The RCA advertisement shown in the immediately preceding post also mentioned its VHF TV valves. 1951 had been a significant year for VHF TV valve developments, in that it saw the introduction of both the double triode cascode RF amplifier and the triode-pentode VHF oscillator mixer.
This RCA advertisement, covering the 6BQ7 double-triode and 6X8 triode-pentode is illustrative:
I suspect that they were parallel developments, with an earlier start for the 6BQ7, as RCA had been working on an improved VHF TV RF amplifier since c.1948. But they ended up being somewhat co-dependent. The pentode mixer became desirable with the introduction of the “40 MHz” IF, which being just below the lowest low-band channels, led to regeneration problems with triode mixers on those channels. The pentode avoided the need for neutralization. On the other hand, the pentode mixer, being relatively noisy, was not such a good idea on the high-band channels, where a triode would be generally satisfactory and quieter. So, if it [the pentode] was to be used, it needed to be preceded by a high-gain, low-noise RF stage, for which purpose the cascode was an excellent fit. Given that the industry already had double triodes (6J6 and 12AT7) for oscillator-mixer service, then clearly a single envelope triode-pentode was desirable, as reversion to a three-valve VHF tuner would have been unwelcome. RCA emphasized that the 6X8 was equivalent to half a 6J6 plus a 6AG5. Both of those were WWII VHF valves that saw extensive use in early American TV receivers. It was not until c.1950 that successors to the 6AG5 arrived in the form of the 6CB5 (mild improvement) and 6CB6 (bigger step change).
GE also had a contender for the cascode amplifier, namely the 6BK7:
It did not offer a companion triode-pentode, though. Perhaps it’s view was that the double-triode would continue to serve in the oscillator-mixer role. In 1947 it had introduced the 12AT7 for frequency changer use in both TV and FM receivers. This valve had lower microphony than the 6J6, so was a better choice for split-sound TV receivers and FM receivers. Even after the triode-pentode arrived, there was some ongoing use of double-triode frequency changers in VHF tuners. That is illustrated by the later development of 600 mA series-string variants, namely the 5J6 and 6AT7.
Tung-Sol had a triode-pentode oscillator-mixer, the 6U8, but not its own cascode valve:
The 6U8 had separate cathodes for the triode and pentode sections, but with the pentode suppressor grid internally connected to the pentode cathode. The RCA 6X8 had a combined cathode, but a separate pinout for the pentode suppressor grid. RCA appeared to have strong views on the benefits of that at the time, and for example its 6CB6 VHF pentode had a separate suppressor pinout whereas the 6AG5 (and 6CB5) had the internal connection. The 6U8 was, I think, more influential on following practice, and it’s interesting to note that the RCA 1954 advertisement attached to the preceding post included the 6U8 as well as the 6X8. The 6U8 became the ECF82 in Europe, from which was developed the PCF82, registered as the 9U8. Later, the 9U8A (with controlled heater warm-up time) was added to the American series when 300 mA series-string valves were introduced.
About the 6U8 Tung-Sol also said:
“The construction and characteristics of the 6U8 provide designers with extremely desirable flexibility in combining circuit functions. The pentode section of the tube may be used as an I. F. amplifier, video amplifier, sound limiter or synchronizing separator. The triode performs satisfactorily as a horizontal or vertical oscillator, or sync clipper.
“Wherever there is need for a triode and a pentacle in a receiver, they can be combined in the 6U8.”
Thus, the 6U8 was also the first multipurpose TV triode-pentode. In American practice, this led to a whole plethora of multipurpose triode-pentodes, distinct from the frequency changer group. Early amongst these, in 1954, were the 6AN8 (RCA) and 6AU8 (GE). RCA also developed the 6AM8 TV diode-pentode. On the other hand, the 6AU8 triode was said to be suitable for operating in diode-strapped mode. In Europe the preference seemed to have been to use the frequency changer triode-pentodes for multiple purposes. Philips had introduced the PCF80 (along with the PCC84 cascode double triode) in 1953, and Mullard had described its frequency changer application in “Valves, Tubes & Circuits” (VT&C) #15 and 16 of 1954 March and April. It followed with VT&C #26 and 27 of 1955 February and March, whose topic was effectively “101 positions for the PCF80”. I suspect that the much larger American TV receiver market allowed a greater range of valve types than could be managed in Europe without diseconomies, and also colour TV receivers benefitted from multi-unit valves if the envelope count was to be kept reasonable.
RCA had not presented its 6X8 as a multipurpose valve. But it did say:
“The RCA-6X8 is especially suitable as an oscillator-mixer in AM/FM receivers.”
That it could be used as an FM frequency changer is not unexpected. But the AM application was at first surprising, considering that post-WWII American AM practice was almost completely based upon the pentagrid frequency changer, mostly the 6BE6. It had eschewed the triode-hexode/heptode type post-WWII, even though those had been used pre-WWII.
But that AM application might be explained by the fact that early American practice in respect of AM/FM receivers was that the same front end could be used for both AM and FM, and for example Zenith certainly built such circuitry. The 6BA6 remote cutoff pentode and 6BE6 heptode were both specified for FM use. The 6SBY7 (octal) heptode was developed by RCA for better FM performance than provided by the 6BE6, and a noval-based version, the 6BA7, followed c.1948. The 6BJ6 remote-cutoff pentode was a bit better at VHF than the 6BA6. But Zenith had adopted the 12AT7 as frequency changer for both FM-only and AM/FM receivers soon after it became available. No doubt it was much better for FM, and presumably acceptable for AM given that it was preceded by an RF amplifier. Perhaps RCA, seeing this and also that its 6BA7 was not likely to have longer-term success, saw an opportunity for its 6X8. This would certainly have been a better AM frequency changer than the 12AT7. In other literature, RCA also noted that for FM, the 6X8 pentode could be operated triode-strapped, with the inference that it would then match a 12AT7 in that role. RCA also offered a 150-mA series-string version, the 19X8, for AC-DC radio receiver use, possibly the only “TV” triode-pentode for which that was done. Consistent with this theme, when RCA embraced the 6U8, it also slated it for AM/FM use, although I don’t think that was done by other makers.
Cheers,