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Re: [Amps] AL-80B reversing its grid current

To: amps@contesting.com
Subject: Re: [Amps] AL-80B reversing its grid current
From: gdaught6@stanford.edu
Date: Sat, 08 Nov 2003 10:42:47 -0800
List-post: <mailto:amps@contesting.com>
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



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