On 06/20/2015 06:55 PM, Roger (K8RI) wrote:
Isn't class A idle current max (tube dissipation) with max signal giving
minimum dissipation? (The tube is never driven "into" cut off) Class B
is at cutoff while class C is deep into cutoff?
No, with class "A" the idling current is in the center of the most
linear portion of the transfer curve.* (roughly half of the maximum
(desired) current. The input signal runs the plate current up on one
half cycle (+) and down with the other half cycle. (-)
The plate dissipation at idle is the same as at full drive.
With class "B" the idling current is set at just above cutoff, to
eliminate the cross over distortion. Other than at crossover only one
tube conducts at a time, each half cycle drives one of the tubes.
The plate dissipation increases with drive. Without drive it is very low.
With class "C" the idling current is set at or (any amount) below
cutoff, frequently increasingly further below cutoff with increases of
drive. (grid leak bias)
The plate dissipation increases with drive. Without drive it is zero.
* I have heard of operating 3-500Z's that idle at about half of the peak
plate current and it be called AB1 when it is actually class "A."
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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