Using the TH628 tetrode-style tube in my amplifier project, I need to pulse the
control grid in and out of conduction for the repetition rate of my amplifier.
This is because the plate dissipation is on order of 350 kW
and there would be extreme cooling and power design if it was CW. RF is needed
for only %10-15 of the time (duty factor). Grid voltage varies from -350 (class
B) to -800 (cutoff) and during the conduction phase has about 5 amps of grid
current.
It is a linear amplifier, grounded grid and grounded screen for RF, with DC
bias on both. I am using Fairchild HGTG11N120 IGBTs, a pair in a totem pole
arrangement connected to two bias voltages. The gates are driven directly from
Avago HCPL-3120 optoisolated IGBT drivers.
It works very well as a switched system, and during the conduction the
amplifier is cathode driven as a linear amp. Its amazing what modern silicon is
doing nowadays. I found a half dozen plastic TO-247 parts that would fit in the
same socket, from various companies, including some MOSFETs.
All are rated 1200 volts at >11 Amps I believe. Very small heatsinks required
in switch mode. For control of screen grid voltage supplies, the same idea
could be extended to linear mode, without the opto driver, and more cooling.
73
John
K5PRO
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