Hello everyone!
I designed a circuit circa 1986 for a screen regulator that has some
features which may be of interest in this current discussion. It was
designed for a 4CX1500B which operates at substantially lower screen
potential and also exhibits marked negative screen current excursions.
The regulator has a FET connected as current limiter in series from a
fairly stiff screen voltage source and the output of this FET is
regulated to ground by a second FET connected as a shunt regulator
controlled by an LM723. The LM723 is ground referenced in this circuit
facilitating remote (microprocessor) control of the screen potential.
The key feature is that there is a small resistor connected in series
with drain of the shunt regulator and a 10V zener is connected between
the drain of the shunt regulator and the gate of the current source so
that when the shunt regulator starts drawing significant current it
reduces the available current from the current source. This greatly
reduces the dissipation in both FETs.
A 1uF bypass on the output provides stabilization of the screen voltage
and assures an orderly startup.
Incidently, though it was built and tested, I did not end up using this
circuit in the final design rather I sourced the screen from the plate
bleed and employed just the shunt regulator portion.
A friend of mine did incorporate this design in a multiple 4CX250
amplifier with good results.
The technique could be extended above the kilovolt level by stacking
FET's in the current source.
I don't think that any FET circuit would survive a flashover... but,
then neither will the screen grids in most of tubes that I've ever used.
:-)
And finally a FET that may be of interest:
Motorola MTP3N120E 3A 1200V 125 watts dissipation
73,
Marv WC6W
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