Unevenly-lit tube filaments

Dr. Toobz · 1353

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Offline Dr. Toobz

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on: July 09, 2013, 05:45:18 AM
I won a pair of NOS RCA 2A3 tubes (grey S-plate variants) on eBay and have them in my Stereomour right now. They sound great, but I have a question about the filaments: each tube essentially has two plates, and therefore, two filaments. Each filament is "hung" from two points, like an inverted v. When lit, this means that four lit "points" can be seen when looking down into the triode assemblies. On one of the tubes, one of these points is lit when turned on, but then fades after a minute or so, leaving just three of the corners lit. Then, it will sometimes get bright again. The other tube does not show this behavior. Is this a problem, or simply the result of uneven heating, impurities in the filament wire, etc?



Offline Paul Birkeland

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Reply #1 on: July 09, 2013, 06:46:05 AM
This may very well be a problem.

You can measure the voltage at pins 1/4 of the 2A3 as that filmanet wire begins to dim.  If that voltage deviates noticeably as the filament brightness changes, I would ask for a replacement tube.

It may also be the case that there's some crud somewhere in the tube, and you just need to let it cook for a few days

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man