mwhouston wrote:
The use of a single Rk to be shared by the power tubes is a little odd. But I wired it the way they suggested. Not sure what it is meant to do.
The requirement for single or dual cathode bias resistors can be confusing. Most power triodes and high transconductance tetrodes require separate cathode bias resistors in push-pull to help ensure the tubes go cleanly into cutoff and hence minimize crossover distortion. Most beam power pentodes are so well behaved in their transitions that a single bias resistor can be used for both tubes and crossover distortion is still acceptable.
Going to UL mode I'm not really sure how they will behave. I would look at the crossover distortion with the single Rk and if it looks ok then you're set. If it looks excessive, run the separate Rks so that the you get a small auto bias current at crossover that cleans up the transition. We can't count on the inductance of the transformer to do all our work for us.
WAIT A MINUTE - THIS IS SINGLE ENDED 
A bypassed single Rk for both channels? The AC is bypassed but the bias voltage is forced to identical values. If the tubes aren't very well matched this is going to lead to channel imbalance. One nice thing about SE amps is no matched tubes. This is just silly, IMHO. I know that Steve Deckert has a reputation, but this is just designing finicky for finicky's sake.
It will be interesting to see how this turns out.
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