[AMPS] re 4-250 A

[Peter Chadwick]

Rich,

I didn't make it at all clear.

It's a funny filament arrangement, in that there is a 5 - 0 - 5 volt
filament xfmr, with the two fils in series across it, with the centre
point of the two fils connected to the centre tap of the xfmr - thus
(hopefully!)they act as two separated filaments each with their own
windings and a common point.  Each filament is then held up to RF by its
individual bifilar wound filament choke on a ferrite toroid. The
filaments are then bypassed with 0.01microfarad across the filament
pins, and then each one fed via a 0.01 microfarad mica capacitor through
the 8.2 ohms to ground. The filaments then have small RF chokes ( about
100microhenry from memory) from the side of the filament going to the 0v
point on the filament xfmr to ground on each tube.

The PA was originally the top of a Marconi naval tx; don't ask me why
the fils are done with a 5 - 0 - 5 xfmr, rather than a 5v 28 A one.
Maybe it's because the copper loss is less. I could have understood it
if there were two transformers and a Scott connected filament.
Originally, it was an AB1 PA, with a 600 ohm grid swamp, and a 5894
driver. However, the bottom line is that it works, and the whole shebang
cost about $60 for 1Kw out.  This meets my criteria of "less than 10
cents a watt".

73

Peter G3RZP

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