[Amps] AL-80B unusual plate current behavior

[Rick Low]

Filament/cathode to grid short is the answer.

Thanks for the info and Ameritron confirms this behavior indicates the
3-500Z is ready to be replaced. The other symptom that I had not previously
seen until my attention was directed to it---grid current goes negative
during this condition.

Ciao 73 de Rick I0/N6CY

-----Original Message-----
From: R. Measures [mailto:r at somis.org]
Sent: Tuesday, March 08, 2005 13:02
To: Rick and Theresa Low
Cc: amps at contesting.com
Subject: Re: [Amps] AL-80B unusual plate current behavior



On Mar 8, 2005, at 1:34 AM, Rick and Theresa Low wrote:

> Several times in the past week I䴜ve observed unusual behavior from
> this single 3-500Z amplifier and need to know if this looks like the
> tube is about to go belly up on me, or if there is some circuit factor
> that䴜s causing this behavior:
>
> 1. First turn on, plate current indicates about 200-250 mA without
> amplifier going into transmit mode.

My guess is a filament to grid short since 250mA on standby sounds like
0v bias and it's normally c. 25v on Standby.  I would test the grid-fil
BD voltage with a high-pot to find out what's what. .

> I turn ON with the standby switch in STBY. Stays this way for about 3
> to 5 seconds, then zero current.
>
> 2. At seemingly random times during T/R operation, plate current meter
> shows same 200-250 mA, input circuit shows HI SWR on the driver rig,
> and of course, no RF output. Cannot clear by cycling T/R relay---PTT.
> I turn off, wait about 10 seconds minimum, then turn on and get the
> same ‰¥þFirst turn on‰¥ÿ behavior above.
>
> De Rick I0/N6CY Gaeta IT JN61sf
>
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org