[ARC5] OK Smart People - Mystery 211s

[Kenneth G. Gordon]

On 15 Jun 2012 at 19:09, Richard Knoppow wrote:

>      Its possible the Eimac tubes have a different kind of 
> filament than the other tubes.

Yes. I am sure they do. They are usually either quite thick or spiral-wound, or 
even made with little "fins" on them. For instance, the 304TL filament 
requires 5 volts at 25 amps, or 125 watts (or 10 volts at 12.5 amps). This is 
substantial.

>  The reactivation works on 
> thoriated tungsten filaments.

As I understand it, the method I mentioned (2.5 x the rated filament voltage 
for 1 minute, etc., ) ONLY works with thoriated-tungsten filamented tubes.

There is a different method that works with pure tungsten filamented tubes, 
and yet another method that works (sometimes) with rare-earth-coated 
filament tubes. Most of these last are receiving-level tubes though.

> I have not looked at the 572B 
> to see what kind of filament it has.

Thoriated-tungsten. But it is very thin and has no real reserve. The filament 
in the 304TL, for instance, is quite thick and spiral-wound.
> 
>      As far as getter, some transmitting tubes appear to 
> have the same kind of getter as receiving tubes. I have 
> somewhere a pair of very old Western Electric 211s that have 
> the typical silvery deposit on the inside of the envelope. 

Both my WWII vintage 211s have a silvery coating inside the bulbs.

> They glowed quite nicely when tried in my BC-375 perhaps 
> forty years ago and were kept for display.  The other 211s 
> all had plain envelopes.  Eimac used materials for plates 
> that did the gettering when they were really hot.  Those 
> tubes were designed to run so that the plate glowed.  I 
> suspect if something has allowed air to get in the gettering 
> will not work before the filament is poisoned.

Undoubtedly true. The various Eimac tubes have coated plates (I cannot 
now remember what that coating is: perhaps zirconium) and are designed, 
purposely, to run with lots of plate color. We ran our 304TL-modified BC-
610s with yellow-hot plates in Class B, which was undoubtedly over their 
normal ratings. But they were quite reliable. We never had to change one. 
Besides, it was SSB, and very cyclical.

I remember one 304TL we had in a modified BC-610 that we accidentally ran 
so hot once that the coating burned off a large spot on one of the plates, 
leaving a large shiny spot there.

The tube still worked quite well for several years afterwards.

Ken W7EKB