Hi Vic,
A power FET or an IGBT (with an internal or external diode) should do for
that spot rather than an old bipolar device.
At least a 600V device would be indicated.
600V IGBT's have become very economical.
A small series R between the screen and the FET / IGBT will protect against
arcs. The device body diode will pass the arc current back to the power
supply... who knows what it will do there!
You'll need an isolated 10V source to drive the gate and probably an
optoisolator to couple the control signal.
But why?
If you're running Class C, with proper grid bias in the absence of drive,
the plate & screen currents will be essentially zero.
The ol' clamp tube scheme was used with "Linear" amplifiers.
73 & Good evening,
Marv WC6W
http://wc6w.50webs.com/
--- On Wed, 4/14/10, Vic Rosenthal <vic@rakefet.com> wrote:
> From: Vic Rosenthal <vic@rakefet.com>
> Subject: [Amps] Series screen gate design help
> To: "Amps reflector" <amps@contesting.com>
> Date: Wednesday, April 14, 2010, 11:29 AM
> I want to update the old-fashioned
> clamp-tube circuit.
>
> I want to create a solid-state switch that will accept a
> voltage input (negative, but it
> can be inverted) derived from the rectified grid current of
> my class-c amplifier. When
> this input is present, I want the switch to connect the
> amplifier screen to its power
> supply; when it is not, I want to disconnect the screen
> voltage.
>
> I am thinking of a circuit similar to the series pass
> transistor that is used in regulated
> power supplies; but of course the transistor will have to
> be rated to handle the 400V
> screen voltage (the current will be 80 ma). My amp has a
> separate screen supply producing
> the 400v.
>
> Unlike the shunt clamp circuit, this won't waste power by
> pulling down the voltage across
> a resistor -- it will just be a switch.
>
> If this makes sense, what would be a good choice for an NPN
> power transistor, something
> like a 2N3055 with a higher voltage rating, that could do
> this? Any other ideas? The
> keywords are cheap and available.
>
> I apologize for still living in a world of 6L6's, etc.
> --
> Vic
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