In article ,
wrote:
John Byrns wrote:
3. TRF receivers have been mentioned, and everyone seems to assume
that a TRF receiver would consist of cascaded single tuned
resonators with RF amplifier stages between. There is no reason why
double tuned circuits, similar to those used in the IF transformers
of a superhetrodyne can't be used in a TRF receiver, with all the
selectivity/bandwidth benefits that brings to the party. For
examples see the Western Electric No. 10A receiver, the J.W. Miller
TRF receiver, the early Altec AM receiver, as well as others.
I did a cursory check on the Internet, but did not yet find any
schematics for the mentioned receivers. Are they online somewhere?
Anyone?
I also found the following article from John posted back in 2000,
where he talks about the double tuned TRFs, such as WE-10A, J.W.
Miller, Collins (which I assume is the same one Volker Tonn
mentioned today), Meissner, and the Weeden (the last of which John
noted to be the best designed of all of them):
http://groups.google.com/groups?selm...&output=gplain
Unfortunately the URLs to the TRF schematics at John's site are not
working.
Try this URL:
http://users.rcn.com/jbyrns/BP-TRFs.html
Another bandpass receiver which uses J.W. Miller coils is the Altec
101(B), the schematic for the 101(B) is available on the "Nostalgia Air"
web site.
After reviewing this thread, and considering that the proposed tuner is to
be an audiophile tuner, and given the great affinity of audiophiles for
Western Electric audio equipment it is clear to me that the tuner of
choice would be an updated version of the Western Electric No. 10A
receiver. The tubes used by Western Electric receiver are obsolete and
should be updated to more modern tubes in the revised design. The
detector in the Western Electric design is also inadequate having high
distortion, and due to the square law characteristic it modulates the AGC
voltage at the syllabic rate of the audio signal. Modifications to update
the tubes, resolve the detector and AGC problems were available by the
early 1950's, or earlier.
There is one remaining problem with the W.E. 10A that hasn't yet been
addressed to the best of my knowledge, and that is the aperiodic antenna
input circuit, that is likely to be a source of RF 3rd order IM problems
which will wipe out some weaker signals if you live near a number of
50,000 Watt flame throwers, or even 5,000 Watt stations. This problem
could be eliminated by deleting the first RF amplifier stage, which is
probably serious overkill with modern tubes anyhow, and making the first
bandpass filter a tuned antenna circuit. This is the reason that I prefer
the Weeden and Altec designs, they both have tuned antenna circuits to
minimize RF IM problems.
If one wanted to duplicate an existing circuit the Altec is probably the
best choice as it has a tuned antenna circuit and uses modern 7 pin
miniature tubes. One addition that would be necessary for any of these
tuners today would be the addition of a "NRSC" deemphassis network to
compensate for the preemphassis that is used in AM broadcasting today.
Regards,
John Byrns
Surf my web pages at,
http://users.rcn.com/jbyrns/