[ARC5] Audio stage impedances.

[Ian Wilson]

The RCA data sheet for the 12A6 (available from Frank's
amazing site, etc - http://www.tubedata.org ) has a handy
chart at the end that shows power output and harmonic
distortion versus the load seen by the tube.

The typical operating parameters in Class A fit fairly well
with the rule-of-thumb for Class A amplifiers:
  Rload =~ plate_V / (plate_I * 1.3)

Note that 'plate resistance' is the small-signal slope of plate
voltage with respect to plate current, whereas the effective
load resistance is a large signal parameter.

73, ian K3IMW