That design is called a "transmission line amplifier". The tube output
capacitance makes it very difficult to create a matched wide bandwidth
amplifier. But, a transmission line can consist of a series of
inductances in series and capacitors in parallel. So string a series of
tubes with inductors daisy chaining between the plates and it makes a
simulated transmission line with wide bandwidth. The impedance of the
line is SQRT( L / C ), where L = inductance between tubes and C = tube
plate capacitance. Design the line for an impedance like 200 Ohms and
put a 4:1 impedance transformer at the output to make 50 Ohms. Play the
same trick with the grids to form the input network.
They probably wanted to run the amplifier in pulse mode. That requires a
lot of volts and peak amps but not much power. Pulses require a lot of
bandwidth.
jeff, wa1hco
Chris Howard wrote:
I dropped by the university surplus warehouse today
and picked up a couple of beat-up RF amps, Amplifier
Research Model 200L. Only one has it's RF deck, and
that has just 11 of it's 12 8122 tubes resident.
I'm a little confused though. From what I can find
out this guy was supposed to do 200 Watts output
over a wide frequency range. Why would they
need 12 tubes?
And the power supply looks pretty hefty, but
the 8122 datasheet says 2000 volts and 300 ma
for the plate. Wouldn't that be (.3 X 12) 4 amps at
2000 volts? I don't think the primary side
leads look like they will carry that much current.
What am I missing here?
Chris
kc0atc
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