The RCA Power Tube book shows plate load impedance= (plt voltage/plt
current)/K factor. Note that plate voltage is the d.c. value from the power
supply
and plate current is as measured by the d.c. plate current meter; not the peak
values seen on a graph or load line.
K factor is shown as 1.8 for class AB. K factor is determined by the
conduction angle of the plate current. A K factor of 1.8 implies an angle of
(1/K) X
360 or 200 degrees. If the class of operation were A then the conduction
angle is 360 degrees and K=1. If the class is C then the conduction angle is
less than 180 degrees and K will be greater than 2 and possibly as much as 3.
K factor is altered by the value of zero drive plate current. As the zero
drive current is made lower by additional grid bias, the conduction angle is
made less, which raises the K factor. Conversely, raising the zero drive plate
current will lower the K factor. This is intuitive since we would be moving
toward class A operation in this case.
The RCA book discusses the proper level of zero bias plate current for class
AB as being related to 1/3 the tube plate dissipation rating. For an 8877
running at 4kv this comes out as .125 amps which is pretty close to what we
typically use.
73 de Gerald/K5GW
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