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RE: EML 45

Posted by Paul Joppa (P) on June 4, 2012 at 21:56:52

In Reply to: EML 45 posted by Frihed89 on June 4, 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.

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