Doctordog wrote:OK, Not trying to contradict anyone, but thankful for the thread and replies, it helps me to try to get a grasp of this too. But, doesn't the OT primary impedance play into this? I was under the impression that it did, that the current draw would be different between say, a pair of 6L6's with a 4K primary and the same tubes with a 6K5 secondary.
I know that for efficiency's sake that a person would match the tubes impedance with the output's, but do these play into the power supply selection?
then, in a fit of pique, Phil_S wrote:I suppose the OT primary plays into the load of the power tubes. I would treat it as a fixed load and it is not very much of a load compared to the internal plate resistance. The load of the tube itself varies with the amount of heat and the signal being passed through it. I'd say that the tube itself, in respect of the OT primary is not modified very much by the OT for this particular determination. As I see it, this particular discussion concerns how to get a grip on the B+ supplied to the plate.
I think DoctorD's question is regarding the power amp current draw and how it is affected by the choice of the load impedance, and yes it will be affected because the current at which the tube saturates will be affected, and this also depends upon the screen supply and how well it is regulated.
The values on the tube data sheets are listed as "plate current at maximum signal," which looks to be the rms current along a load line that passes through the knee assuming a fixed plate and screen supply voltage. This seems reasonable- a little conservative because there will be some sag, maybe a lot of sag, and a little non-conservative because it is possible to drive a guitar amp to square wave.