"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
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