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.
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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!