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24.19.48.62
In Reply to: RE: EML 45 posted by Frihed89 on June 04, 2012 at 05:48:34
On their web page, EML says "They have 4 four original V-shaped filaments like found in vintage 45 tubes."
The nonlinearity due to uneven cathode-grid spacing is a real effect, as Dennis alludes to. However, you must consider two further things: 1) it's the active surface of the space-charge electron cloud, rather than the emitting surface per se, that is important, and 2) other sources of nonlinearity include the mechanical precision of the grid (EML says "Wolfram grids allow the most precise grid wire distance, because wolfram is an extreme hard metal. This ensures better grid precision, which gives uniformity and better linearity at the low end of the tube curves...").
I would add uniformity of grid-plate spacing on theoretical grounds, but as long as the geometry is either flat or cylindrical (as in most 45s, 2A3s, 300Bs, and 845s), I believe that parameter is more easily controlled. Where you see it crop up is when the plate is a box construction while the grid is oval and the cathode cylindrical - many small signal triodes are built this way.
As for the RCA reference, EML says only "This tube has fully original specifications of the American RCA Type." Those specs do not include distortion, basically just mu, gm, and rp. EML does provide plenty of evidence that their tube is extremely linear, which RCA never did as far as I know. For what it's worth, the WE 300B original published curves are also extremely linear.
Incidentally, when I say "linear" I really mean that the mu does not vary significantly with operating point. All tubes still follow the 3/2 power law imposed by the physics; constant mu is as close to linear as tubes can get.
Follow Ups:
Just for the sake of clarity, Wolfram is Tungsten. Tungsten's symbol is "W" after the original name: Wolfram.
Wolfram may sound more exotic and by mentioning in their blurb that it's extremely hard, they give the impression that they're using a different material than most other manufacturers, which is not the case. Tungsten filaments are also used in light bulbs - which require very precise manufacture:)
The standard material for oxide-coated vacuum tube filaments is nickel. Tungsten is used in light bulbs because it has a much higher melting point, but oxide-coated tube filaments operate at a low enough temperature for nickel. Tungsten, thoriated or not, is used in high-voltage tubes where oxide coatings are too fragile to withstand high-velocity bombardment from stray ions - at least, that's the way I understand it.
I don't know the details, but I gather that when recent high-end manufacturers went away from round wire to a more flat construction, there were some problems with breaking filaments. It would be educational if Jac or someone who knows the real history would post, but it may be that there are proprietary technologies involved - and the internet is NOT a good place to keep industrial secrets!
Hi Paul. I know you don't like to have multiple posts per topic but as an example, Shugang Psvane and Treasure series are using tungsten filaments as do most transmitting tubes like most 10's, 845 etc. I was only mentioning the material as a difference between marketing and innovation. Also, using the name "wolfram" is misleading by giving the impression that it's a special or unique material when it's tungsten with or without special additives.
Good point - I just assumed that Wolfram was the word used in Czech, as it is in most Germanic and Slavic languages. So I figured it was more a translation issue than a marketing one.
Wikipedia has a pretty interesting article on the element. Among pure elements, only carbon has a higher melting point.
Yes, the material is called Wolfram in Czech (and other languages).
That is why its chemical symbol is W. English calls it differently -
Tungsten. So AVVT and EML call it by its international name.
The reason to use Wolfram (Tungsten) in filaments is as mentioned:
the Tungsten filaments do not burn as easily as Nickel filaments.
In any case, EML is talking about using it for grids, not for filaments. All their tubes use oxide-coated filaments, as do any 45s. Tungsten for Grids is probably not all that rare, I'd speculate. Rare grids were the gold-plated ones used by Western Electric, and later by a few modern tube manufacturers. (reduces secondary emission from the grid)
-Ed
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