## Paul, you are not going from full class C cut off to normal operating
ssb bias.
That’s the old alpha bi-state ebs scheme. With tri-state bias, its more
like toggling
between 20 ma to 180ma on a 8877. Its only ever cut-off bias when in RX.
Jim,
To your last point -- in the case of VOX operation, I think most tri-state
EBS systems rapidly go from class C in STBY, to class B on PTT key line
activation, then to class AB upon RF sense - and back again during the VOX
T=>R sequence. Under normal hand/foot PTT operation, the amp is only
switching between the last two states and not three like it is under VOX.
In the case of the Alpha 77Dx, a 50V supply supplies cathode bias and sets
the amp in class C during STBY. When the amp key line goes low, bias goes
to 20V and produces a slight amount of Ip. This state/voltage is adjustable
via a bias control rheostat. Finally, upon detection of 100 mW of input RF,
the amp becomes AB biased and cathode bias goes to 8.1V via a switched Zener
diode. I have my 77Dx biased at stage two to produce about 30 mA of current
which is a bit more Ip idle current than the stock factory setting. So, the
77Sx/Dx does indeed use a tri-state EBS system.
Paul, W9AC
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