In 2009-2012 I designed some very large tetrode amplifiers that used
grounded grid configuration. Not tied directly to ground, but grounded
for RF via cavity circuits that have built in blocking capacitors to
allow normal bias voltages. Like their amateur siblings ("tubes with
handles" as R. Measures used to say), the input match is highly
dependent on the electron beam, specifically the level of DC cathode
current. I have a cascade of amplifiers, one 150 kW tetrode driving the
final stage of several MW. Being pulsed, the grid bias is switched from
cutoff to conduction just before RF drive is pulsed on, to avoid 100 kW
of pulsed plate dissipation in the final (about 12 kW average power). It
took a while before my design team figured out best way to protect
things, as if the final power supply tripped offline, the IPA would be
driving into a horrible mismatch. So lots of fast electronics are
involved, fast logic, that shuts off the RF drive and so forth. Also,
timing of the conduction bias pulses had to be right, so that the RF
drive didn't come up too many microseconds before the beam was there.
Tying to sweep the input match of these amplifiers while they are
pulsing is not easy. I tested them with a Hp8753 VNA, at 10 mW power,
and sweep across the bandpass of the amplifier cavity. As Bias is
cranked up, and a few amperes of quiescent (but pulsed) plate current
flows, the input return loss goes from < 1 dB to 10 dB or more and a
definite dip occurs. It is a chopped waveform due to pulsing, as I don't
have the ability to run these things in CW key down. I bring this up as
it parallels what we hams have to deal with using GG amplifiers. Not
having cavity circuits for HF, we use L and C and try and get some Q,
without narrowbanding the thing - broadbanding the input tuning at least
a ham bands width.
We have to drive the thing with the normal level, and get the current up
to know the match. Lots of compromises can be made, and those amplifiers
that require a length of cable, have input matches that were fine for
tube exciters but these days not so good for solid state drivers. Some
of the problem is the cyclic fluctuation of the beam from cutoff to
conduction across each cycle of RF voltage, as others have suggested.
For HF ham stuff I have the trick now. I can use a Delta Electronics
OIB-2 operating impedance bridge. This thing can take 1 kW through
power, and will read the input Z of an antenna, line or amplifier, while
driven. It reads the R and X component to 5%, and from this some
suitable matching can be developed to get it to 50 ohms. I now have two
of them, so one will be made available FS.
73
John
K5PRO
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