November 7, 2003
AL-80B reversing its grid current
de VO1TV
Our club station, VO1CRC, uses the AL-80B amp with the 3-500Z tube. Quite
suddenly, the amp stopped transmitting and the Ig meter actually reversed
itself and pinned on the bottom of the plastic case. I shut off the amp and
then turned it on again. All OK... Sure enough, after a short time the amp
stopped transmitting and the Ig meter reverse-pinned its little self.
Now, the amp will do the reverse-pin routine even before we transmit. The
problem is getting worse. Because the amp stops transmitting, I conclude the
Ig meter is accurately telling us that there is a problem. I don’t think it’
s just a metering problem.
Any idea what the problem might be?? I’d like to get some collective wisdom
before opening the case and fiddling with the innards...
I do have an old tube tester, the Heathkit IT-17. Unfortunately, the
Heathkit tube manual I purchased in the 1980's was written in January 1978
and does not list the 3-500Z tube, so I don’t know how to test it. The
manual also covers the Heathkit IT-21 and Heathkit IT-3117 tube testers. By
a miracle, does any soul out there have the proper testing settings for the
3-500Z tube for either of these Heathkit tube testers?? I’d greatly
appreciate receiving the tube settings.
73's de Tom, VO1TV.
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