[Amps] LK550 neg grid ... and bandswitch woes

[Ken Nicely]

If the plate current is 400 ma, then at least one tube is conducting.  Mine
registers slightly negative grid current when it is not transmitting, so
that is not the issue.

I would say that it is oscillating. I had a similar problem, but I was
actually getting arching in the tubes.  I would suspect that you will find
that either the plate choke has partially melted and shorting out between
windings.

If not that, then I would suspect a bad tube.  Take one tube out at a time
and see if the problem goes away.

Ken

On Thu, Nov 25, 2010 at 12:46 AM, Gary Hinson <Gary at isect.com> wrote:

> Gents,
>
> Any idea what fault would cause my old and previously faithful Amp Supply
> Co
> LK550 sometimes to register ~400mA of plate current and negative grid
> current, even with no PTT and no drive?
>
> First time it happened it took me a while to notice and, judging by the hot
> smell, something inside very nearly let out the smoke.  After leaving it
> off
> for a few days to cool off while I mulled over what might be wrong (without
> any inspiration  :-( ), I plucked up courage to switch it back on and it's
> working OK now, most of the time, but I've noticed the neg current thing
> again, including one time immediately after switched it on, even before the
> ALO trip circuit would normally let me transmit.  This time it cleared
> itself.
>
> [I know I should pop the lid and look for charcoal but first I'm trying to
> comprehend what the fault would be.  It's part of my 'self training in
> amateur radio'!]
>
> 73
> Gary  ZL2iFB
>
> PS  Any idea where I might find a new bandswitch for the old amp?  I
> changed
> the padder caps recently but it still maxes-out on tune and or plate C on
> the low bands, so I strongly suspect the switch is no longer connecting the
> padders.
>
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