Presently I am working on a cathode-driven
grounded-grid/grounded-screen tetrode PA for 201 MHz. The cavity is
very large, and we (mechanical guy and I) developed a small tuning
short that sits between the screen and control grid 1/2 lambda away.
They are concentric lines, as most transmission line cavities are.
The short will be tuned with an S21 measurement from input to output,
filament lit, but the HV off. This is similar to the result from
neutralization, but it will accomplish minimum feedthru by setting
the transmission line between the screen and grid connections,
reflecting the RF grounded - short - to them inside the tube. Of
course, blocking capacitors (made from heat shrinkable PTFE tubing
around concentric sleeves) are incorporated in both grids so that we
can have DC screen voltage and pulse the control grid bias to keep
the Class AB on time to a minimum. Cannot handle more than about 300
kW of plate dissipation due to the size of the cooling plant.
At this power level, I cannot bias the cathode and earth the screen
as Collins radio did in some of their VHF amplifiers. The bias power
supply would have to carry several hundred amperes of peak cathode
current in addition to being a several KV.
Photos and results should be forthcoming, by summer I hope.
John
K5PRO
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