Thread: EF91 to EF80
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Old 16th Oct 2019, 12:40 am   #60
Synchrodyne
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Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
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Default Re: EF91 to EF80

Something I have not yet found is “hard” information as to the origin of the EF91 valve group. My best estimate is that the Osram Z77 was the parent valve, the circumstantial evidence supporting this being follows.

In the post-WWII period, the major UK valve makers, Mullard (Philips), Osram, Mazda and Brimar were faced with the need to introduce miniature domestic receiving valve ranges, including high-slope pentodes suitable for TV applications.

First though was the choice of miniature type, which at the time was between the American B7G and Philips Rimlock. Mullard, as a Philips subsidiary, chose Rimlock, as did Mazda. Brimar, whose valve range was based upon American practice, chose the B7G form. Osram also opted for the B7G form.

In respect of the “TV” pentodes, Mullard had the Philips EF42 from the Rimlock range, and Mazda developed the similar 6F13.

Osram’s TV pentode offering was the Z77, as part of its “77” series (which included some “78” suffix valves.) This range included some clones of American types, such as the DH77 (6AT6) and X77 (6BE6), and some own designs, such as the W77 and X78. The Z77 had no counterpart in the American series, and so was either an own-design or a clone of another American or European valve.

Brimar mostly chose American types, but did develop its own, or clone British/European types where there was no American type suitable for the job. Its B7G radio receiving range was essentially American (6BA6, 6BE6, 6AT6, etc.), but the American TV pentode of the time, the 6AG5 (slope around 5 mA/V), may have been seen as not being fully competitive with what was expected from the other British and European valvemakers. (Probably the general features of the Philips EF42, including its very high slope, were known by late 1946, even if production was yet to start.) Thus Brimar offered the 8D3 (later renamed as the 6AM6) as its TV pentode, this being essentially the same valve as the Z77.

The two previous paragraphs point to the Z77/8D3 as being either an Osram or a Brimar development, cloned whichever of the two did not develop it.

Notwithstanding that Mullard and Mazda opted for the Rimlock type for their respective domestic receiving ranges, it appears to have been the case that industrial/commercial and military users would not adopt the Rimlock for general use, but preferred the B7G type. This meant that Mullard and Mazda would need to offer an adequate range of B7G types additional to their Rimlock ranges, including high-slope pentodes.

The minimum effort approach would been to simply develop B7G versions of their respective Rimlock valves, the EF42 and 6F13. On the other hand, the military market in particular liked standardization, and so there was a benefit in cloning whatever their B7G-oriented competitors were offering as high-slope valves. In that case the Mullard EF91 and Mazda 6F12 would have been more-or-less copies of the Z77/8D3.

Had either Mullard or Mazda been the lead developer, therefore with the Z77/8D3 characteristics unknown, it might have been expected that they would have closely followed their Rimlock prototypes, rather than develop valves with somewhat different characteristics. So on the balance of probabilities, I’d say that they were the followers in this case.

That leaves us with either Osram or Brimar as the developer. Osram generally developed its own valves except where American or industry types were a good fit, so it probably expected to have to develop its own TV pentode both for domestic receiving and (Marconi) studio/transmitter equipment (in which I think the Z77 was widely used). Z77 development might well have been the major activity in the whole “77” programme. Brimar on the other hand generally followed American practice, with own development being the exception. It may well have earmarked the 6AG5 as its TV pentode until it saw what its UK competitors were doing. Combined with the military preference for standardization, cloning could well have been seen as the optimum pathway.

So again on balance of probabilities, the Osram Z77 looks more likely as the parent valve than the Brimar 8D3.

Presumably the BVA would have facilitated the interchange of information necessary for cloning. Apparently there was tentative agreement on a new standard receiving range – including high-slope TV pentodes) most on the Rimlock (B8A) base, reported in Wireless World 1946 November, p.375,but events did not turn out quite as planned.

First mentions in Wireless World (that I can find) for some of the above TV pentodes were:

Mazda 6F13 - 1947 June p.228
Osram Z77 - 1947 June p.199
Mullard EF42 & EF91 - 1947 November p.435
Mazda 6F12 – 1949 January p.13 (I think that this was somewhat after it was released.)

American registration for the Z77/EF91, as the 6AM6, was done by Cossor in 1949. (What designation, if any, Cossor may have used before this I don’t know). Brimar changed from 8D3 to 6AM6 soon thereafter.

Although the Mullard EF91 and Mazda 6F12 might have been intended for commercial/industrial and military applications, they were also used in domestic receivers and so each of these valvemakers had two horses, as it were, in the TV pentode stakes of the late 1940s, one B7G and one Rimlock.


Cheers,
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