close they are, but it does not have a plate voltage setting.

 

[SpiderWars]

Am I correct that - all factors being equal - as you lower the B+ on an amp the idle bias will read higher?

Typically just the opposite. Think of the bias and B+ as a ratio. If you set idle current to 70% max dissipation at a higher B+ and then lower the B+ the current will drop as well. But now you're way below 70% dissipation because both the B+ dropped and the current dropped too.

Does that bias probe you're using account for screen current? That's usually around 5-7mA and it all flows thru the cathode. In my 2204 which is only 400vdc I set cathode current at around 46-47mA.