ST70 bias redux

The sticky on bias recommends .40v for a 40ma current.

Ignoring tube life for a moment, isn’t this significantly underbiasing? The EL34 data sheet shows a max bias of 150ma and a “typical” bias of 100ma. The old 50ma recommendation was already only half the manufacturer’s recommendation.

Now I admit I can’t hear the difference but I wonder if a change in distortion can be measured. Theoretically any departure from the ideal bias results in distortion.

Not being argumentative. Just trying to learn.

Jeff

jeffjmr wrote:The sticky on bias recommends .40v for a 40ma current.

Ignoring tube life for a moment, isn’t this significantly underbiasing? The EL34 data sheet shows a max bias of 150ma and a “typical” bias of 100ma. The old 50ma recommendation was already only half the manufacturer’s recommendation.

Now I admit I can’t hear the difference but I wonder if a change in distortion can be measured. Theoretically any departure from the ideal bias results in distortion.

Not being argumentative. Just trying to learn.

Jeff

Thet depends on which circuit one uses. In the push-pull amp like ST70 one have 430Volt across the plate <> cathode. The max dissipation is 25W which you get close to with 58mA ( per tube)
50mA gives 21W 40mA 17W ( which is close to general recommendation)

In addition to that the rectifier used GZ34 is close to it's maximum current in a ST70, thus not allowing much more then 50mA/tube

I have measure dist in the interval 40mA to 60mA and the difference was really small, 45mA is a good target.

There’s the piece I missed: max dissipation. 21w doesn’t seem excessive given the 25w limit, but I know the output transformer figures in here as well. My ST70 manufacturer recommends 1.00v for the pair (10 ohm bias resistor, 50ma each) but now I understand .800 to .900v might be better advised.

Thanks for clarifying that for me.

Jeff