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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