History of the AA5 (All American 5ive) AM tube radio
History of the AA5 
AA5 AM radio using miniature tubes.
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 Superheterodyne 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 Five."
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 (- George Gonzalez). 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. Either 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) power plug.
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+ 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 superheterodyne 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 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 superheterodyne 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 "superheterodyne") 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 volume 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 lived 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 tubes.
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 uL and similar safety agencies didn't like this. The 50C5 was a rearrangement of the pinout to solve this safety concern (- George Gonzalez.) 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 (- Doug Houston). You sometimes find AA5 radios using a mix of octals and loktals, or octals and mini's. Doug MacDonald tells of an AA5 that used a mix of octals, loktals and mini's. A 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. See Electronics World Oct 1960 article page 48 and page 49 on these. A radio would have used 2 tubes, a converter/ IF pentode tube "40xx10" (my guess at a likely tube number had it went into production) and a detector/audio driver & output/rectifier "70xx11". 100 ma heaters. 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. See a web page of an AM radio I built using compactrons. 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. See my web page for speculation on how such a radio might have taken shape.
The end of the AA5 radio was in the early 1970's, as stocks of tubes and related parts were depleted by manufacturers and sellers. 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
34
6A8 6S7 6Q7 6K6 5Y3 1935
6K8 6K7 First octal
sockets
6L6 6X5 1937
0Z4
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
1T5 sockets
1LA6 1LG5 1LD5 1LA4 117Z3
1LH4
12BE6 12BA6 12AV6 50B5 35W4 1946
12AT6 50C5
14B8 14A7 14B6 50A5 35Y4
12GA6 12EA6 12FM6 mid to late 50's
12AD6 12AC6 12AJ6 12V B+ tubes for
12AG6 12AF6 12FK6 car radios, same
12FA6 12BL6 12AE6 pinouts as their
12EG6 12DZ6 12FT6 AA5 counterparts
12CX6
12EK6
12CY6
12EZ6
12CN5 12EL6 different pinouts
12DK5
18FX6 18FW6 18FY6 32ET5 36AM3 early '60's
18GD6 34GD5 100 mA heaters
26D6 26A6 26C6 Military tubes
26CG6 26BK6 70 ma heaters
56R9 150 mA compactron triode/ power pentode tube, early '70's?
GE compactrons never released:
"40xx10" converter/IF "70xx11" detector/audio driver & output/rectifier
100ma heaters See Electronics World Oct 1960 article page 48 and
page 49 on these.
Foreign types replacable by AA5 or audio types:
foreign American
foreign American
1C1 1R5
1F3 1T4
1H33 1R5
1P10 3S4
1P11 3V4
6B32 6AL5
6BC32 6AV6
6D2 6AL5
6F31 6BA6
6F33 6AS6
6H31 6BE6
6L13 12AX7
6P15 6BQ5
6R-HH2 6BS8
10PL12 50BM8
12BC32 12AV6
12F31 12BA6
12H31 12BE6
13D2 6SN7
19M-R9 18FW6A
19M-R10 18GD6
A2900 12AT7
B36 12SN7
B65 6SN7
B152 12AT7
B309 12AT7
B329 12AU7
B339 12AX7A
B739 12AT7
B749 12AU7A
B759 12AX7A
CC81E 12AT7WC
CV133 6C4
CV452 6AT6
CV453 6BE6
CV454 6BA6
CV455 12AT7
CV491 12AU7A
CV492 12AX7A
CV537 12SA7
CV538 12SA7GT
CV543 12SK7
CV544 12SK7GT
CV546 12SQ7
CV547 12SQ7GT
CV551 25L6GT
CV552 25L6
CV561 35L6
CV568 35Z5GT
CV571 50L6GT
CV586 6L6GT
CV609 42
CV614 75
CV782 1R5
CV852 6C4
CV925 12SN7GTA
CV1075 6L6GC
CV1286 6L6
CV1287 25L6GT
CV1928 12BA6
CV1947 6L6GC
CV1948 6L6
CV1959 50C5
CV1961 12AU6
CV1966 6SA7
CV1967 6SA7GT
CV1981 6SK7
CV1982 6SK7GT
CV1988 6SN7GT
CV1990 6SQ7
CV1991 6SQ7GT
CV2007 12AU7A
CV2016 12AT7
CV2024 6BE6
CV2026 6BA6
CV2526 6AV6
CV2527 6BA7
CV2534 50L6GT
CV3508 12AT7WA
CV4004 12AX7A
CV4012 6BE6
CV4029 5902
CV4058 6C4WA
CV5037 6BA6W
CV5212 12AT7
CV5358 6DJ8
CV8069 6BQ5
CV8154 12AT7
CV8155 12AU7A
CV8156 12AX7A
CV8201 6BE6
CV8202 6BA6
CV8221 12AU7A
CV8222 12AX7A
D152 6AL5
D2M9 6AL5
D717 6AL5
D77 6AL5
DAF92 1U5
DD6 6AL5
DF60 5678
DF62 1AD4
DF904 1U4
DF91 1T4
DH77 6AT6
DK91 1R5
DL37 6L6GT
E81CC 12AT7WC
E99F 6BJ6
E2157 12AT7
E2163 12AU7A
E2164 12AX7A
E81CC 12AT7, 6201
E82CC 12AU7
EAA91 6AL5
EBC90 6AT6
EBC91 6AV6
EC90 6C4
ECC186 12AU7
ECC32 6SN7GTB
ECC70 12AT7WC
ECC82 12AU7A
ECC83 12AX7A
ECC88 6DJ8
ECC801 12AT7W
ECC802 6189
ECC803 12AX7A
ECC863 12AX7A
EF72 5840
EF730 5636
EF732 5840, 5901
EF734 6205
EF93 6BA6
EK90 6BE6
EL37 6L6GC
EL71 5902
EL84 6BQ5
EZ4 6C4
HAA91 12AL5
HABC80 19T8
HBC80 19T8A
HBC90 12AT6
HBC91 12AV6
HCC85 17EW8
HF93 12BA6
HF94 12AU6
HK90 12BE6
HL92 50C5
HL94 35C5
HM04 6BE6
HY90 35W4
KT32 25L6GT, 25W6
KT71 50L6GT
KT88 6550
L77 6C4
LN119 50BM8
M8121 5840
M8136 6189
M8137 12AX7A
M8162 12AT7WC
N308 25E5
N709 6BQ5
OBC3 12SQ7
OSW3104 6SA7
OSW3105 6SQ7
OSW3111 6SK7
PM04 6BA6
QA2406 12AT7WC
QB65 6NS7GTA
QB309 12AT7
QL77 6C4
QS2406 12AT7WC
UF89 12AD6
UCL82 50BM8
W17 1T4
W727 6BA6
X17 1R5
X77 6BE6
X107 18FX6
X727 6BE6
Mil to AA5 or audio types:
VT33 33
VT49 39/44
VT58 58
VT54 34
VT75 75
VT78 78
VT80 89
VT86 6K7
VT92 6Q7
VT103 6SQ7
VT104 12SQ7
VT115 6L6
VT117 6SK7
VT131 12SK7
VT150 6SA7
VT161 12SA7
VT173 1T4
VT174 3S4
VT199 6SS7
VT201 25L6
VT209 12SG7
VT211 6SG7
VT231 6SN7
VT264 3Q4
VT288 12SH7
1622 6L6
5749 6BA6
5750 6BE6
5881 6L6GT
6046 25L6GT
6057 12AX7
6060 12AT7
6100 6C4
6137 6SK7
6189 12AU7
6201 12AT7WC
6662 6BJ6
6679 12AT7
6680 12AU7
6681 12AX7
7025 12AX7
7036 6BE6
7320 6BQ5
7581 6L6
8426A 12AU6
NOTE: Not all subs will work in all circuits!