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[AMPS] 3-500Z grid to filament resistance?

To: <amps@contesting.com>
Subject: [AMPS] 3-500Z grid to filament resistance?
From: mike@g0mjw.freeserve.co.uk (G0MJW)
Date: Fri, 28 Apr 2000 08:46:38 +0100
>Greetings,
>
>I suspect that one or both of the 3-500z's in my SB-220 (original Eimac
>bottles, date code 1977!) may have a grid to filament short.  I've
>measured about 11 Kohms with my DVOM while the tubes are still in the
>socket.  I've heard that I should be seeing around 100 Kohms.
>
>Can someone confirm what the grid to filament resistance of a good
>3-500z should be?

It should be very high - with the valve "cold". With the valve warm, it will
presumably be just like a diode and the resistance you measure will depend
on the polarity. However, 11k is high enough as the 3-500Z is zero bias. Are
you sure you are not just measuring something in the PSU ? There needs to be
some path from Grid to Cathode after all.

Is there some fault ?




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