Re: 4-1000A Screen Biasing...

["David Sharpe" <sccr4us-at-erols-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

A method we used on an DC powered RF VTTC exciter for a CO2 laser using
6-807WA in push-pull parallel was a full wave
bridge rectifier driving (5) electrolytic
capacitors in series.  The SG tap was #2 (ie 40% of Vbb+ voltage).  Each
capacitor had independent 20Kohm 50W bleeder.  This connection guarantees:
1. SG voltage coincident with and scaled to anode voltage.
2. Plate current swamps SG current and prevents reverse current flow
    through SG. SG voltage has very low impedance (relatively speaking) to
cathode
    ground with this connection.
3. Theoretically difficult/impossible to have only SG and no anode voltage.
This connection was simple, robust and worked reliably.  But will not work
if driving a tetrode with AC.  In that case, a separate properly
phased much smaller plate transformer with half wave rectifier could be used.
You will still need a SG OC relay to trip SG supply off with loss of plate
voltage.  A circuit to perform this function is shown in ARRL handbook.
Just my $0.02.

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA:

> Original poster: "Ed Phillips" <evp-at-pacbell-dot-net>
>
> > This one comes up a lot in high power audio amps. There is
> > really no substitute for a well regulated standalone screen
> > supply which is usually derived from a separate transformer
> > winding where screen current is high, or in cases where it is
> > required to be half the anode supply, the half point of a full
> > wave voltage doubler.
> >      The screen current should be self-limiting if the correct
> > anode supply and operating point is used.
> >
> > Regards,
> > Malcolm
>
>         True, BUT.......  If the separate supply is left on after the plate
> voltage is removed or turned on before it is applied, all of the cathode
> emission will be intercepted by the grid and the tube may be wrecked.
>
> Ed