One point I found absolutely vital was a separate bias pot for each tube. For a
given bias voltage, I've measured a plate current variation from 2.5 to 45 mA on
sweep tubes - about twice the variation on 6146Bs - so especially if there's
going to be grid current, bias derived from a low impedance source is necessary.
If you don't bias them at all, you really need to make sure the tubes are fairly
well matched, otehrwise you can find one tube doing all the work, and the
linearity being poor. I've used 1kohm bias pots across 15 volts or so, but if I
were doing it again, I'd use an op amp and power transistor to make a shunt
regulator for each tube.
Otherwise, I can't add to Skipp's comments.
73
Peter G3RZP
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