On Wed, 16 Nov 2011, Kenneth Scharf wrote:


Still, my question remains: can one build a decent superhet radio out of
nothing but 12ax7 dual triodes?

At least for the BCB yes, though the AVC control ability of the 12AX7 is
questionable. For the IF stage use both haves of the tube connected in
series with the first section as a common cathode amp and the second section
as a cathode driven amp (common grid, NOT grounded grid as the second section
grid is grounded for RF via a capacitor and is connected via a resistor to
the plate of the first section). For the converter stage the two sections
are again in series, the lower section is a Hartley oscillator (cathode to
tapped coil) and the upper section is the mixer, grid connected via a
resistor to the plate of the lower section and cap coupled to the antenna
coil. Use one section of a 12aX7 diode connected as the detector (or as a
triode infinite impedance type detector). One or two triodes as a phase
inverter driving (get this) 4 or 6 12AX7's in a zero bias class B push pull
parallel amp (should be good for a couple of watts).

Don't you mean a cascode for the IF amp?

You could get fancy. A dual-triode as a balanced mixer. A similar
arrangement for the IF stages, in the IC world the differential amplifier
is pretty common, and one reason seems to be the ability to control gain.
Have a balanced mixer as the detector, and feed the "bfo" input with the
incoming signal via a limiter.

The reality is, people have pretty much followed what came before. Once
in a while someone comes along with something "radically" different and
then there's another period of following along.

So the solid stated direct conversion receiver came along (the concept
existed before solid state, but I don't think "direct conversion" was used
before that 1968 article by Hayward. So instead of building regens, direct
conversion receivers were commonplace, despite the limitations. It took
some time before someone realized a better mixer would help, endless
articles about better mixers. It took almost 20 years from that Hayward
article before someone really started building good direct conversion
receivers, Roy Llewellyn using a diode DBM but actually
terminating it properly (somehing done in VHF converters and I think by
then in ham receivers for the first mixer, but not migrated to direct
conversion receivers until about 1985 or so). Then Gary Breed extended
the concept, adding phasing (not a new concept, not even to direct
conversion receivers, but still a leap forward) followed by Rick
Campbell's various works on the subject.

Somewhere in there, superhets came back, because suddenly everyon was
writing about crystal ladder filters. They'd been around before the full
wave hit, but something happened to cause a turn. And with cheap filters,
the superhet became inexpensive again, and actually simpler than some of
those fancier direct conversion receivers (though often their
complications came in the audio section).

It's hard to stretch the limits, but somehow and sometime, it happens, and
then there's some new development, followed by it becoming the status quo,
until someone stretches things again.

Michael VE2BVW