I have an AL-82 that periodically blows the zener diodes in the biasing
circuit. When it happens, I hear a pop, after which the plate current
increases and the tubes run hot. I generally operate on the same frequency
(on 75m), so I don't have to retune the amp very often. It has happened
when running the amp at anywhere from 500W to 1500W. I most often run at
about 500W (SSB). I tune as directed with 100W drive then lower drive to
obtain desired output. Some other trivial facts are I am using a resonant
dipole and SWR is under 1.2:1. After replacing the diodes, the amp will
run fine anywhere from a week to 3 weeks and then at some given point, I'll
hear a pop and know that another zener blew. I checked/tightened all the
grounds on the power supply board and also the diodes on that board. I
also blew out the amp with compressed air to be sure there was no dust
causing anything to arc. Tubes do not appear to be gassy and the amp will
put out full power. The amp has Amperex tubes. Any ideas?
Joe, N1FCJ
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