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Old 29th Nov 2017, 8:34 am   #21
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 1,897
Default Re: 6BE6 = EK90 valve.

Originally Posted by TonyDuell View Post

I believe :

H = hexode or heptode designed for a separate local oscillator

K = heptode or octode with a 'phantom cathode' local oscillator section.

In general a 'K' can be used as a frequency changer on its own, an 'H' needs a 'C' (triode) as well.

That’s also my understanding of the intent, although practice does deviate. Either kind could be sharp or remote cutoff, but mostly I think that one would find:

Standalone “H” valves were sharp cutoff;

“CH” valves were mostly remote cutoff with respect to grid 1 of the “H” section, but a minority were sharp cutoff, such as the ECH84.

“K” valves were standalone, and remote cutoff with respect to the signal grid, usually grid 3 or grid 4 according to configuration.

Note though that the American 6L7 was a remote cutoff (grid 1) standalone “H”-type valve, although it never received a Pro-Electron designation. The concept was not repeated in the post-WWII miniature series; rather the 6BE6 did double-duty in that it was intended for use as a separately excited mixer as a self-oscillating mixer.

The EH90 had an interesting history. As the 6SC6, it was originally developed (by Sylvania in 1953) for use as a TV noise-gated sync separator. This concept appears to have its origins in work done by Zenith, starting with the 6BN6 gated beam valve of the late 1940s. The latter was developed (the idea by Zenith and the valve by GE) in order to realize a single-valve combined limiter and FM quadrature demodulator. Zenith saw that the 6BN6 made a good sync separator, and taking advantage of the fact that it was a dual-control valve, segued that into a noise-gated sync separator. As the 6BN6 was relatively costly (but justified in its FM role because it replaced two or three valves in conventional circuitry), Zenith looked for alternatives. The ubiquitous 6BE6 heptode from the radio valve series was found to do a reasonable job, so was used initially. Presumably the large and growing American TV receiver market made it attractive for the valve makers to introduce dedicated sharp cutoff valves which did a better job in the sync separator role than the 6BE6. As well as Sylvania with the 6CS6, RCA introduced the 6BY6 heptode at about the same time. Initially though the 6BY6 found more employment as a colour TV subcarrier synchronous demodulator.

Perhaps unsurprisingly, other uses were also found for these valves. Drake used the 6BY6 as a self-oscillating 3rd mixer in its 1-A HF SSB receiver. The irony was that the 6BE6 1st and 2nd mixers in that receiver were externally excited. The Heathkit Mohawk HF receiver employed the 6CS6 in the 1st and 2nd mixer positions, in both cases externally excited.

Then later on Philips adopted the EH90 (6CS6) for use as a locked-oscillator FM demodulator in TV receivers. I don’t know if Philips itself used it much, but Mullard certainly made a feature of it at the beginning of the UK dual-standard era. In this role it was of course self-oscillating. Effectively this was a reincarnation of the Philco locked-in oscillator FM demodulator of the late 1940s, for which Sylvania had developed a special sharp cutoff heptode, the FM1000. As with the Zenith 6BN6, this was another approach to realizing a single-valve limiter and FM demodulator. Quite possibly the Sylvania 6CS6 was a miniature derivative of the FM1000 (Loctal base). The EH90 circuit differed in detail from the original Philco circuit, but was the same in principle.

At the time that Mullard was promoting the EH90 as a locked-oscillator demodulator, the quadrature type was still in widespread use in American TV receiver sound channel practice. The Philco circuit did not seem to have lasted all that long, and was said to have some problems. The 6BN6 was still in use, along with a mid-1950s variant developed by RCA and which initially used a 6DT6 dual-control pentode, this being less costly than the 6BN6. The RCA circuit functioned as a locked-oscillator at low signal levels, and as a non-oscillating quadrature demodulator at higher signal levels.

The use of the EH90 as an FM quadrature demodulator completed the circle on a roundabout journey, detouring via sync separator country, that started with the 6BN6 as an FM quadrature demodulator.

Philips/Mullard did not appear to have used the EH90 as a noise-gated sync separator, though. Philips had earlier used the heptode section of the ECH81 radio oscillator-mixer valve in that role, which may be seen as being parallel to Zenith’s use of the 6BE6. Later it also made use of the ECH83 from the 12-volt HT car radio series, perhaps because of the rather low HT voltage at which these sync separator valves often operated. (That brings up the unresolved debate as to whether the ECH83 was simply a renamed ECH81, or a subset of the ECH81 selected for its better low-voltage behaviour. The use of the ECH83 as a TV sync separator suggests the latter.) The next step was the dedicated ECH84 with a sharp cutoff heptode section. Presumably the triode section of the ECH81 had been otherwise gainfully employed in TV circuitry, and Philips wanted to retain that facility in any successor valve.

At the beginning of the UK dual-standard era, Mullard was advertising the EH90 as an FM demodulator and the ECH84 as a noise-gated sync-separator, even though the two heptodes were functionally interchangeable. For example, Mullard suggested that either could be used as synchronous demodulators in one of its early FM multiplex stereo decoder circuits.

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