Sorry I press send too early, The 5.7 volts on the filament will eat tubes in
a hurry, also, (In the Dark) look for plate choke shorting out,
I find weak points are the Bias circuit and the Bandswitch. I see many of them
that come in for repair with these being the most common problems.
But the 5.7V is a good start, I just can't see tubes being all bad. 73 Chris
VE3NGW/W4 Florida

"Chuck (Jack) Hawley" wrote:

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