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OOOPS! APOLOGIES!! I take it back, you're right. Here's the scoop:

of the RF tubes sometimes used in SET's, the 845 and the 813 have thoriated tungsten DH'd filaments.

The 2A3, 300B and 45 are DH'd filaments, BUT they are oxide coated tungsten - and, yes, operate at lower temps.

So, it appears that the only real difference between DHT's with oxide coated filaments and indirectly heated cathode tubes - with oxide coating over nickel over ceramic over tungsten filament - is that in the DHT with oxide coating the coating is directly over the tungsten, whereas in the indirectly heated cathode the oxide is over nickel, which in turn is over a ceramic insulator, within which at the center is a tungsten filament (and the tungsten filament, interestingly, is coated with aluminum oxide in order to prevent shorts where the filament is bent or kinked and might contact itself).

I think when I get the time to really get this all in order I'll make a master list that divides tubes into tungsten, thoriated tungsten, oxide coated tungsten, and indirectly heated oxide coated nickel over ceramic over tungsten.

I still don't know what the 10 has, but I suspect it may be pure tungsten (the difference between bright and dull emitters PERHAPS being the difference between pure tungsten and thoriated tungsten, respectively).


"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.

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  Kimber Kable  

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