I'm just trying to achieve efficiency between the impedance of the tubes + transformer + speakers. I AM using the power-grid feature (half power setting) as intended. This setting only uses two of the four power tubes as it is "true" half-power, therefore I just pulled the outer tubes. No use in lighting them up for nothing. Besides, this way, I'll have a fresh matched pair if anything happens (since it was a matched quad). It's also two less heaters to tax the power-supply. Pulling the outer power-tubes isn't necessary, however. I don't know if this makes any more since? My question, being "true" half-power, should I switch the resistance on the OT to 8? You need to effectively double the speaker impedance to compensate for the doubled tube impedance (since the pairs are in parallel, all four have half the impedance of two).
I guess it's not absolutely essential to switch it, all that happens if you don't is that the remaining tubes are running into half their matching impedance, which wears them faster. The transformer is less efficient due to the mismatch. I assume this applies to any 4-tube amp that you run with only 2 tubes, unless the half-power switch, if Bruce designed it this way, also compensates for this??
