[AMPS] The Monster Breathes

[Ron Angle (AB6ZX)]

Some of you have been so gracious as to provide advice and support during my amplifier rebuilding projects of the last six months. Now it is time for an update.

The Monster, a 40-inch high homebrew linear, was constructed in the mid-sixties. It uses a pair of 4-400A's in grounded grid with very old circuitry. No safety features, completely ungrounded with floating AC, bad bleeders and all.

Tonight I fired it up for the first time. It breathed, groaned, and glowed in the dark! The one tube with the greenish glow at its base seemed to cook properly (more on this in a future post). The plate HV meter does not indicate the correct voltage so I am not sure of the full load plate voltage. I was able to get an accurate reading of 1200 VDC on the PS with 35 VAC on the primary supply from the Variac. Some extrapolation puts the plate voltage possibly near 4 KV.

With initial loading the amp drew approximately 400 ma with a grid current over 100 ma. The indicated key down output was about 600 watts. 

Question:  What would be the expected efficiency of this amp, considering the tubes may be 15-20 years old?

I am pleased with the initial results and brief on-air tests on 75M SSB yielded good reports on the signal quality with very low drive (MIC gain setting of 2 on my TS-820S). Besides the need to completely redo the control wiring, relays, safety features,etc., the only major problem is heat. Idling current is 135 ma which produces considerable heat; a zener in the filament return line is my next project.

Again, thanks to all for their posts. I compare this project to rebuilding a sixties' big bore Mustang. It's big, ugly, but still fun!

73

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