This thread is a spinoff from the thread Valve Items - Philips/Mullard Rimlock-to-Noval Transition
https://www.vintage-radio.net/forum/...d.php?t=159120, wherein the question arose – why were both the EAA91 and EB91 issued, when they appear to be functionally quite similar?
A first pass look at the easily accessible data does not produce a definitive answer.
As a starting point, what were probably the final Philips datasheet issues for these valves were dated respectively 1999 June 12 for the EAA91 and 1999 June 14 for the EB91.
That for the EB91 is quite brief. No quantitative performance data was provided, simply the note
“For further data refer to type EAA91.”
From those datasheets, the only discernible difference between the two was a dimensional one. The overall height (top pip to pin ends) of the EAA91 was shown as 44.5 mm maximum whereas that for the EB91 was 54 mm maximum. In that the EB91 referred back to the EAA91, Philips may have viewed the latter as the primary type.
An earlier reference is the Philips Australia 1962 Technical Data Book. This provides a tabular listing of a very wide range of valves (including the Innoval types not mentioned in Philips Netherlands sources.) Both the EAA91 and EB91 were listed, and in both cases with the comment
“For data and notes refer type 6AL5”. (No dimensional data was provided in this case.)
At this stage one could say that in later years at least he EAA91 and EB91 were functionally the same valve, differing only in overall length. Whether that difference was deliberate or happenstance is unknown.
Earlier still was a Philips (international) data pocket book, apparently from 1954. This appeared to cover most of the Philips range, including miniatures, Rimlocks and novals, although not all of the Australian Innovals were listed. The apparent date of 1954 seems about right. For example the EF89 (thought to date from c.1954) was included but the EBF89 (thought to date from c.1955) was not. The double diodes listed were the listed the EAA91, EB91 and 6AL5, as well as the EB41 and UB41. Each had its own brief dataset (no physical dimensions) except for the 6AL5, for which it was said
“see EAA91”. Furthermore, the EAA91 was the only double diode in the “preferred types” summary at the front of the book. That could have been a change of position by Philips, as the double diode in the TV series, as delineated in Book IIIC, was the EB91.
The 6AL5 was the standard double diode in the American miniature series. It had been introduced as a WWII valve by RCA in 1944. It was found that the 6J6 miniature double triode (dating from 1942) was being used in the triode-strapped mode where a high-perveance diode was required for wideband demodulation. Thus RCA developed a high-perveance double diode with performance characteristics very similar to that of the triode-strapped 6J6.
The 6AL5 became a part of the initial American B7G-based AC miniature receiving valve range in 1945, one of its recommended applications being as an FM receiver demodulator. The 12AL5 derivative, with 150 mA heater making it suitable for use in AC-DC radio receivers with 150 mA series-string heaters was added in 1947. Then the 3AL5, with 600 mA controlled warm-up time heater was added in 1954 for use in TV receivers with series-string heaters. The overall height of the 6AL5 & co. was 1.75 inches maximum, that is 44.5 mm. That was the same as for the EAA91.
From that one might deduce that the EAA91 was simply the European version of the 6AL5, with the EB91 being an electrically similar but physically different (for some reason) variant. But it does appear that the EB91 preceded the EAA91.
The EB91 was included in the 1949 Mullard publication “Valves for Industry and Communications”. So it was in place at least by 1949. The same publication also included the EB41 and UB41 Rimlock double diodes. The EB91 had a maximum height of 55 mm, and the EB41 and UB41 57 mm.
Whilst the exact release dates for the EB41 and UB41 are unknown, the double diode suitable for FM and TV purposes was certainly within the scope of the original Rimlock receiving valve range as discussed in Philips Technical Review 1946 October. Evidently Philips saw these as its principal post-WWII receiving double diode types. However, in 1949-50, when it decided to adopt the noval base for most of its standard TV receiving valves, it did not develop a noval double diode, but rather took the existing EB91 into its TV range. Whether Philips itself had previously offered the EB91 is unknown; perhaps it was drawn from the Mullard range.
One may then ask, why did Mullard (by 1949) offer both the EF41 and EF91, which were very similar in performance characteristics. A possible answer may be found in the Wireless World 1947 November report on the valves shown at that year’s Radiolympia. In respect of Mullard it was said:
“Miniature all-glass valves shown by Mullard are mainly for use in industrial equipment. There were a few samples of the valves that will eventually be produced for domestic receivers, with the small B8A pin arrangement.” (The associated picture included both the EF42 (B8A) and EF91 (B7G) high-slope pentodes, another duality that has been covered in the recent thread EF91 to EF80,
https://www.vintage-radio.net/forum/...d.php?t=157399.) In 1947 the term “miniature” was mostly used for the B7G types, the Rimlocks being identified as such or as B8A.
Thus Mullard probably saw it as necessary that its B7G industrial range should include a double diode, hence the EB91. The competitive situation may also have indicated this. Osram had opted for the B7G base for both industrial and domestic applications, and its double diode was the D77, part of its “77” range. Mazda, whilst a strong proponent of the Rimlock base, had nevertheless offered a small number of B7G valves for industrial applications, including the 6F12 high-slope pentode and the 6D2 double diode. That latter also doubled as its domestic double diode; unlike Philips/Mullard, it did not release a Rimlock version.
The D77 and 6D2 both had properties that were more-or-less the same as those of the 6AL5. According to a 1948 May datasheet, the 6D2 had a maximum height of 54 mm, the same as was later specified for the EB91. The D77 had a maximum height of 49 to 55 mm, according to a 1951 October datasheet. Brimar’s offering in this class was, unsurprisingly, the 6AL5 itself. Brimar Manual #7 gave the height of its 6AL5 as 1.875 inches maximum, 47.6 mm, different to that of the original American 6AL5.
The EAA91 appears to date from 1951. Perhaps it was intended to be an exact match, both electrically and physically, for the (American) 6AL5.
A Mazda datasheet dated 1961 December showed its miniature double diode as being the 6D2/EB91, with 54 mm maximum overall height.
A Mazda Belvu datasheet dated 1968 January had what it called the 6AL5/EB91 with a 45.24 mm maximum height
A 1956 GEC summary leaflet listed its double diode as the D77/6AL5, no physical dimensions given.
Condensing the foregoing, we can say:
The 6AL5, EB91 and EAA91, released in that order, were functionally the same valve. So were the Mazda 6D2 and Osram D77 functionally the same as these three.
The original American 6AL5 had a maximum height of 44.5 mm, but some European issues were taller.
The original EB91 was taller, up to 55 mm, but some later issues were shorter.
The EAA91 appears to have conformed to the original 6AL5 physical specification.
But that still does not produce a “hard” answer as to why both the EAA91 and EB91 were necessary. One could say that the “short” version might have been needed for some compact equipment that had been designed around its dimensions, and in which the “tall” version would not fit. But that just moves the unanswered question to :why was the (functionally identical) taller version developed in the first place?
Cheers,