In article , Patrick Turner
wrote:

Steven Swift wrote:

If you are willing to live with about 5-10% THD, then you can use more
common circuits.

True, but onje doesn't have to live with 5-10%.
linearize the IF amp and detector, and thd plummets.

However, there are dozens of "Hi Fi" AM circuits published

by the hobby magazines, tube vendors and kit makers. Have a look at them.

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.

RDH4 has the circuit for the Selsted and Smith "infinite impedance "
detector, where a 12AU7 performs as credible detector, and as a diode,
but I think I'll stick with a germanium diode fet by a 12AU7 CF.

The "Selsted and Smith" detector is not the same thing as the so called
"infinite impedance" detector. The "infinite impedance" or "reflex"
detector was designed by RCA, while the "Selsted and Smith" detector was
designed by, well "Selsted and Smith", or at least "Selsted" who is still
around, or was a year or two ago.

The "Selsted and Smith" detector differs from the "infinite impedance"
detector in that it has a diode in series with the grid, and also a diode
load resistor. There is no peak detection capacitor across the diode
load, so the diode does not act as an ordinary diode peak detector, and
the triode doesn't act as a cathode follower. The triode is the actual
detector operating in a fashion similar to the "infinite impedance"
detector, with the diode apparently serving to linearize the "infinite
impedance" detector. The input impedance of the "Selsted and Smith"
detector is not infinite due to the presence of the diode load resistor.
For that matter the input impedance of the so called "infinite impedance"
detector is also not infinite, and can even have a negative resistance
component which can cause stability problems. The negative resistance
effect can occur when circuit conditions are right, similar to the
conditions that can cause oscillation in cathode and emitter follower
circuits if you aren't careful.

But does the RC-19 have enough tuned circuits to give over 70 dB rejection of
signals which are 50 kHz away from the wanted station at any place on
the band?

The 6BA6 is a variable U tube, with a non linear
transfer curve.

There is nothing wrong with the 6BA6, it was specifically designed for
this service and has very low odd order distortion which is all that
matters since the even order distortion products can't get through the
IFT. I hope I got that the right way around, if not it is explained in
some detail in some of the old texts, I think "Radio Receiver Design" by
Sturley is one that explains it. You only get in trouble if you try to
run the tube at a very high signal level, simultaneously with a high AGC
voltage applied for a large gain reduction. This is mainly a problem in
the stage driving the diode detector, so it is best to avoid AGC on that
stage, but in a minimal radio that is of course problematic. This is one
of the many topics that the RDH4 gives short shrift. The 6BA6 is even
usable as a gain control element in audio circuits where even order
distortion does matter. IIRC the peak limiter at a radio station where I
once worked used four 6BA6s in the audio path, where they were connected
in push pull, presumably to cancel the even order nonlinearities which are
inherent in the design of the tube.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/