[Synchrodyne]

In the domestic equipment sphere, the EF91 and EF80 were usually found in TV receivers and FM receivers and tuners, where their apparent relative merits have already been discussed.

But to a lesser extent, they were also found in AM receivers and tuners, and also in AF roles. Following are a few examples.

There was a viewpoint that for domestic receivers that included SW bands, lower noise at the higher frequencies was achievable by using a high-slope VHF pentode as RF amplifier rather than a conventional HF remote cutoff pentode. One example was the Ambassador Viscount (original version), which had an EF80 RF amplifier in what was a mostly Rimlock valve line-up. Others using this technique were Murphy (Mazda 6F1 in the TA160) and Armstrong (Mazda 6F13 in the EXP119 and early BS125.) In these cases I suspect that the valve choice had less to do with the subtleties of the various high-slope valves than with what each setmaker’s established valve suppliers were offering at the time. Ambassador appeared to be in the Mullard camp, and was probably already using the EF80 in its TV receivers. Murphy was in the Mazda camp, and the timing of the TA160 (later 1951) was such that by then the improved 6F1 was available. Armstrong was Mazda oriented at the time it moved to miniature small-signal valves late in 1949, and the EXP119 was probably developed when the 6F13 was its high-slope offering, and just before the 6F1 became available.

On the industrial side, it may be noted that the same technique was also favoured by Marconi, who used an EF91 1st RF amplifier (with an EF92 2nd RF amplifier) in many of its point-to-point receivers.

Another approach to lower noise in the higher SW frequencies was to use an aperiodic grounded grid triode input stage. Dynatron did this with its T139 tuner/control unit of 1954, using a triode-strapped Z77 (EF91). Dynatron had moved to Osram valves when it went to miniatures with the T99 in 1951, and was already using the Z77 elsewhere in its equipment. So commonality was probably a reason for its choice in this case. Not only that, the Z77 was specified for triode as well as pentode operation. The T139 was designed for use with an outboard FM tuner, the FM1, which was released at the same time or very soon after the T139. The FM1 used no fewer than 5 Z719 (EF80) valves. So had Dynatron seen the Z719 as a better choice than the Z77 for the T139, it would not have been a logistical problem.

As an aside, it could be that the aperiodic grounded-grid RF input stage, as well as being beneficial from a noise viewpoint, also helped with selectivity, in that the initial tuned circuit (following it) would not need to be such a trade-off between noise and selectivity, nor would it need to be configured to accommodate a quasi-random range of aerial impedances.

Dynatron also used triode-strapped Z77 in both the driver and phase splitter positions in its LF512, LF612 and LF613 audio power amplifiers. The T139 also included a triode-strapped Z77 in its tone control stage, and I think that the same applied to the T99. The T139 also had a triode-strapped Z729 (EF86) for the gramophone input stage. One could infer from this deployment that the triode-strapped Z77 made a competent “line” level AF amplifier, suitable for high quality work, such that it was not necessary to go to the Z729 outside of low signal level situations.

Rogers used a 6AM6 (EF91) as AGC side-chain IF amplifier in its RD Junior AM tuner of 1953. The main IF amplifier was a 9D6 (EF92). In this case the choice of 6AM6 probably came about because Rogers was using Brimar valves for this unit, and the 6AM6 was then a mainstream Brimar valve.

I don’t think that the foregoing helps much with the basic EF91 vs. EF80 question, but if nothing else, it does demonstrate the versatility of the EF91. Probably the EF80 would also have fitted the AM and AF applications for the EF91 delineated above, but perhaps without real advantage and it might also have been seen as overkill.


Cheers,