My testing environment is coming along...
(generic circuit mostly complete)
I've got a Fender Champ preamp (basic two-stage) set up there, and the back-end is coming along.
I think I've figured out the power section values I'm shooting for, for a first test. With a class A setup there's a fairly narrow set of choices about how to do things because, based on what I've read, (a) I want the tube(s) to run at full power (at the plate) all the time, to a target output in watts (e.g. 23W for a 5881 tube), and (b) for reasons I don't yet fully understand, I want to leave a margin of positive voltage swing on the grid (meaning, big signal, overdrive situation) to protect the tube and circuit. Achieving both, given the constraints and flexibility of my PT and OT, means there are just a few options. I found an online calculator that really helps visualize the relationships, and home in on the optimal values.
Here's a graph of the configuration I'll try first, showing plate dissipation (straight red line) remains below the max rated dissipation for this tube (red dotted curve), and that at a 2500 ohm output transformer load (affecting the slope of the plate dissipation line) and screen voltage 30 volts lower then the plate (affecting the blue grid-voltage gradient lines) should allow operation up to around 13 peak signal volts (which can be easily produced by a two-stage pre-amp) to the tube's grid:
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