Hello Mark,
The solution that I would recommend is a grid-block keying circuit
that biases the grid into the cut-off region when the key is up.
This is easy to handle if you drive the grid bias circuit through a source
follower
or a reasonably low-impedance potential divider.
By using a heavy duty FET optoisolator (Clare PLA140) you can switch the
operating point with
a very small delay (a few 100's of microseconds), allowing sequencing circuits
to work properly.
This concept was used a few years ago for QSK-ing a Collins 208U-10 amplifier
that
used a 6S4 cathode follower and a reed relay switching circuit.
The 6S4 was replaced by a 1000V 3A MOSFET source follower, and the reed relay
with the PLA140.
The 208U-10 is somewhat special, in the respect that the screen grid is
directly grounded, the cathode
is then at -1250V respective the screen grid. Consequently the grid bias supply
is floated at -1250 V
and the isolation requirements become quite stringent.
When there are flash-overs in the 4CX10000D, the MOSFET and/or PLA140 sometimes
get vaporized.
Compared to what it used to cost to replace the 6S4, reed relay and 6S4
filament transformer,
the solid state replacement is a bargain.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Mark Hill" <g4fph@mjha.co.uk>
To: "Amps Reflector" <amps@contesting.com>
Sent: Monday, November 01, 2004 5:56 PM
Subject: [Amps] 4CX1500B QSK
> Folks,
>
> I'm looking to QSK an ex-commercial amplifier which uses a 4CX1500B. The
> amplifier has no changeover mechanism at all at present (it was part of a
> TX system that was on 24/7 with remote receivers).
>
> I intend to engineer the aerial c/o using a vacuum relay - the rest I would
> like to do in solid state. Any experience on the line regarding the
> relative merits and de-merits of different methods of taking the valve
> between 'correctly biased ready to go' and 'cut off with no noise in the
> RX' states? To cut the valve off, my options would seem to be:
>
> >Shift the grid bias from operating (normally about -35V from a pot across
> a Zener) to some greater negative potential; or
> >Remove screen voltage (normally about 225V from a string of Zeners) and
> ground the screen terminal of the valve; or
> >Both!
>
> Having been a 'triode in GG' station for over 20 years, I thought I would
> try an extra grid and see if it's worth a candle! All input appreciated.
>
> Regards.
>
> Mark.
>
>
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