I have built 4 amplifiers using the 4CX-250B-350R tubes in the so called
"Super Cathode Driven" mode, tying the control grid to the cathode and
grounding the screen grid....This design was in some old Bill Orr notes
but I am now unable to find the reference, however I think this has been
covered in this reflector some years back..
Off the top of my head I remember the idle current for two tubes is
around 30ma..I used 2500 volts on the anodes and with about 120 watts of
drive I could get around 650-750 watts output....Not a lot if you
consider feed through power...I also remember the tuned Pi input circuit
had to be spot on to get maximum efficiency from the amplifiers..One
other thing was when fully loaded the Grid current (Screen) was only 2-3ma..
I don't remember the input circuit parameters but it was fairly critical...
You know while typing this it brings back quite a lot of memories..
The amplifiers were very stable and reliable and as far as I know are
all still in use today...Considering that the 4CX250/350 tube pulls are
a dime a dozen it is still a cheap amplifier and the tubes seem to last
forever...
For sockets I used 9 pin loctal types or similar and made up a mounting
plate with a good air flow around the sockets through the tube
anodes...I coupled the cathode and grid pins all together using thin
copper strap...
These days I prefer to play with the readily available Russian triodes
also very cheap tough and reliable and I wouldn't bother with the 250B
type tubes again but at the time when that was all I had to play with
they were good solid workers..
73.
Bob VK3ZL..
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