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Re: [Amps] 4CX1500B QSK

To: Karl-Arne Markström <>
Subject: Re: [Amps] 4CX1500B QSK
From: R.Measures <>
Date: Mon, 1 Nov 2004 10:34:44 -0800
List-post: <>

On Nov 1, 2004, at 9:48 AM, Karl-Arne Markström wrote:

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,

This is virtually the only circuit configuration possible since Eimac does not make a screen-bypas C for the SK-300A.

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.

Indeed, Karl-Arne. An even less expensive solution is to guard the DC-floating cathode circuitry with 2, 800piv 3A diodes in series to gnd. Thus, if the HV+ arcs to gnd, the potential across the 2 diodes tries to rise from 1250v-neg. to > 1600v-neg., the diodes fail-shorted. and the HV+ glitch-R dissipates the joules stored in the HV filter-C until the mains-breakers trip.



----- Original Message -----
From: "Mark Hill" <>
To: "Amps Reflector" <>
Sent: Monday, November 01, 2004 5:56 PM
Subject: [Amps] 4CX1500B QSK


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

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.



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