Advantages of ECC85 over ECC81?

[Synchrodyne]

The attached selection of advertisements from early 1950s Wireless World issues might add something to the chronology.

In January 1950, Brimar was advertising the 12AT7, 12AU7 and 12AX7 double triodes. That they are noval-based was featured, this being at a time when Mullard was still B8A-oriented. The 12AT7 was billed for use as a frequency changer up to 450 MHz, and as an oscillator up to 600 MHz.

In September 1952, Mullard listed the ECC81 as a “replacement” for the “American” 12AT7. So the ECC81 was available by then. As the ECC82 and ECC83 were not listed, it seems reasonable to infer that Mullard had not yet released them.

In October 1953, Mullard was advertising “three new double triodes”, namely the ECC81, ECC82 and ECC83.

The ECC81 was noted as being suitable for use in RF amplifiers up to 300 MHz. The ECC82 was suggested for oscillator and frequency multiplier work, whilst the ECC83 was billed as an AF valve. The noval base aspect was featured – by that time Mullard had evidently moved from B8A to B9A as its standard base.

In the same issue, Mullard also advertised noval-based audio valves, including the ECC81, ECC82 and ECC83 in this group. Nothing is said about low hum in respect of the ECC83, although it is mentioned for the EF86.

In September 1954, Brimar was still featuring the 12AT7 as a TV frequency changer. By then its days as such must have been numbered, as the advent of the higher (34.65 MHz) standard IF concomitant with the need for Band III reception indicated a move to pentode mixers, hence the ECF80/PCF80. (Paradoxical that, as the 12AT7/ECC81 would have been fine – and quieter - for Band III, but the pentode mixer was now preferred for Band I because the IF was so close to the signal frequencies.)

Unfortunately I cannot find any similar advertisements relating to the ECC85 (or ECC84, for that matter).

Regarding the ECC83 and DC heating in AF applications, as far as I know the conventional wisdom of the late 1950s/early 1960s was that input amplifiers for magnetic cartridges (or tape heads) could have a sensitivity down to around 4 mV (before noise became obtrusive) when using an EF86 with shunt feedback. Better sensitivity, down to 2 to 2.5 mV could be had by using an ECC83 with series feedback, because of its lower noise floor, but then DC heating became necessary (or highly desirable), in part because series feedback meant that the first stage cathode resistor was unbypassed, which then provided a path for hum transfer from the heater. I think that DC heating for ECC83 input amplifiers was used by Brenell in some of its tape amplifiers.

The 12AT7 was used as the frequency changer (oscillator-mixer) in the original Quad FM tuner, and shown as such in the schematic (A series). For the various B-series iterations it was shown as ECC81/12AT7, and finally as 12AT7/ECC81 for the C-series (FMII).

Eddystone also referred to the 12AT7, not ECC81 for its 770U UHF receiver, where it was used as a shunt cascode 1st IF (50 MHz) amplifier, following a grounded grid RF amplifier and germanium diode mixer. For the Mark II version the same valve was noted as 12AT7 or ECC81.

Why both Quad and Eddystone started with the 12AT7 and not the ECC81 is unknown but they both ended up seeing them as being interchangeable.

Back to the original question, it seems that when the12AT7/ECC81 was first designed, whereas amplifier, oscillator and mixer applications, on a one-section-per-function basis were envisaged, the single-valve FM front end, combining RF amplifier and self-oscillating mixer, was not a key application, maybe not even considered, and so it was not ideal in this role due to its lack of interstage screening. Once economic FM receiver design strongly indicated the preference for the single-valve front end, the addition of an interstage screen was justified, hence the ECC85. As GH8QP Dave has outpointed, the available evidence indicates that the screen was the major difference, any others seemingly being points of detail, perhaps even happenstance that were nevertheless featured in order to maximize the case for the ECC85.

Cheers,