[Amps] Muting 813's

[John Irwin]

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 at 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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