[BC] 316F beanstalks

[clive@citiria.com]

>If it had transistors it was the 316F. The transistors drove two 4CX15,000A
>tubes in the final amplifier. This was the FIRST two-tube 10 kW. transmitter
>and predated the Gates MW-5 and MW-10 series transmitters. Agreed the direct
>coupled transistors would all avalance every now & then.

- Aha! Interesting, the things I find out here.
Ron Bradbrook (Marconi Bldg 46, retired now I am pretty certain) designed the
Marconi 50KW Doherty with twin tubes and the screens were driven by a pair of
'beanstalk' solid-state amps. Some have said the design was copied from
Continental, and by the sound of it, I suspect this might be true! Because
exactly the same thing would happen.
The cause was anode-screen flashovers in the 4CX35000 tubes.

The spike would blow every transistor in the beanstalk and after a while I got
really, really cheesed off with repairing them.
We wanted to have a two tube transmitter but in the event it was too unreliable,
so the design was changed and a pair of 3CX1000's or whatever, I really cannot
recall the tube type, were installed as cathode followers. This completely solved
the problem and the Marconi 50 became a very reliable unit after that. I
regularly tested it past 125% modulation without any trouble.
The BBC still have quite a few in use I believe, connected as triplets (3 x 50KW
= 150KW output) using star combiners.
- Clive