Re: [Amps] 4CX1500B QSK

[R.Measures <r@somis.org>]

On Nov 1, 2004, at 8:56 AM, Mark Hill wrote:

> 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!

Increasing the grid bias V on Rx requires a lower-V device than 
increasing the screen-V on Tx.  An electronic grid bias switch is shown 
at:
http://www.somis.org/D_a_04.GIF
> 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.

Mark -- The extra grid allows using:  the grid-driven config. -- which 
means simpler input circuitry than G-G, and makes it somewhat easier to 
cover a wide freq. range.  The disadvantage of a having a screen is 
providing stable screen-V.  However, with a 4cx1500B, screen-I&E is so 
small that the bias for the screen's shunt regulator can be obtained 
from the low-potential end of the HV bleeder R. ///  note --  If you 
require less power than the 4cx1500B's cathode is capable of producing, 
reducing the screen  potential from 400v to 225v will reduce the power 
out to c. 43% of what the tube is capable of.
> Regards.
>
> Mark.
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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