[ARC5] Tube Testing Weirdness

[David Stinson]

Howdy, y'all.

I'm working on developing settings for the TV-7
tube tester so I can test U.K.-nomenclature
tubes for the Marconi R1155 receivers.

While developing a valid set-up for the
VR-100/KTW61/CV1100 (6.3V Fil, HS0-3485)
and exploring bias range "B" (because that's
the range used by a close U.S. equiv, 6K7),
I'm seeing an odd behaviour.
While rotating the BIAS pot from 0 to full
clockwise, the test meter climbs, peaks and
reverses, going down-scale again.  I don't
see this behaviour on 6K7 or others.
Video:
https://photos.app.goo.gl/G4js8opd91N4kGTZ9

Has anyone an idea of what's happening here?

TNX OM ES 73 DE Dave AB5S

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