17-04-2018, 03:14 AM
Thanks, Trevor.
It’s surprising what unrelated information comes out when one embarks upon an initially focussed search that quickly turns into a “rabbit-chase”. Here is a bit more from that meandering….
Still in the field of American AM-FM radio receiver practice, I gleaned the impression that not all that widely used was a heptode, such as a 6BE6, as both AM mixer-oscillator and FM 1st amplifier. That would be the American counterpart to the European (extensive) use of the ECH81 in the same dual role, adjusted for the American preference of a self-oscillating heptode rather than a triode-heptode as AM mixer-oscillator.
Also not widely used in the early days was the single-valve FM front end, for which the 12AT7 seems to have been the valve of choice.
The designed-for-purpose – with enhanced internal screening - ECC85 was a European development. I think it came from Telefunken, although I am not sure. It does not seem to have been a Philips original, though.
The single-valve front end was adopted more widely later in American practice. Apparently, that adoption stemmed from the influence of the mid-1950s German table receiver imports from Telefunken, Grundig, etc., which mostly used the ECC85. American makers used the ECC85 under its American registration of 6AQ8, and the home-grown 6DT8 was released late in 1957. Some of the established American TV tuner makers, such as General Instrument, also then offered single-valve front end tuners to the setmakers.
But whereas European practice did not advance beyond the ECC85, there were further developments in the USA. The 6EZ8 was a triple triode (on a noval base, with two cathodes tied to one heater leg) that could be used as combined RF amplifier, mixer and oscillator, presumably for those makers that preferred to avoid self-oscillating mixers. It could also be used as a combined mixer, oscillator and AFC reactance valve (with a separate RF amplifier) in two-valve FM front ends. A later triple triode, the 6GY8, was intended to combine the RF amplifier, self-oscillating mixer and AFC reactance functions. This had one cathode tied to a heater leg. AFC was quite important in American FM practice, and the single-valve front end had arrived on the cusp of the adoption of the diode as AFC reactance element, hence the interest in triple triode combinations.
The 6J9 triple triode, on a decar base, avoided the need to tie any electrode to a heater leg. It was intended to undertake the RF amplifier, mixer and oscillator functions. By the time it was released, diode-based AFC was in available.
On another vector was the 6JK8/17JK8 double triode. This had a frame-grid RF section, for better performance, and a conventional triode for the self-oscillating mixer. Again, by the time this was released, diode-based AFC was established, hence there was no need for a third triode.
And different again was the 6C9/17C9 double tetrode, on a decar base. One tetrode served as RF amplifier, and the other as self-oscillating mixer. It was used by RCA, for example. It probably stemmed from the use of very low-screen current tetrodes as RF amplifiers in VHF TV tuners. Along with earlier specialized VHF triodes (e.g. guided-grid), these were evidently the outcome of a search for a more economical VHF TV RF amplifier than the established cascode double triode. Tetrodes avoided the need for neutralization (which increased component count and was another production adjustment) as compared with triodes.
Also used in FM front ends was the triode-pentode, with pentode as RF amplifier and triode as self-oscillating mixer. The 6JN8/12JN8 was used for this purpose, although whether it was designed just for that, or was purloined from the rather large American triode-pentode inventory as being the best-suited I do not know. But that there was a 12JN8 variant (150 mA heater) does suggest that it was conceived with radio applications in mind. And I am not aware that there were 450 mA or 600 mA variants, of which at least the latter was normal for American TV valves, in addition to the baseline 6.3-volt version.
Some setmakers appeared to prefer pentode FM RF amplifiers, GE amongst them. The GE T-2000 receiver mentioned upthread had a 12JN8 as FM RF amplifier and self-oscillating mixer, with a diode for AFC. The pentode section of another 12JN8 was used as FM limiter. The triode section was used in the stereo decoder, along with a whole third 12JN8. The “12” prefix valves in this case did not indicate an AC-DC chassis, though. It had an isolated chassis with a mix of (mainly) 12.6-volt and 6.3-volt valves, supplied from a centre-tapped heater winding.
The (presumed to be slightly earlier) GE T-1000 AM-FM Stereo had used a 6EW6 pentode as FM RF amplifier, and a 6EZ8 triple-triode as mixer, oscillator and AFC reactance valve. So it looks as if GE acceptance of the self-oscillating FM mixer came quite late, and then only when it could retain a pentode RF amplifier.
The Zenith MJ1035W AM-FM Stereo receiver mentioned upthread, assumed to be of similar vintage to the GE T-1000, had a 6JK8 single-valve FM front end with diode AFC.
So not only were American setmakers late in adopting single-valve FM front ends with their implied self-oscillating mixers, but to some extent they may also have been wary of the latter. On the other hand, the European setmakers quickly embraced the concept. Apart from the early (c.1954) development of the ECC85 valve, in its early literature on the ECH81 (from c.1952) Philips indicated that its triode section was intended to be used as a self-oscillating FM mixer, as well as an AM oscillator (its evident primary function) and an extra AF voltage amplifier on FM, when the triode was not otherwise occupied).
That was quite the opposite of the AM situation, where the self-oscillating heptode mixer was nearly universal for American domestic receiving equipment, even where shortwaves were involved, and even for hi-fi tuners that had “quality” AM sections. (The apparent salient exceptions seem to have been on those AM-FM receivers that had combined or partially combined front ends.)
On the other hand, the European makers largely eschewed self-oscillating AM mixers, much preferring the triode-hexode/heptode type. That preference caused Osram, who had adopted the B7G base for its initial miniature domestic receiving valve range, to offer the X78/X108 triode-heptode on a B7G base, with the cathode tied to one heater leg. (This was in addition to its X77/X107 heptode. The X77 was the American 6BE6, later renamed by Osram as the X727 as part of its “727” series. The X107 was a 100 mA heater variant that had no American prototype.) As soon as the B9A base was available and seen as being a new standard, Osram released the X79/X109 noval counterparts. Brimar, unsurprisingly, had released the 6BE6/12BE6, but in deference to the European preference, soon issued the noval-based 12AH8 triode-heptode, which had a dual heater system suitable for both 6.3-volt and 150 mA operation, and which had no American prototype.
The only American miniature triode-heptode issue that I can trace was the 12FX8(A) of the early 1960s. This was in the 12-volt HT car radio series. Its heptode section was intended for use as a self-oscillating AM mixer, in the usual American way. Its triode section was intended for use as an RF amplifier. So it was not a conventional triode-heptode.
Cheers,
Steve
It’s surprising what unrelated information comes out when one embarks upon an initially focussed search that quickly turns into a “rabbit-chase”. Here is a bit more from that meandering….
Still in the field of American AM-FM radio receiver practice, I gleaned the impression that not all that widely used was a heptode, such as a 6BE6, as both AM mixer-oscillator and FM 1st amplifier. That would be the American counterpart to the European (extensive) use of the ECH81 in the same dual role, adjusted for the American preference of a self-oscillating heptode rather than a triode-heptode as AM mixer-oscillator.
Also not widely used in the early days was the single-valve FM front end, for which the 12AT7 seems to have been the valve of choice.
The designed-for-purpose – with enhanced internal screening - ECC85 was a European development. I think it came from Telefunken, although I am not sure. It does not seem to have been a Philips original, though.
The single-valve front end was adopted more widely later in American practice. Apparently, that adoption stemmed from the influence of the mid-1950s German table receiver imports from Telefunken, Grundig, etc., which mostly used the ECC85. American makers used the ECC85 under its American registration of 6AQ8, and the home-grown 6DT8 was released late in 1957. Some of the established American TV tuner makers, such as General Instrument, also then offered single-valve front end tuners to the setmakers.
But whereas European practice did not advance beyond the ECC85, there were further developments in the USA. The 6EZ8 was a triple triode (on a noval base, with two cathodes tied to one heater leg) that could be used as combined RF amplifier, mixer and oscillator, presumably for those makers that preferred to avoid self-oscillating mixers. It could also be used as a combined mixer, oscillator and AFC reactance valve (with a separate RF amplifier) in two-valve FM front ends. A later triple triode, the 6GY8, was intended to combine the RF amplifier, self-oscillating mixer and AFC reactance functions. This had one cathode tied to a heater leg. AFC was quite important in American FM practice, and the single-valve front end had arrived on the cusp of the adoption of the diode as AFC reactance element, hence the interest in triple triode combinations.
The 6J9 triple triode, on a decar base, avoided the need to tie any electrode to a heater leg. It was intended to undertake the RF amplifier, mixer and oscillator functions. By the time it was released, diode-based AFC was in available.
On another vector was the 6JK8/17JK8 double triode. This had a frame-grid RF section, for better performance, and a conventional triode for the self-oscillating mixer. Again, by the time this was released, diode-based AFC was established, hence there was no need for a third triode.
And different again was the 6C9/17C9 double tetrode, on a decar base. One tetrode served as RF amplifier, and the other as self-oscillating mixer. It was used by RCA, for example. It probably stemmed from the use of very low-screen current tetrodes as RF amplifiers in VHF TV tuners. Along with earlier specialized VHF triodes (e.g. guided-grid), these were evidently the outcome of a search for a more economical VHF TV RF amplifier than the established cascode double triode. Tetrodes avoided the need for neutralization (which increased component count and was another production adjustment) as compared with triodes.
Also used in FM front ends was the triode-pentode, with pentode as RF amplifier and triode as self-oscillating mixer. The 6JN8/12JN8 was used for this purpose, although whether it was designed just for that, or was purloined from the rather large American triode-pentode inventory as being the best-suited I do not know. But that there was a 12JN8 variant (150 mA heater) does suggest that it was conceived with radio applications in mind. And I am not aware that there were 450 mA or 600 mA variants, of which at least the latter was normal for American TV valves, in addition to the baseline 6.3-volt version.
Some setmakers appeared to prefer pentode FM RF amplifiers, GE amongst them. The GE T-2000 receiver mentioned upthread had a 12JN8 as FM RF amplifier and self-oscillating mixer, with a diode for AFC. The pentode section of another 12JN8 was used as FM limiter. The triode section was used in the stereo decoder, along with a whole third 12JN8. The “12” prefix valves in this case did not indicate an AC-DC chassis, though. It had an isolated chassis with a mix of (mainly) 12.6-volt and 6.3-volt valves, supplied from a centre-tapped heater winding.
The (presumed to be slightly earlier) GE T-1000 AM-FM Stereo had used a 6EW6 pentode as FM RF amplifier, and a 6EZ8 triple-triode as mixer, oscillator and AFC reactance valve. So it looks as if GE acceptance of the self-oscillating FM mixer came quite late, and then only when it could retain a pentode RF amplifier.
The Zenith MJ1035W AM-FM Stereo receiver mentioned upthread, assumed to be of similar vintage to the GE T-1000, had a 6JK8 single-valve FM front end with diode AFC.
So not only were American setmakers late in adopting single-valve FM front ends with their implied self-oscillating mixers, but to some extent they may also have been wary of the latter. On the other hand, the European setmakers quickly embraced the concept. Apart from the early (c.1954) development of the ECC85 valve, in its early literature on the ECH81 (from c.1952) Philips indicated that its triode section was intended to be used as a self-oscillating FM mixer, as well as an AM oscillator (its evident primary function) and an extra AF voltage amplifier on FM, when the triode was not otherwise occupied).
That was quite the opposite of the AM situation, where the self-oscillating heptode mixer was nearly universal for American domestic receiving equipment, even where shortwaves were involved, and even for hi-fi tuners that had “quality” AM sections. (The apparent salient exceptions seem to have been on those AM-FM receivers that had combined or partially combined front ends.)
On the other hand, the European makers largely eschewed self-oscillating AM mixers, much preferring the triode-hexode/heptode type. That preference caused Osram, who had adopted the B7G base for its initial miniature domestic receiving valve range, to offer the X78/X108 triode-heptode on a B7G base, with the cathode tied to one heater leg. (This was in addition to its X77/X107 heptode. The X77 was the American 6BE6, later renamed by Osram as the X727 as part of its “727” series. The X107 was a 100 mA heater variant that had no American prototype.) As soon as the B9A base was available and seen as being a new standard, Osram released the X79/X109 noval counterparts. Brimar, unsurprisingly, had released the 6BE6/12BE6, but in deference to the European preference, soon issued the noval-based 12AH8 triode-heptode, which had a dual heater system suitable for both 6.3-volt and 150 mA operation, and which had no American prototype.
The only American miniature triode-heptode issue that I can trace was the 12FX8(A) of the early 1960s. This was in the 12-volt HT car radio series. Its heptode section was intended for use as a self-oscillating AM mixer, in the usual American way. Its triode section was intended for use as an RF amplifier. So it was not a conventional triode-heptode.
Cheers,
Steve
