I just got a used 76A and it had a melted plate choke. This was a
handwound
> choke off the vertical rf choke - guess abot 30 turns of #24 on 1/2 "
> tephlon form mounted on top of vertical rf choke. Looked homebrew - I
> called Alpha and they said it was not their stock design - anybody venture
a
> guess what it was for? Was in series with stock rf choke. Regular
> parasitic resistors were ok - but tubes seem to be flat above 40 meters
(only 250
> watts out on 20m and above, 500 watts on 40, 80, 160). All prior wattages
measured with bird wattmeter. Higher drive power just gives higher grid
current with no additional power out. Grid current looks "normal" and
driving power is about 35 watts, plate current is about 700 ma at 500 watts,
350 at 250 watts with plate voltage on ssb position or about 1.6 to 1.8KV.
Another curious observation was the forward and reflected meter readings
were identical - it looked like all the forward power was identical to the
reflected meter amounts, but the amplifier was feeding a heath cantenna and
the bird wattmeter indicated no reflected power and all forward power as
noted above. Wonder if some vhf or other currents are circulating in tank
circuit and messing up the metering and/or causing the low output on higher
frequencies.
>
I have worked on many amps but not alphas, so need some guidance on what to
do next. Seems like flat tubes wouldn't fall off that fast in frequency
since this tube works up into VHF range. WIthout a different know good set
of tubes don't know if tube problem or rf/vhf problem in tank circuit - any
help would be appreciated.
> Bob, AK6R
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