Description
This article is from the Antique Radios
And Phonographs FAQ, by Hank van Cleef vancleef@netcom with
numerous contributions by others.
54 Dating Old Radios By Their Tube Complement: p2 Older radios
Problems with secondary emission from the 24 were "cured," more or less,
by processing the plate material to reduce this emission. This produced
the 24A. However, a more permanent fix was to include a third grid to
"suppress" the reverse current resulting when plate voltage was lower
than screen voltage. The 57 and 58 pentodes were the result. Both have
2.5 volt indirectly-heated cathodes. However, the 58 has a
characteristic known as "variable-mu." Actually, with pentodes, one
considers transconductance, and what "variable-mu" actually does is to
reduce the transconductance as the tube is more heavily biased. The
feature is desirable in circuits with AVC. These pentodes showed up
around 1931. The pentode power amplifier was also introduced around the
same time, with the 47 replacing the 45 in many designed of the 1932-34
era.
The last significant development in tube design for AM broadcast radios
was the development of a single tube with two control grids to serve as
a self-exciting local oscillator and mixer amplifier. The 2A7, quickly
replaced by the 6-volt-heater equivalent 6A7, was the predominant
design, and the 6A7 was used very commonly until after 1940. The 6L7
also was introduced fairly early. This is a mixer that is not designed
to operate as a self-oscillator, and was used, particularly in
communications sets, with a separate local oscillator, until the
1950's.
Availability of a single tube for the superheterodyne oscillator-mixer
function was essentially the death-knell for TRF designs. Another
contemporary development which entered production in 1933 was the 2E5
"tuning eye" tube, which varied a shadow area on a visible target as an
inverse function of the control grid voltage. TRF sets were built into
the 1950's, but are not very common. They tend to be either very cheap
radios for use in metropolitan areas with strong signals or in high end
sets where the broad bandpass allowed "high fidelity" (though the
AM stations actually only transmit a signal that has 5KC as the 3db
half-power point in the modulation).
Availability of components for a vibrator power supply made automobile
sets operating from 6 volts DC practical. There was a wholesale switch
from 2.5 volt heaters to 6.3 volt heaters in 1934. The 2.5 volt heater
series of tubes quickly became obsolete. The switch to 6.3 volt 300
ma. filaments was parallelled by development of a two-diode rectifier
and an output tube with 25-volt 300 ma. heaters, making series string
wiring of the heater circuit practical. These are the 300 ma. heater
transformerless sets described above, which date from about 1934.
Octal-based tubes enter the picture in 1936. Many of the original
designs were built in self-shielding steel envelopes. Metal octal tubes
were built with a flat "button" glass seal, which allowed much shorter
electrode lead connections. Early glass octal tubes continued to use
the older "press" design, with relatively long leads. RF and AF tubes
in the original octal series had small top caps for connection to their
control grids. It was not until about 1939 that single-ended tubes
entered production.
Continue to:
Share and Enjoy
Bookmark this story so others can enjoy it:
Tags
hobby, recreation, old radio, old phono, antique, radios, phonographs, tools, test equipment, resource, repair, identification, books, components
