[Amps] RE : Interesting Item...........................

[Ian White GM3SEK]

Peter Chadwick wrote:
>The 4CX250Bs were selected for some parameter which I can't remember: I 
>know that the M-O Valve company and STC (ITT) never succesfully managed 
>to duplicate the required characteristics - at least, at acceptable cost.

You can see from the photographs that they are 4CX250BMs, an OEM version 
made for Marconi (hence the M). They were specially selected (and/or 
processed) for low secondary emission from the screen grid.

A friend who used to babysit a similar distributed amplifier using 8122s 
said that due to the unusually low anode voltage, screen currents were 
always very high,  and the screens were always close to the limit of 
thermal runaway.

This isn't a problem at our normal anode voltages around 2000V, but the 
'BM still makes an excellent replacement for the regular '250B, with 
much better DC operating stability.


-- 
73 from Ian GM3SEK