[Amps] 4CX1500B QSK

[Karl-Arne Markström]

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 at mjha.co.uk>
To: "Amps Reflector" <amps at 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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