Description
This article is from the Antique Radios
And Phonographs FAQ, by Hank van Cleef vancleef@netcom with
numerous contributions by others.
50 Dating Old Radios By Their Tube Complement p2
The tube complements are:
a. First version, built primarily 1938-40.
(note: this design is similar to the 19-4 example, but is its immediate
prececessor, so has a few substantial differences, noted below).
12A8 RF-converter, 12K7 IF amplifier, 12Q7 detector-audio, 35L6 power
output, and 35Z5 rectifier. The first three tubes had small top caps
for the signal grid connections, with either metal or glass envelopes.
The original glass tubes had a "G" suffix, indicating use of an ST-12
stepped bulb envelope. The major difference between this design and
that shown in example 19-4 is the use of a 12A8, which uses a slightly
different oscillator circuit than the 12SA7, 14Q7, or 12BE6. The other
top-cap tubes are very similar to the single-ended octal tubes which
followed, varying primarily in mechanical construction. 12J8 and 12K8
were sometimes used as converters as well. RC-19 unfortunately omits
any circuits for these converter tubes. This version uses a series
resistor in the heater circuit because the heater voltages do not add up
to "near 120"). The proper place for this resistor, electrically,
is between the rectifier heater and the power amplifier heater.
b. Second version, built 1939-ca. 1960
12SA7 RF-converter, 12SK7 IF amplifier, 12SQ7 detector-audio, 50L6 power
output, 35Z5 rectifier. This is almost the same radio, but using
single-ended tubes in the first three stages and a power output tube
with a 50-volt heater. The major difference is in use of a 12SA7 in
place of the 12A8---these tubes are different internally. Note that the
sum of the nominal heater voltages adds up to 122.8 volts, allowing
operation without need for any series resistor in the heater circuit.
c. Postwar version, 1945-mid '60's
12BE6 RF-converter, 12BA6 IF amplifier, 12AT6 detector-audio, 50B5 power
output, 35W4 rectifier. The only difference from b., above,is the use of
seven-pin miniature tubes. All are electrically identical to the octal
versions above. Some sets were built using a mix of seven-pin miniature
and octal tubes, however, the presence of seven-pin miniature tubes
indicates that the set is postwar production.
d. Loctal tube version, 1940-ca. 1960
14Q7 RF-converter, 14A7 IF, 14X7 detector-audio, 50C5 power output, 35Y4
rectifier. Once again, the same radio as version b., using loctal-base
tubes in place of octal. Philco and GE were fond of using loctal tubes.
Note that some radios used a 14B8 converter, which is the same
configuration in a circuit as the 12A8.
The six-tube configuration used the same tube type for both RF
preamplifier and IF amplifier, and the 35 volt heater version of the
output tube. In most cases the RF preamplifier is resistance-coupled to
the RF-converter stage, and the radio used a two-stage tuning capacitor.
Some later versions used movable slug tuning in place of a variable
capacitor. This variation began around 1947, and became more common
during the next decade.
2. Five or six tube AC-DC transformerless radios using 300 ma heaters
wired in series.
These radios were the precursors of the 150 ma. series heater
radios. Some of these radios also included a tuning eye indicator,
typically a 6E5. Total voltage drop of the series heater string was
68-74-82 volts requiring an external voltage dropping resistor of
some sort. These radios often used "ballast" tubes or resistance wire
in the line cord for this purpose.
a. Version using large-base 5, 6, or 7-pin tubes, 1935-50.
6A7 RF-converter, 78 or 6D6 IF, 75 detector-audio, 43 power
output, 25Z5 rectifier. Most of these sets were built before 1938,
although a few manufacturers built them in the early postwar era.
There are more variations on this design than on the 150 ma. heater
designs described above. As noted, some sets had 6E5 tuning eye tubes.
Sets with shortwave often had a 76 triode as a separate local oscillator
for the 6A7.
b. Version using top-cap octal tubes, 1936-1950's.
6A8 RF-converter, 6K7 IF, 6Q7 detector-audio, 25A6 or 25L6
audio, 25Z6 rectifier. This reflects the switch to octal tubes in 1936.
The first three tubes had small top caps for signal grid connection.
The 25A6 is an octal version of the 43; the 25L6 is a 25 volt heater
beam power tube identical, except for heater, to the 35L6 and 50L6. The
25Z5 is a full-wave rectifier (two diode sections), and was usually
connected with the two sections in parallel. However, some
manufacturers, notably Philco, used the two sections to provide voltage
doubling for B+. Radios with voltage doubler power supplies are
AC-only, as a voltage doubler requires alternating current to "pump" the
doubler circuit.
c. Version using single-ended octal tubes, 1939-50's.
6SA7 RF-converter, 6SK7 IF, 6SQ7 detector-audio, 25L6 output, 25Z6
rectifier. Once again, this is a "switch," this time to single-ended
octal tubes. Major circuit difference is in the 6SA7 circuit because of
differences internally between the 6SA7 and 6A8.
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