[Amps] 4CX1500B QSK

[R.Measures]

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.

> 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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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org