Design of a 400 W Power Amplifier for 432 MHz using 8874.
Background:This project started long time ago, inspired partly from an article in the 4/1979 DUBUS magasine by DL7YC for "A 70cm High Power Amplifier" using a 4CX250B, and partly from the QST Oct. 1979 article by K1FO "A Grounded-Grid Kilowatt Amplifier for 432 MHz" using a 8874.
Having access to the EIMAC 8874 Triode I decided to build an amplifier based on the electrical circuit from K1FO and the mechanical design of DL7YC.
The design:The amplifier is build from two identical aluminum boxes measuring 307x192x76 mm with the top box being the plate compartment and the lower box having a small room for the input (cathode) circuit and space for filament transformer, power for the blower and metering, keying and bias circuits.
Even though it´s a grounded grid design I decided use the proper EIMAC grid decoupling collet. The DC connection is made through a low ohmic resistor, in this manner I can measure the grid current directly across the resistor.
Plate circuit is a stripline 210x110 mm made from 1,5 mm copper sheat. The tuning flapper is grounded at the end wall and moved by a teflon string going to the lower box. The output flapper is placed at the rear side and adjusted by a teflon screw through the top cover.
The input circuit was copied from K1FO, but as I had difficulty obtaining a proper input match, I had to modify into much lower L's. The picture shows the series L as a straight wire abt. 50 mm long and the shunt L as a half turn, ø25mm diameter, 5mm wide strip.
Have a look at the diagram (206k).
Results:With 25 W drive and abt. 1700 V plate voltage the output is more than 400 W. As the 8874 has a maximum voltage rating of 2200 V more power is available. K1FO has reported 570 W output with 35 to 38 W input.
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