Hey-Hey!!!,
With sweep tubes, one can treat petodes plate Z the same as one would a triode's...it's high, but get the right OPTx, no?
Take the 6CB5A, octal base, anode cap, 2.5A of heater. Plate Z is ~5kOhm. As a triode, its plate Z is 430R. Go 2.5 this number for approx U-L and we're at 1k.
There are other valves that perform in a similar fashion. High gm, low u-g1g2...
Take a good 3k a-a OPTx and apply a little bit of plate-to-grid NFB and you'll have a sweet amp that has power output from 20-40 wpc depending on the valve chosen. No way I am going to build with 845's when it's this easy.
cheers,
Douglas