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52 Dating Old Radios By Their Tube Complement: History and Other Designs




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This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.

52 Dating Old Radios By Their Tube Complement: History and Other Designs

This ends the "most common" AC-DC section. Now we will consider
history, and some of the other designs.

Example 19-1 in RC-19 shows a later battery-operated portable, using
7-pin miniature tubes. This design was built after about 1934,
originally using 5-6 pin tubes in ST-12 bulbs; later, octal or loctal
tubes. This circuit also is the basis for most later battery-operated
"farm" sets, some of which were built as floor consoles. Close study of
the circuit will show its resemblance to the 19-4 example. A very
significant difference is the use of filament tubes, and the method of
using a back-bias resistor (R10 in the example) to develop grid bias
voltage for the output tube. Note also that a different local
oscillator circuit is used for the 1R5. This circuit was often used in
the "All American Five" design as well, and is not unique to the battery
design. Resistance values in example 19-1 have been chosen for
operating with a 67.5 volt B battery; otherwise, the circuit is suitable
for operating with a 90 volt B battery.

Example 19-2 shows a typical three-way portable. The term "three-way"
may seem confusing, when the radio can be operated either from the power
line or from batteries. However, the fact that it could operated from
110 volts DC as well as from AC lines was considered noteworthy when DC
domestic service was common; thus "AC or DC or internal battery" are the
"three ways." Note that a modern ricebox radio operating on an internal
battery or with an AC adapter is not "three way" as it will not operate
from a DC line.

Once again, this is the Hazeltine-RCA standard circuit used in examples
19-1 through 19-5, with specific provisions for the three way feature.
Example 19-2 also shows use of a double-tuned RF preamplifier.
Notable are the use of series connection of the receiver filaments,
provision of a rectifier, and a changeover switch. In practise, many
manufacturers provided a dummy line-cord outlet inside the receiver.
Plugging the line cord into this outlet would mechanically actuate the
changeover switch, placing the receiver on battery operation. When
studying this circuit, note in particular the order in which the tube
filaments are wired, and the use of an 1800-ohm resistor (R14) in the
3V4 filament circuit to provide a shunt-feed balance current. The order
of connection of series-wired heaters and filaments is significant in
series-string sets. In this case, the 3V4 is connected to the high end
to provide grid bias for operating, and the shunt resistor provides some
of the plate and screen currents for the tube. The rectifier circuit
shown is typical, although three way portables may use a 35Z5 or a
selenium rectifier. DC output from the rectifier is around 120 volts,
depending on the rectifier used, which requires a large dropping
resistor to feed the receiver filaments. Note the use of two large
electrolytic filter capacitors, C28 and C29, connected to either end of
the 3V4 filament. Small filament tubes require "clean" DC power, thus
these two capacitors filter out both residual ripple from the half-wave
rectifier and audio-frequency variations caused by varying power draw of
the power tube. This circuit arrangement is critical. If any filament
opens, one or both of those capacitors will charge up to the rectifier
output voltage. Also, the design assumes that the rectifier is part of
the voltage-dropping string, and 1.5V filament tubes are limited in
their ability to handle out-of-tolerance filament voltage.

The circuit shown in figure 19-3 for an AC-operated receiver is the same
as that in figure 19-4, with several upscale features, and resistance
values selected for operation at 250 volts B+ rather than 120. Note
that the circuits for the 6BE6 converter, 6BA6 IF, and 6AV6
detector-audio stages have the same configuration as those shown for
those three stages in figure 19-4. An additional 6BA6 RF preamplifier is
provided for higher gain and better selectivity. A pair of 6AQ5 tubes
provides push-pull output. The second 6AV6 placed ahead of the lower
6AQ5 grid circuit inverts the audio signal for grid drive, with
"approximately unity gain," determined by the tapped grid leak
(470K/8200 ohms) in the top 6AQ5 circuit. This particular circuit is a
classic example of older home entertainment engineering, and there is
much to criticize in its selection over the use of a twin-triode
balanced paraphase using a 12AX7 or a 6SN7. Why was it chosen? Habit,
probably---it was a good choice for 1932.

The main feature of this set which differs from AC-DC configuration is,
of course, the use of a power transformer and a 5Y3 full-wave rectifier.
The configuration of the rectifier circuit was one of the earliest and
most durable circuits in the history of tube-type home entertainment
radio. This later configuration uses a 5Y3 instead of an 80, has larger
filter capacitors (20 mfd rather than 8 or 10 mfd), and a resistor in
place of an inductance between the two filter sections. Older radios
most often used a speaker field coil between the two filter sections,
partly because Alnico magnets were not available until the late
thirties, and partly because inductance at this point compensates for
using smaller capacitance values to get good filtering.

Note the configuration of the screen circuit for the 6BE6 and two
6BA6's. All three screens are connected together. This is poor design,
and likely to cause parasitic oscillations. The circuit in figure 19-4
also shows the screens connected together, but in this instance, there
are only two screen, in stages that operate in opposite phase, so any
coupling between the two stages has a negative feedback effect.

 

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