[Amps] Alpha 76A "hums on 160" problem resolved

[Jim Barber]

The Alpha 76A "hums on 160m" problem has been resolved.
Thanks to all that responded, on an off the list.

The actual problem turned out to be a plate bypass cap that measured 
good out of circuit with a 100kHz capacitance meter, but was essentially 
open to any RF current on 160. (not a typical failure mode)

For those who care to know, here's what I ended up doing:

. Replaced the 15uH 1A cathode choke with a 100uH 2A unit

. Replaced the cathode bypass cap (.02uF) with .04uF (four .01 500V in 
parallel)

. Replaced the two .002uF caps at the "cold" end of the larger plate 
choke with two .01uf 6KV blue Chinese caps in parallel. (It's what I 
had, don't tell Carl...)

. Replaced the two .002uF caps at the "cold" end of the smaller 15uh 
plate choke with two .01uf 6KV blue Chinese caps in parallel, as above.

. and leading up to this, I had already replaced the filter caps and 
bleeder/equalizing resistors, as well as the other carbon-comp resistors 
on the control board, which were largely out of tolerance.
...

I'm happy to say the amp shows no hum at all now and loads up normally 
on 160/80/40/20/17/15/10 meters. The two 8874's are 30+ years old, but
still load up to ~ 1100 watts with 100 watts drive from a solid-state 
rig. (a little less on 10m, there appears to be some inefficiency in the 
input circuit there) The little amp runs quiet and cool at around 800 
watts out, which is exactly what I want it for.

Thanks again es 73,
Jim N7CXI