Description
This article is from the Antique Radios
And Phonographs FAQ, by Hank van Cleef vancleef@netcom with
numerous contributions by others.
51 Dating Old Radios By Their Tube Complement p3
This version was generally not built as a "price leader" inexpensive
table radio because of the availabity of 150 ma. tubes that didn't
require a dropping resistor in the heater circuit. It was very often
used as the basis for an upscale AC-DC radio. Some configurations that
you may run across:
1. Shortwave receiver using an additional RF preamplifier,
separate local oscillator, and second IF stage. The 6SK7 was used for
the RF and IF stages, and a 6J5 as a local oscillator.
2. Push-pull audio output, using two 25L6 tubes and a 6J5 as a
phase inverter. This may be combined with the RF-IF additions, above,
and a tuning eye tube (6E5 usually).
Note that use of rectified line voltage gives a relatively low B+, a
major limitation in the transformerless design. The primary market for
a "full house" receiver that had all of these features would have been
the DC service metropolitan areas, particularly New York City, and that
is the general area where most "odd-ball" configurations of
transformerless sets can be found today. In summary, all of the designs
identified in items 1 and 2 above either used the circuit shown in RC-19
example 19-4, or fairly simple variations of the design. There are very
few radios with these tube complements that vary markedly from the
design, which was established around 1932, and licensed to builders
through Hazeltine and RCA patent licenses. In general, the sets that
deviate markedly from the standard circuit are a few Philcos and
Zeniths, and some off-brand sets that may have been marketed through
chain stores with chain store brand names.
3. Postwar AM-FM sets, 1945-up. These were made in two
configurations: separate FM front end, and common front end (i.e, RF,
IF, mixer, and IF amplifiers. There are many variations on both
designs, using 7-pin miniature tubes, loctal tubes, or "hot" octal
tubes. The 6SB7Y was a "hot" 6SA7-type tube capable of self-exciting
oscillation at FM frequencies, and the 6SG7 a "hot" replacement for the
6SK7. The presence of 88-108 MC FM in a radio always means that it is a
postwar set, as this band was not assigned to FM until April, 1945.
Manual RC-19 shows an example of an FM tuner in example 19-9. Many
AM-FM sets "merged" AM capability into the FM tuner design by using a
bandswitch in the RF and converter stages, and by connecting IF
transformer coils for 455KC and 10.7 Mc. in series, the idea being that
the desired frequency will cause one or the other to resonate (high
impedance) and the other will appear as a low DC resistance. The
bandswich would also select which IF fed the AM detector, and which
detector's output was used to feed the audio section. Example 19-9 also
shows both the limiter-discriminator and the ratio detector designs
commonly used in FM-capable sets.
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