[Synchrodyne]

Some thoughts as to why the ECL80 was used as a TV frequency-changer were recorded in post #30 upthread.

As well, the ECL80 was designed to be a multipurpose valve, and as well as AF and field output applications, the pentode section was intended for use in functions where an “EF” rather than an “EL” type might normally be employed, such as sync separator and half of a line timebase multivibrator.

Some output pentodes turned out to have quite good RF properties. The EL85 was the noval-based successor to the EL42 “small” radio output valve (from the initial Philips Rimlock series) rebottled, rebased, and given a separate suppressor grid pinout. It was then specified for RF use up to 100 MHz, as well as for AF output purposes. The empirical evidence is that the ECL80 pentode was similarly capable in RF terms.

In post #22, addressing inter alia the 6U8 and 6X8 American triode-pentode TV frequency changer valves of 1951, I noted that RCA also described the 6X8 as being suitable for AM and FM radio frequency changer applications. I have since confirmed that RCA did use the 6X8 (and its 19X8 150 mA series-string counterpart) for this purpose in some of its FM-AM radio receivers of the period. In these, the 6X8 pentode was used as such as a mixer on AM, but triode-strapped on FM. Previously, RCA had used the 6J6/19J6 for this purpose.

The 6J6, developed by RCA during WWII for military VHF applications, had been the de facto standard frequency changer valve in early post-WWII American TV practice, and was also sometimes used for RF amplifiers. In 1947, GE released the 12AT7 for TV and FM front end applications. Apparently, this was preferred over the 6J6 for split-sound TV receivers because of its lower microphony. One would infer that likewise it was a better choice for FM receivers, but perhaps RCA preferred to use its own 6J6 rather than the GE-origin 12AT7, at least until the 6X8 was available. Zenith adopted the 12AT7 as frequency changer on both bands for its FM-AM receivers. GE did a bit differently; it used the 12AT7 as an FM frequency changer, but for AM, one half was used as an oscillator feeding the 1st FM IF pentode (often a 6AU6/12AU6) which doubled as an AM mixer.

Notwithstanding the general change to the cascode-plus-triode pentode combination for VHF TV tuners for use with the 40 MHz “high” IF, some of the tuner makers continued to offer the option of pentode RF amplifiers (such as the 6AG5) and/or double-triode frequency changers through the 1950s. For the latter, the 6J6 seems to have been the valve of choice, with the 5J6 being introduced c.1954-55 for 600 mA series-string applications. With its 450-mA heater, the 6J6 may have afforded a more robust oscillator section than the 300 mA 12AT7, so was preferred on that basis, at least where microphony was not an issue. (Both the 6U8 and 6X8 had 450 mA heaters.) The 12AT7 was of course designed with a “split” heater to allow one valve type to fit both 6.3-volt parallel and 150 mA series-string heater systems. It also fitted 300 mA series-strings and 12.6-volt parallel systems.

VHF tuners with pentode RF amplifiers and/or double-triode frequency changers were probably aimed at the lower cost receivers, where performance expectations were lower. The reminder occasioned by the continued existence of the simple pentode RF amplifier alongside the cascode may have added impetus to the search for something nearly as good as the cascode and nearly as simple as the pentode, hence the neutrode and tetrode.


Cheers,