Quote:
Originally Posted by Roger Jones
Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger
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For the curious & others the following is lifted from "History of the British Radio Valve to 1940" by Keith R Thrower. As excellant resource for anyone interested in vacuum tubes.
...
It is generally not known that the beam tetrode was invented in Britain by two EMI engineers, Cabot S Bull & Sidney Rodda, in an attempt to find an alternative to the pentode, the patent for which was owned by Philips. Unfortunately MOV, which was half owned by EMI, did not believe the valve could be manufactured & the design was passed to RCA under an exchange agreement between the two companies. The invention is clearly set out in a patent application made in Aug 1933.
(Description of the claim)
An interesting alternative to the beam tetrode was the critical distance tetrode that was introduced by Hivac in 1935 & was the brainchild of JH Owen Harries. It was found that if the spacing between the anode & screen was increased, there was a critical separation (about 3 cm) where there was no kink in the anode characteristic & the valve had exceptionally good linearity.
END OF QUOTE
Cheers to all, John