dumb guy's bias question

[arfy]

I read in the TW pages that KF biases his amps so that there is -30 V at the grids, so far so good. But my Express clone was made with Toneslut transformers, etc, by me, it's a different amp although I've tried hard to follow the known design, and I'm biasing it to hit the typical 70% idle dissipation and the negative voltage at the grid winds up at about -32 V plus change to reach that dissipation level.
So did KF intend to run the power tubes hotter than normal and that's why he prescribes the -30 bias? I had been assuming that he tweaked the amps to hit the common 70% dissipation but maybe that wasn't his aim and I've missed the point.
If I'm understanding the posts I see, most of you are running your
Rxpress copies with -30 bias voltage and I'm doing it wrong, but mind tiny brain is stuck on the 70% dissipation idea.
What are your amps showing as idle plate dissipation with -30 V bias voltage?

[Fischerman]

You need to get off of that 70% dissipation rule. That's just a number that somebody thought was a good number...high enough to usually get out of the 'crossover notch' yet not so high as to over-dissipate the tubes. It's like somebody asked; "If you were going to tell someone that was clueless what to bias a guitar amp to...what % plate dissipation would you choose?" And the answer was 70%. Many, many people prefer the sound of their amp biased colder than 70%.

Regarding the "KF value of -30v"...I'm skeptical that this was taken in the right context. If you bias two amps to -30vdc bias voltage; the amp with a plate voltage of 250vdc would be biased pretty cold but the amp with the plate voltage of 500vdc would be biased pretty hot. We're not so much interested in the bias voltage...but rather the idle current...which is heavily influenced by plate voltage, screen voltage, bias voltage, tube type (and even a range of tubes within each type), etc.

Also, how are you biasing the amp? What are you measuring? If you're measuring cathode current (using 1 ohm cathode resistors) then remember that the screen current is included in that measurement...it's not much but it might make a little difference.

[arfy]

I measure the current by dividing the voltage drop across each side of the OT by the resistance across that same side of the OT. Multiplying that number by the voltage at the plate gives the power dissipation at idle.
Should I be doing it differently.

[Fischerman]

No, that method works too and doesn't have any screen current included.