[Amps] Amps Digest, Vol 269, Issue 3

[Chris]

I'm not familiar with the SB-200, but I know the Yaesu FL-2100 series 
pretty well, and they are pretty  much the same design.

Although negative grid current can be caused by grid emission, which is 
caused by heating of the grid due to contamination, I think that is 
unlikely here because you are seeing it only on the upper bands. That 
kind of thing you would see everywhere. While you can test the 572B on a 
tube tester, you cannot pull normal currents on a typical tester.

I those are CETRON 572Bs, don't mess with them: they are golden!

You don't indicate any other metering observations, which might help 
diagnose the problem.

My best guess would be parasitic oscillation.  Inspect everything around 
for evidence of components getting too hot.  In particular, the 
parasitic suppressors in series with the plates are known go bad in 
these things. If you see cracking or darkening of the resistors, replace 
the suppressors.

My personal opinion on the Yaesu anyway, is the ones they used had too 
much inductance, making them vulnerable to overheating on the higher 
bands. I cannot say if the SB200 has the same suppressors.

There are lots for sale on EBAY.

Chris, AB6QK


------ Original Message ------
>From amps-request at contesting.com
To amps at contesting.com
Date 5/17/2025 09:00:01
Subject Amps Digest, Vol 269, Issue 3

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>    1. SB 200 negative grid current on 15 and 10 meters (Alton McConnell)
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>----------------------------------------------------------------------
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>Message: 1
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>Date: Sat, 17 May 2025 15:56:25 +0000
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>From: Alton McConnell <amcconnell3 at hotmail.com>
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>To: "amps at contesting.com" <amps at contesting.com>
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>Subject: [Amps] SB 200 negative grid current on 15 and 10 meters
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>Message-ID:
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>Content-Type: text/plain; charset="iso-8859-1"
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>Hello everyone!
>
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>I am working on a friends SB 200 amp.  Works on 75, 40 and 20m.  Has a slightly negative grid current on 15 m and low output.  A very negative grid current and lower output on 10 meters.
>
>
>
>Tubes are new ebay units.  Unknown quality.  The SB 200 uses a pair of 572Bs.
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>
>
>
>
>   1.
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>  My tube tester will not test 572Bs  but I can test 811As.  Can I use the same tube tester set up to test the 572Bs?
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>   2.
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>What is likely causing the negative grid current?
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>
>
>Thanks!
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>
>
>Alton, NU8L
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