"aa4", "aa6"

[Synchrodyne]

From the thread "Help with 4 valve mantle superhet", https://www.vintage-radio.net/forum/...84#post1288484, and so as not to hijack it:

Originally Posted by usradcoll1

That is what I'm used to seeing! No if amplifier, the F/C transformer coupled to the detector and A/F amplifier stage.
RCA, Motorola and Zenith never seemed to build a short superhet.
Every other US manufacturer did.
Dave, USradcoll1

I have the impression that Zenith was interested in good RF performance for its domestic radio receivers, and so tended to look upwards rather than downwards in that department.

For its valved FM-AM radio chassis from the late 1940s and into the early 1960s, it seemed to offer versions both with (3-gang) and without (2-gang) an AM RF amplifier. The FM side always seemed to have RF amplifier (often a 6BJ6), mixer-oscillator (often a 12AT7), 3 IF stages and a Foster-Seeley discriminator (often a 19T8). I imagine that the version with an AM RF amplifier was useful in rural areas, for example in the skywave outer service areas of clear channel transmitters. But my experience generally is that the RF stage and the 3rd gang was noticeably beneficial even in strong signal areas, and very much so if you wanted to receive a distant weak signal in the presence of nearby strong signals. The better overall AGC curve conferred by the RF amplifier also seemed to delay the onset of modulation rise distortion as compared with the no RF-amplifier case.

Those Zenith FM-AM chassis also had two AM IF stages, which probably helped with selectivity. I suspect that two were used mostly because they were there for the taking because of the FM complement, as Zenith used only one in its AM-only receivers, and the two-stages seem to have been operated at lowish gains. Where the AM RF stage was not included, with the AM aerial going directly to the 12AT7 frequency changer, I wonder if oscillator reradiation was on the high side? RCA’s use of the 6X8/19X8, pentode mode for AM and triode-strapped for FM, strikes me as having been a better idea. (Although it had also used the 6J6 in this role.)

As far as I know, as well as the AA5 type, Zenith also had some “AA6” AM-only chassis, with RF amplifiers and 3 gangs. That is, it looked “up” from the AA5 to the “AA6”, and not down to the “AA4”.


That GE “AA4” mentioned previously was notable in that it used a 12AU6 pentode as a self-oscillating mixer rather than the customary 12BE6 pentagrid. I wonder if that was to obtain lower noise and/or higher conversion gain. But GE seemed to like pentodes for the AM frequency changing job. In its FM-AM receivers where it used the 12AT7 as an FM mixer-oscillator, it used only the oscillator section for AM, with the 1st pentode in the IF chain serving as AM mixer (as well as FM 1st IF amplifier). Later, when it moved on to different forms of FM-only front ends, that 1st IF pentode was a self-oscillating mixer on AM.

I’d guess though that the markets for “AA4” and “AA6” types were much smaller than for the AA5.


Cheers,

[FStephenMasek]

This really follows the TRFs with different numbers of stages. Another collector I know has a three valve TRF radio, with a one gang tuning capacitor!

[Herald1360]

Det, 2LF was a fairly common arrangement in the early days.....

[usradcoll1]

Originally Posted by Synchrodyne

From the thread "Help with 4 valve mantle superhet", https://www.vintage-radio.net/forum/...84#post1288484, and so as not to hijack it:

I have the impression that Zenith was interested in good RF performance for its domestic radio receivers, and so tended to look upwards rather than downwards in that department.

For its valved FM-AM radio chassis from the late 1940s and into the early 1960s, it seemed to offer versions both with (3-gang) and without (2-gang) an AM RF amplifier. The FM side always seemed to have RF amplifier (often a 6BJ6), mixer-oscillator (often a 12AT7), 3 IF stages and a Foster-Seeley discriminator (often a 19T8). I imagine that the version with an AM RF amplifier was useful in rural areas, for example in the skywave outer service areas of clear channel transmitters. But my experience generally is that the RF stage and the 3rd gang was noticeably beneficial even in strong signal areas, and very much so if you wanted to receive a distant weak signal in the presence of nearby strong signals. The better overall AGC curve conferred by the RF amplifier also seemed to delay the onset of modulation rise distortion as compared with the no RF-amplifier case.

Those Zenith FM-AM chassis also had two AM IF stages, which probably helped with selectivity. I suspect that two were used mostly because they were there for the taking because of the FM complement, as Zenith used only one in its AM-only receivers, and the two-stages seem to have been operated at lowish gains. Where the AM RF stage was not included, with the AM aerial going directly to the 12AT7 frequency changer, I wonder if oscillator reradiation was on the high side? RCA’s use of the 6X8/19X8, pentode mode for AM and triode-strapped for FM, strikes me as having been a better idea. (Although it had also used the 6J6 in this role.)

As far as I know, as well as the AA5 type, Zenith also had some “AA6” AM-only chassis, with RF amplifiers and 3 gangs. That is, it looked “up” from the AA5 to the “AA6”, and not down to the “AA4”.


That GE “AA4” mentioned previously was notable in that it used a 12AU6 pentode as a self-oscillating mixer rather than the customary 12BE6 pentagrid. I wonder if that was to obtain lower noise and/or higher conversion gain. But GE seemed to like pentodes for the AM frequency changing job. In its FM-AM receivers where it used the 12AT7 as an FM mixer-oscillator, it used only the oscillator section for AM, with the 1st pentode in the IF chain serving as AM mixer (as well as FM 1st IF amplifier). Later, when it moved on to different forms of FM-only front ends, that 1st IF pentode was a self-oscillating mixer on AM.

I’d guess though that the markets for “AA4” and “AA6” types were much smaller than for the AA5.


Cheers,

As being from "down under" you're well versed in US radio design!
Zenith had to build a radio to maintain the reputation as the "Long Distance Radio". I even have a early 60's AM only transistor that has a 3 gang tuner.
Dave, USradcoll1

[Synchrodyne]

Well, in the “underworld” we had both American and British/European influences when it came to radio receivers and the valves used therein. It makes the subject much more interesting! (But I did live in the USA for around 14 years, so that helped to put US practices into context.)

Thus, in terms of 5-valve receivers, we’d find various valve lines-up, such as 6BE6/6BA6/6AV6/6AQ5/6X4, ECH42/EF41/EBC41/EL41/EZ40, and ECH81/EF89/EBC81/EL84/EZ80. Actually, the second would apply to New Zealand only, as Rimlock valves were used quite a bit during the 1950s. In Australia, Rimlocks were not much used, and, for example, Philips went direct to its “Innoval” series, for which an early line-up would have been 6AN7/6BH5/6BD7/6M5/6V4.

Re Zenith, an AA4 would thus have been a direct contradiction of its “Long-Distance Radio” platform.

Were any valves developed specially for the AA4 receivers, or was it a case of simply selecting from the AA5-oriented range? I suspect that the economics associated with the AA4 type indicated only that mass-produced standard components be used.


Cheers,