Going by memory.....you have a switched bias circuit in that amp. It comes
on and off with voice peakes. Enables a cooler operation. That circuit is
having a problem. Ive heard of this before. But since I don't have an amp
like that I can only go by what I remembered.

Dan/W4NTI

"Chuck (Jack) Hawley" wrote in message
...
A friend has an AL80B (bought new) and is on his fourth 3-500 in two years
or so. It just developed a grid-cath short. Supposedly there was a batch
of
tubes that were prone to go bad, but the latest ones from MFJ are supposed
to be ok. Apparently not or something is wrong with the amp. He's a good
operator and seems to be tuning and operating within safe limits for the
AL80B. The latest tube is an Amperex as was the first ones in the reported
bad batch. He reduced the filament voltage last time from 5.7 (from the
factory) to about 5 before the present tube was installed.
Any ideas?
Chuck, KE9UW