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

[Colin Lamb]

"Tokyo High Power marketed a cathode driven 2 X 4X150/4CX250 amp that had IMD 
on par with grid driven. The price was high for the power and didnt sell 
well in the US. The few I worked on were well built and looked acceptable on 
the SA for IMD......on par with the exciters.

http://bama.edebris.com/manuals/tokyohpl/hl1ka"

I downloaded the manual and it has a nice schematic.  As Carl said, it is cathode driven.  It is not grounded grid.  The screen voltage would be about 380 volts (regulated only by a heavy bleeder resistor from a capacitor only filtered power supply). The grid is bypassed to ground and fed from an adjustable bias supply.  The cathode is fed through an attenuator.  By operating the screen at normal voltage, the gain of the tube will remain relatively high.  

Grounded grid configuration should have some improvement in imd over grounded cathode because of the negative feedback inherent in that circuit.  I have seen quotes of approximately 3-5 db or so.  Additionally, adding a tuned input would pick up a few additional db.  The measurement of this improvement, however, would be difficult, because the tuned input would improve the imd of the transmitter driving the amp (or, to be more specific, the lack of a tuned input could decrease the imd of the transmitter).

All this is speculation without adequate tests and measurments to confirm it.  I wish I had the time, energy and workspace to make such tests.

73,  Colin  K7FM