Still another question about biasing tube amps

Both the tube type and the amplifier topology (Class A or A/B). To keep it simple: it's maximizing power, minimizing distortion and maximizing tube life. From there the bias is a function of the plate voltage and impedance of the OPT primary.

When I built my KT150 amplifier, I needed an OPT wound for 6K at 3 ohms and 65 Watts since my speakers have a flat 2.5-ohm impedance. That determined Class A/B operation. Selecting a plate voltage of 460V I set the bias at the recommended 70% (110mA) and turned it down from there. Sounded great until about 95mA and then turned it down to the final setting of 85mA to prolong the life of those $150 tubes. That's what I would recommend if I was selling.

Maybe Ralph from Atmashpere can comment... knows more about this than me.

The OEM calculates the bias for you but they have to assume a tube type.  I recommend you learn how to calculate your own bias.  It just takes a bit of googling and reading.  You'll need to know the plate voltage on your amp.  

As GS says, it is probably set at 70% of the max which you can get from googling the tube data sheet.  You can probably back calculate the plate voltage from the tube data sheet assuming your OEM used the 70% rule of thumb.  P=IV. P= power in watts.  I= current in Amps, V= voltage.   so if P=50 watts from the data sheet and I is 70 mA, the your plate voltatage is 500V.  use 70% of 50 watts = 35 watts.  500V x .070A= 35 watts.

But the answer to your question is:  If you change tubes, and the new tube has the same plate dissipation rating (say 30 watta) then the bias will be the same.  If it is more or less you can change your bias by the ratio of the number.  For example, if your recommended tube has a 30 watt plate dissipation and you swap in a 15 watt tube, then cut yoru bias current in half.  easy calc to do.

Jerry

Thank you.

I did find a tube data sheet (from Svetlanta) for the EL34 that I can read, and I am thinking that the bias that is read on the MM reflects "plate current, idle"?