49 Dating Old Radios By Their Tube Complement
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
49 Dating Old Radios By Their Tube Complement
The development of vacuum tubes, both electrically and mechanically,
advanced at a rapid pace between about 1925 and 1950. The vast majority
of radios sold for home entertainment between 1920 and the late 1950's
were built to various standard circuits. In most cases, checking out
what tubes are used in the radio will place it's date of manufacture
within a few years, identify which of the standard circuits it used, and
give a some indication of the quality of the set. Most radio repair
technicians in the 1930-60 era did not need to look at schematics most
of the time, even when the problem was not a burned-out vacuum tube
heater or filament.
The tube complement is not always an accurate guide, except insofar as
the presence of a given tube indicates that the set was built after that
tube was placed in production. You won't find any 1932 radios using
tubes with octal bases or 6.3 volt filament heaters, and you won't find
any prewar radios with 7-pin miniature tubes. But you may find a 1946
table radio built to a 1935 design. There are also a few other design
features that are very obvious on casual inspection; I'll mention some
of them as we go along.
(New 12-94) In the following discussion, there are references to the
example circuits shown in the RCA Receiving Tube Manual RC-19, dated
1959. This manual is available in reprint from Antique Electronic
Supply. Examples 19-1 through 19-4 in particular show examples of four
standard circuits that were used, either identically or with minor
modifications, in the majority of the smaller "collectible" radios built
from the mid-1930's on.
1. The five or six-tube AC-DC radio with 150 ma. tube heaters wired in
series. Example circuit 19-4 shows one of these radios, using 7-pin
miniature tubes. This design is colloquially called the "All-American
Five" by some of us. The design was first built in 1939, using octal
tubes (i.e., 35Z5 and 50L6 in place of 35W4 and 50C5), so it is also
called by some a "35Z5 radio" or a "50L6 radio." I list this design
first, not only because it dominated home entertainment radio production
for over 20 years, but because it is a very simple superheterodyne
circuit. If you study this circuit and know what every component's
function is, and study an example radio of this design, you'll be
prepared to trouble-shoot and repair most post-1935 radios.
These sets do not have a power transformer, and could operate
in places like mid-Manhattan, which had 110 volts DC as its primary
electrical service. Most of these were built as table radios, although
some were installed in small consoles and radio-phonograph combinations.
Virtually all clock radios use this circuit. These are generally
AM-broadcast-only. The tube set shown in the example is one of three
common sets, having either octal, loctal, or 7-pin mechanical design,
but electrically equivalent. Some sets, particularly in the early
postwar period, were built with mixtures of tube mechanical types,
because of tube shortages and availability, and some sets used more than
one configuration during their production runs.
The six-tube version had an RF preamplifier, and was more sensitive than
the five-tube. Example circuit 19-3 shows the same
basic design with an RF preamplifier stage, with tuned output
(three-section tuning capacitor). Many of the six-tube versions used
resistance coupling between the RF preamplifier and the converter stage
(see Diagram no. 3, p. 339, in RC-19, for a resistance-coupled pentode
circuit). The six-tube version was often called a "35L6 radio" because
a 35L6, 35A5, or 35C5 was used, allowing connection of one more 12-volt
heater in the series heater string. In the fifties, some of these radios
were built with a selenium rectifier, omitting the rectifier tube.
Also, a few manufacturers built a four-tube version that omitted any IF
amplification.
Several low-end "boatanchor" communications sets used this circuit,
adding multiple tuning coils and provisions for a beat-frequency
oscillator. Notable examples are the Hallicrafters S-38, S-41, S-119,
S-120, and Ecophone EC-1 series; and the National NC-46 and SW-54.
Continue to:
My Books
