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Re: EML / AVVT / KRON or KR / EAT output tubes

Posted by unclestu52 (D) on May 7, 2007 at 23:08:44

In Reply to: EML / AVVT / KRON or KR / EAT output tubes posted by drlowmu on May 7, 2007 at 15:58:21:

Much of this is nothing new.

In the case of 300B's, the genuine WE built tubes, while employing a M shaped filament structure, had a a filament tap at the physical center and then the outer two filament leads were siamesed. This was not true for the imitators, including other US manufacturers. These makers would tap one end of the filament and then tap the far end. There is a significant voltage drop across the filament lead and thus the cathode emission would vary more from side to side, perhaps one reason why many prefer AC heated filaments.

As for the 'harp' filament structure, one reason why they were discontinued was that, in addition to being expensive to manufacture, they had a spring loaded structure on the top to tension the filaments. Those little springs did have a tendency to break. You see similar tensioning done on early rectifier tuned and large triodes: 211. 845, and the like. Many have been the tubes I have received through the mail, pristine, except for the fact that the little springs had broken off leaving the filament hanging loosely. The tensioners seem to work better on tubes tilizing ceramic spacers rather than the mica sheets or the glass rods used in the early designs (Bendix sound familiar here?).

Filament tension is quite overlooked by many modern designers. Older makers used spring loaded mechanisms and when you apply filament power you can visually see the filament structure deflect, very much like a a fuse. While newer tube designs employ thicker filament wiring, the older tubes seem to use smaller gauge wiring, but better tensioning systems (when they haven't failed). This is very noticeable on the rectifier designs.

Incidentally, I pulled my single plate 2A3's out to examine and their filament structure is actually very similar to WE 200B. The harp structure is divided in half, with 5 filament loops per side paralleled to each of the two filament leads. They are are all joined at the top bar, so in effect one half of the plate will have a slightly different voltage from the other half, factoring the resistance of the filament.

Stu

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