I have inherited an old homebrew 4 x 811A amp which I am
trying to get back into working shape (my friend has dubbed
it "The Widowmaker" as it had +400V on the case of the power
supply when I first turned it on - high impedance thank God).
I was wondering what the "preferred" method for biasing would
be? The amplifier uses a grounded grid configuration in which the
grids are RF bypassed to ground via ceramic disc capacitors (220pf
I think). All four grids are tied together via four 47 ohm resistors
(one to each grid). This common point is fed to the antenna
relay thru an RF choke. During standby, the RF choke is
switched to a 170 volt supply which presumeably cuts off
the tubes from conduction. When the antenna relay switches
from standby to operate, this node is disconnected from the
cutoff bias, and shunted to ground thru the grid current
metering circuit. This scheme presumeably provides a negative
"self bias" proportional to the envelope of the drive signal.
My question is, what class of operation does this sort of
biasing scheme produce with 811A's? Is it linear enough
for SSB service? Is there a better alternative?
Thanks,
Mike,
W4EF...........................................................................................
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