Based on additional info below, will take the 5x magifying glass and examine
the circuitry/boards for additional shorting paths that affect the filament
bias. Also, removing the tube from its socket and measuring resistance from
filament to grid should also confirm or eliminate the tube as source of problem.
Ciao 73 de Rick I0/N6CY
-----Forwarded Message-----
From: "R. Measures" <r@somis.org>
Sent: Mar 10, 2005 12:37 AM
To: n6cy@earthlink.net
Cc: amps@contesting.com
Subject: Re: [Amps] AL-80B unusual plate current behavior
On Mar 9, 2005, at 10:34 AM, Rick Low wrote:
> 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.
This condx is not the tube's fault. I'm using an Amperex tube that
was manufactured in 1967, the filament helix is still straight, and it
still delivers the suds. .
> 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@somis.org]
> Sent: Tuesday, March 08, 2005 13:02
> To: Rick and Theresa Low
> Cc: amps@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?¥úve 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?¥ús 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
>>
>>
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>
> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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