[AMPS] grounded grids

John T. M. Lyles jtml@lanl.gov
Thu, 1 Feb 2001 12:07:04 -0700

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.

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