History of the AA5
(All American 5ive) AM tube radio
Possibly the longest lived consumer electronic product design was the
five tube “AC/DC” AM radio. Virtually every household had at least a few
over the years. These radios were low cost, and one expensive item designed
out was the power transformer. Thus the series heater string, and using
the powerline directly rectified for B+ power. No power transformer also
made it possible for smaller and lighter sets to be made.
—– (This portion quoted from an article published in the Michigan Antique
Radio Club newsletter by John Reinicke)
In the 20’s the crystal set and then the Tuned Radio Frequency,
or TRF, set would provide adequate performance. The complexity and cost
of the Superhet receiver was simply not required. As a result, the Superhet
design appeared only in the most expensive receivers. See
a brief description of the Superhetrodyne radio. In the 30’s, the situation
rapidly changed. Radio had enjoyed explosive growth and the number of transmitters
on the air exceeded the selectivity of the TRf sets. The 30’s also saw
an extraordinary economic circumstance and the manufacturers of radios
realized the need to produce low cost, high performance, receivers. It
was now evident the only design that would provide adequate performance
was the Superhet. In order to reduce the number of tubes required to support
the Superhet, manufacturers designed multipurpose tubes. In April 1933,
RCA introduced the 2A7. The 2A7 was the first pentagrid converter which
combined the functions of Rf amplifier, mixer, and oscillator in a single
envelope. This tube could then be used with a pentode as an If amplifier,
a combination diode-triode as a detector-first audio amplifier and a audio
power amplifier to make a complete receiver. Add to this a rectifier to
power the set and you have a high performance receiver with but 5 tubes.
To further improve the receiver, a remote cut off pentode could be used
in the If amplifier so the If amplifier could be used as a part of the
automatic volume control circuit. The tube line up for this 5 tube receiver
would then be: 2A7, Rf amplifier, converter; 58, Remote cut off pentode,
If amplifier; 55, Diode-triode, Detector-first audio; 59, Audio output;
and 80 for a rectifier. This arrangement uses 2.5 volt filaments and therefore
required the use of a power transformer. This was the prototype “All American
1934 saw the introduction of the 6A7 and a whole series of 6 volt tubes
to go with it. It was now possible to build an automobile radio or to combine
with a 25Z5 rectifier to build a set without a power transformer. (See
the March 1990 Chronicle article, Ballast). With the elimination of the
power transformer, it was now possible to have a truly low cost, high performance
receiver. There are those who argue the series filament version became
the classic all American Five.
(end quote) —–
The tube heaters were wired in series, sometimes with a “ballast” resistance
added to make the total voltage drop across the entire string add up to
that of the powerline, around 120V. All the tubes needed to have the same
heater requirement for this to work. All tubes had indirectly heated cathodes.
Early versions of the five tube radio used the same small signal tubes
(RF, IF, non-power audio) as transformer sets used. Tubes like 6A7, 6D6,
75, 6F7 and newer ones like 6SK7, 6SQ7. And an extra “tuning eye” 6E5 tube
if desired. All these had 300 ma heaters.
To make an
“AC/DC” radio, you would just need specially designed audio output and
rectifier tubes. Like the 43, 25L6, 25Z5 and 25Z6. That would be only two
new tubes to be developed (per radio chassis design) to make an AC/DC set.
These had higher voltage heaters, but the same current (300mA) as the small
signal tubes above. Power handling tubes like audio outputs and rectifiers
need bigger cathodes and more heater power to operate. If current is the
limiting design factor, increase the voltage to get more heater power.
But all the heaters in a series string in the above didn’t add up to
enough voltage to be fed directly off the powerline. So some sort of additional
voltage dropping resistance was used. Eighter a power resistor, “ballast
tube” or resistive wire in the power cord was used. I don’t know if anyone
used a power resistor housed in a “wall wart” (calculator charger style)
One of the above mentioned rectifier tubes, the 25Z6, is a pair of diodes,
used in a voltage double circuit. This gets you a B+ power supply of around 250 – 300
volts. Might make “translating” a design from a power transformer design
to a “hot chassis” design. Not “AC/DC”, voltage doublers won’t work off
of a DC supply.
Later on, to reduce waste heat in ballast tubes or resistors, the 150
ma tubes were developed. By this time, the 5 tube AC/DC radio was a popular
product, so it was worth while to create new tube designs. Basically, the
6V, 300mA heater signal tubes had their heaters replaced with ones that
needed 12V at 150mA. “Tuning eye” tubes at 150 mA heater for consumer radios
did exist. There’s the 6AB5 / 6N5. Heater of 6.3V @ 150ma. And the 1629,
heater of 12.6V @ 150ma. But they were rarely used. The Airline model 93WG602B
used the 6AB5. But the common 150ma AA5 tubes used the same power as the
300ma AA5 tubes. And the 25L6 became a 50L6 the same way, 2x voltage, 1/2
current. A new design overall was the rectifier tube, the 35Z5, with a
tap on the heater to operate a pilot light. And the total added up to the
powerline voltage, so no wasted heater string current thru a dropping ballast.
Saved 18 watts of power that used to be 18 watts of heat to get rid of.
And conserved some energy, but noone worried about that until the mid seventies.
It looks like this occurred in about 1940. All these were octal socket
tubes. Loktal versions appeared at about the same time, also.
Brief superhetrodyne description
So much for the heaters for now. Early sets were TRF’s (tuned
radio frequency) that just amplified the radio station’s carrier frequency,
detected it down to audio, and amplified it. This design would need to
have 3 or so LC (L stands for inductor, C for capacitor)
circuits that would “track” each other as you tuned across
the band. And with gain stages between, you had to be careful that the
amplified signal at the detector didn’t leak back into the antenna, or
else you’d hear yourself instead of a signal. Later on, the superhetrodyne
radio was invented, and is still the preferred architecture for modern
radio receivers. A basic superhet receives the radio station with an antenna
LC circuit, heterodynes it with a supersonic (thus “superhetrodyne”) locally
generated frequency, and the difference of the station carrier frequency
and the local oscillator would be the intermediate frequency (IF). After
this conversion, a narrow fixed bandwidth and frequency gain stage was
designed to amplify the signal. Easier to design such a stage instead of
a TRF circuit of the same gain. It also helps that leakage form the IF
won’t be “heard” by the front end antenna LC circuit, because it’s a way
different frequency. Special frequency changing tubes were developed to
generate and mix the local oscillator frequency with the radio station
carrier to generate the IF. The 6A7, 6A8, and 6SA7, and later the 12SA7
are “pentagrid” converter tubes for this purpose.
Tubes with variable gain were used in IF amp stages, so automatic
control (AVC) could be done. Decrease the gain on strong stations so you
don’t get blasted out when tuning from a weaker station, and also avoid
distortion overload from the strong station. Tubes like 6K7, 6D6, 6SK7,
and later 12SK7 were variable gain tubes. Usually called “remote cutoff”
pentodes, as the tube wouldn’t linearly cutoff current flow like a constant
gain tube (“sharp cutoff”) would. Yes, these remote cutoff tubes would
not be usable in an audio amp, but these tubes were used in IF strips, where
only a narrow bandwidth of frequencies were to be amplified, and harmonic
distortion products fell outside the bandwidth of the output IF filter,
and were thus ignored. The audio detector tube would also measure the signal
level, and thus could be fed back to the remote cutoff pentode IF tube.
And also to any variable gain tubes at the front end of the radio. The
audio detector diode was arranged to create more negative voltage for strong
signals, and more negative voltage reduces the gain of the remote cutoff
Once the audio is detected, it needs to be power amplified to drive
a speaker at reasonable volume levels. A triode signal gain stage feeds
the power tube, to generate about 1 watt of audio power to the speaker.
The audio bandwidth is narrower than modern hi-fi stereos. And the speaker
was fairly efficient, so not much power was needed. To a casual listener,
if you limit the low frequencies and the highs at the same time, the listener
won’t really notice. The extreme example of this is the telephone, 300
to 3000 Hz. AA5 radios do about 150 to 5000 Hz. Hi-Fi stereos do about
20 to 20000 Hz.
The five tube AM radio didn’t much vary after the 150 mA heater tubes
were introduced around 1940 or so. Those were the octal series of tubes.
The 12SA7 converter, 12SK7 IF amp, 12SQ7 audio detector and signal amp,
50L6 audio power, and 35Z5 rectifier. Just after WW2, the miniature 7 pin
tubes were introduced. Miniature tubes were used in the war, but didn’t
hit the consumer market until after. The 12BE6 converter, 12BA6 IF amp,
12AT6 audio detector and signal amp, 50B5 audio power, and 35W4 rectifier.
The 50B5 had its plate next to the heater, but that made for too much voltage
between these pins and Underwriter’s Laboratories and similar safety agencies didn’t
like this. The 50C5 was a rearrangement of the pinout to solve this safety
concern. Another variation, the “loktal” tube, had
its own versions of these, 14Q7, 14A7, 14B7, 50A5, and 35Y4, respective
functions. By this time, the AA5 acquired its designation, the “All American
5″ from ww2 surplus tube dealers who sold to hobbyists.
You sometimes find AA5 radios using a mix of octals and loktals, or octals
and mini’s. An example of an AA5 that used a mix of octals, loktals
and mini’s is the Philco 81-122, using 7A8, 12BA6, 14B6, 50L6 and 35Z5. Most
likely reason for this grouping of tubes was what they could purchase enough
of inexpensively to make radios at the time.
The last version of the AA5 tube line-up was the 100 mA heater string,
introduced in the early sixties. Saved an extra 6 watts of heater power,
but the tubes took a little longer to warm up, and the audio output power
was a bit less. The signal handling tubes were 18V at 100 mA heaters, so
those used the same amount of power as the 12V tubes on the heaters. All
had the same pinouts as the 150 mA versions. But these had slight differences
with the 150 mA tubes, so they were assigned their own designations instead
of being called 18BE6 or 18BA6. They were: 18FX6 converter, 18FW6 IF amp,
18FY6 audio detector and signal amp, 32ET5 or 34GD5 audio power, and 36AM3
rectifier (which the RCA tube manual (RC24) says cannot be used to operate
a pilot light, but the Sylvania tube manual (1968) says it can operate
a pilot light). As you can see, the audio out tube had less heater power
than the 50V at 150 mA version had to heat the cathode, thus less audio
power output. Also the rectifier was also had less heater power, but the
audio stage drawing less current allowed a less current capable rectifier
to be used.
A compactron tube version was in development, but turned out it would
have cost more to make than the miniature 7 pin tubes already out. The
56R9, a compactron triode and power pentode, is listed in the 1973 edition
of GE’s “Essential Characteristics” manual, page 212. With a heater current
of 150mA, this may have been to be an “AA5” compactron.
After you make a few tens of millions
of something, you find ways of squeezing the cost to a bare minimum, which
is usually just a bit more than the cost of raw materials.
Sub-miniature tubes were used by the military, but were too expensive
to make for use in AA5 type radios.
The end of the AA5 radio was around 1968 or so. By then, many were made
in Japan, and Japanese AA5 tubes were also made in Japan used by American
radio and TV manufacturers. After that, solid state radios, many using
a high voltage audio output resistor and thus were also “hot chassis”,
became the preferred technology.
ALL AMERICAN FIVE – TUBE COMPLEMENT (list by John Reinicke)
CONVERTER IF AMP DETECTOR/ AUDIO RECTIFIER YEAR
1st Audio OUTPUT
2A7 58 55 59 80 1933
6A7 39/44 75 42 1934
78 43 25Z5 300 mA heaters
1A6 1A4 1B5 33
6A8 6S7 6Q7 6K6 5Y3 1935
6K8 6K7 First octal
6L6 6X5 1937
6SA7 6SK7 6SQ7 1939
12A8 12K7 12Q7 35L6 35Z4 150 mA heaters
7A8 7B7 7C6 35A5 35Z3 First Loktal
7A7 7B6 sockets
1A7 1A4 1H5 3Q5
12SA7 12SK7 12SQ7 50L6 35Z5 1940
1R5 1T4 1U5 1S4 First Miniature
1LA6 1LG5 1LD5 1LA4 117Z3
12BE6 12BA6 12AV6 50B5 35W4 1946
14B8 14A7 14B6 50A5 35Y4
12GA6 12EA6 12FM6 mid ’50’s
12AD6 12AC6 12AJ6 12V B+ tubes for
12AG6 12AF6 12FK6 car radios, same
12FA6 12BL6 12AE6 pinouts as their
12FT6 AA5 counterparts
18FX6 18FW6 18FY6 32ET5 36AM3 early ’60’s
34GD5 100 mA heaters
56R9 150 mA compactron triode/ power pentode tube, early ’70’s?
from an article published in the Michigan Antique
Radio Club newsletter Apr 1997, by John Reinicke
_Essential Characteristics_, General Electric Corp, 1973.
_RCA Tube Manual, RCA CORP, 1971.
RCA Radiotron Designer’s Handbook, RCA Corp, 1943.