When I was younger -35 years ago-, I have observed the glowing of a NEW VR tube (it was a 0C2) for few hours (ok, I had nothing more interesting to do then..) and seen that at low currents, especially, the ionisation path moves itself randomly between the tubes elements (the ionisation density is never equally distributed vs the element's surface) and that this displacement was translated as small variations of the operating voltage accross. An "old" one does not behaved like this, seeming to have a "preference path" between it's elements. But, interestingly, both were sensitive to magnetic fields (I disturbed the ionisation path at will with a small magnet, creating suddent voltage jumps across the tube).
To sum up, it seems that if you want a stable VR tube, you should age it a few hundred hours and shield it against magnetic fields. Is it making any sense to you ? I do not see why the 5651 cannot behaves similarly. Was your "bad" tube a never used one ?
Have fun, but beware of the gases !
J14 |
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