[Boatanchors] Grounded Gid Amp Bias?

[Price Smith]

Hi Keith,

Lift the center-tap of the filament transformer from ground.
Put a 10K to 25K ohm, 5 or 10 watt resistor from the center-tap to ground. 
Short the
resistor with a relay contact that is actuated during transmit, PTT relay. 
The resistor
will have enough voltage to cut the plate current during standby to almost 
zero. You won't
even see it on the plate meter.

Regards......Price W0RI


Subject: [Boatanchors] Grounded Gid Amp Bias?


> Hi All,
> I am working on refurbishing  a homebrew grounded grid amplifier. It runs 
> 2
> x  3-500Z's. I would like to be able to cut off the tubes idle current
> during no drive condition but with HV applied.
> I see the Heath SB-200 amp uses a positive bias on the filaments  to cut 
> off
> tube current. I like this idea but here is my question. The way this set 
> is
> wired is that the filament transformer is 10 volts not 5. The center tap 
> of
> the filament transformer is grounded and each of the other two  legs go 
> via
> an rf choke to one side of a 3-500 filament. The two other filament
> connections  are joined, but not grounded. Filament voltage is nearly
> identical on each tube.  My guess is that if I lifted the center tap off
> ground and fed 100 VDC to this point, the tubes will stop conducting. I
> obviously don't want to experiment so maybe someone could tell me if this
> will work, won't work, is insane????
>
> Thanks All,
> 73,
> Keith ve3ts
>
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