[Amps] AL-80B reversing its grid current

[gdaught6 at stanford.edu]

On 7 Nov 2003 at 21:43, Tom Turner wrote:

> 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...

It sounds like a filament-to-ground short.  I had an LK550 that did 
the same thing.  It turned out that the tubes were all OK.  There was 
an intermittent short between a filament choke lead and the chassis 
that would remove the bias on the tubes.  When I finally found it, the 
amp worked as is should.

I can't help with the tube-tester info you're seeking, but my point is 
that it might not be the tube.

73,

George T. Daughters, K6GT