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@contesting.com
To amps@contesting.com
Date 5/17/2025 09:00:01
Subject Amps Digest, Vol 269, Issue 3
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Today's Topics:
1. SB 200 negative grid current on 15 and 10 meters (Alton McConnell)
----------------------------------------------------------------------
Message: 1
Date: Sat, 17 May 2025 15:56:25 +0000
From: Alton McConnell <amcconnell3@hotmail.com>
To: "amps@contesting.com" <amps@contesting.com>
Subject: [Amps] SB 200 negative grid current on 15 and 10 meters
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<MW4PR20MB53217896570C2FE9A237538DFA92A@MW4PR20MB5321.namprd20.prod.outlook.com>
Content-Type: text/plain; charset="iso-8859-1"
Hello everyone!
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.
1.
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?
2.
What is likely causing the negative grid current?
Thanks!
Alton, NU8L
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