[Synchrodyne]

A reverse lookup on RMorg shows generally a higher application count for the EAA91 then the EB91 once the former became available. A cursory inspection suggests that the EAA91 was used by European setmakers, whereas the EB91 was used by UK setmakers.

Here is a postulate, or perhaps a “straw man” as to why things turned out the way they did, at least in terms of what Philips/Mullard did.

1. Philips developed the EB41 (and UB41) as part of its new Rimlock receiving range. The EB41 essentially replicated the properties of the American 6AL5, but otherwise was built according to Rimlock precepts. As a result of this, or perhaps the result of limiting the number of height variants in the Rimlock range, it was somewhat taller than the 6AL5.
   
2. Mullard at least, and perhaps other Philips branches required a double diode on the B7G base for industrial applications. Speculation, with as yet no supporting evidence; thus was developed the EB91, essentially the EB41 on a B7G base, and perhaps with Rimlock structural features inside. It came out somewhat longer than the 6AL5.
   
3. In 1948-49 Philips decided to adopt the noval base in place of the Rimlock. This change had several facets, as follows:
   
   • a. New radio valves would be issued as novals, e.g. the EBF80.
   • b. As well as the EBF80, a few existing Rimlocks would immediately be reissued in noval form, and with American registrations and designations, for the Australian market, under the “Innoval” name.
   • c. A new range of noval TV valves would be issued, the EF80 and ECL80 being early examples.
4. In connection with 3c, Philips decided that it was not appropriate to reissue the EB41 in noval form. Rather it took the existing EB91 into the new TV range as its standard double diode. Insofar as the B7G and B9A bases ran in parallel, with mixes thereof being acceptable to the setmakers, this was a logical move. The EB91 was also to displace the EB41 in radio applications.
   
5. Purely speculative: from 3b, Philips would have needed an American designation for whatever double diode it chose to sell in Australia. AWV, I think the dominant Australian valve producer, and with a mainly RCA-derived range, had already made available the 6AL5. Philips might have obtained an American designation for the EB91, but saw offering the 6AL5 as such as being the better pathway in this case. The 6AL5 was physically at least not quite the same as the EB91, and for Philips manufacture, it needed a Pro-Electron designation, so EAA91 was chosen to differentiate it from the EB91.
   
6. Still speculative, Philips Netherlands (and in Europe generally) may not have been heavily invested in the EB91, and so decided to make the EAA91 its standard double diode. That could explain why it was shown as the preferred type in the 1954 pocket book. De facto then, the EAA91 directly superseded the EB41 in European radio receivers.
   
7. This is empirically supported: Mullard, already heavily invested in the EB91 as part of its industrial range, retained that variant as its standard double diode in the radio and TV receiving ranges as the Rimlocks faded out.

If there were any performance differences between the EAA91 and EB91 – and it does seem at least possible that there were - these have not been mentioned in any of the datasheets and literature that I have seen.


Cheers,