[Amps] Grounded Grid (Screen) 4CX250B power gain

[robert briggs]

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..