a resistor in the center tap to ground,created cathode bias, also known as
automatic bias. larger resistor will nearly cut off plate current flow,
cannot bias beyond cutoff with resistor, had article from qst,,just got it
from 1961 or so with a nice adjustable bias supply and large resistor, switched
by relay that was keyed by vox. This was for 2 813 tubes in grounded grid,
I can get issue year and month for you if you need it. The cathode resistor
was a common form of bias a while back, also can use a zener diode for a more
predictable amount of bias voltage, voltage across resistor subtracts from
plate voltage.same amount.
john kb9tc
Gary Schafer <garyschafer@comcast.net> wrote:I have an old amp with 813's in
grounded grid. All grids are grounded
directly. To cut the tubes off during standby I raise the center tap of
the filament transformer above ground. There is a resistor to ground
that stays during standby.
There is about 2600 volts on the plates at standby. There is a small
amount of plate current indicated. I think around 20 ma.
I am going to raise the plate voltage to around 3000 or so.
I am assuming that the standby current I am seeing now may be plate to
grid or plate to screen current?
Is there a more preferred way to cut those tubes off during standby?
Thanks
Gary K4FMX
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