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History Of The American Vacuum Tube AM History Of The American Vacuum Tube AM RadioEngineeringAA 5 History AA 5 History History of the AA 5 (All American 5 ive) 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 2 A 7. The 2 A 7 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: 2 A 7, 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 6 A 7 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 25 Z 5 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 120 V. 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 lik 75, 6 F 7 and newer ones like 6 SK 7, 6 SQ 7. And an extra tuning eye 6 E 5 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, 25 L 6, 25 Z 5 and 25 Z 6.
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 (300 mA) 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 didnt 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 dont 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 25 Z 6, 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 doubler's wont 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 6 V, 300 mA heater signal tubes had their heaters replaced with ones that needed 12 V at 150 mA.
Tuning eye tubes at 150 mA heater for consumer radios did exist. Theres the 6 AB 5 / 6 N 5. Heater of 6. 3 V @ 150 ma. And the 1629, heater of 12. 6 V @ 150 ma.
But they were rarely used. The Airline model 93 WG 602 B used the 6 AB 5. But the common 150 ma AA 5 tubes used the same power as the 300 ma AA 5 tubes. And the 25 L 6 became a 50 L 6 the same way, 2 x voltage, 1 / 2 current.
A new design overall was the rectifier tube, the 35 Z 5, 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.
Local 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 stations 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 didnt leak back into the antenna, or else youd 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 wont be heard by the front end antenna LC circuit, because its 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 6 A 7, 6 A 8, and 6 SA 7, and later the 12 SA 7 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 dont get blasted out when tuning from a weaker station, and also avoid distortion overload from the strong station.
Tubes lik SK 7, and later 12 SK 7 were variable gain tubes. Usually called remote cutoff pentodes, as the tube wouldnt 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 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 wont really notice. The extreme example of this is the telephone, 300 to 3000 Hz.
AA 5 radios do about 150 to 5000 Hz. Hi-Fi stereos do about 20 to 20000 Hz. The five tube AM radio didnt much vary after the 150 mA heater tubes were introduced around 1940 or so. Those were the octal series of tubes. The 12 SA 7 converter, 12 SK 7 IF amp, 12 SQ 7 audio detector and signal amp, 50 L 6 audio power, and 35 Z 5 rectifier. Just after WWII, the miniature 7 pin tubes were introduced.
Miniature tubes were used in the war, but didnt hit the consumer market until after. The 12 BE 6 converter, 12 BA 6 IF amp, 12 AT 6 audio detector and signal amp, 50 B 5 audio power, and 35 W 4 rectifier. The 50 B 5 had its plate next to the heater, but that made for too much voltage between these pins and Underwriters Laboratories and similar safety agencies didnt like this. The 50 C 5 was a rearrangement of the pinout to solve this safety concern. Another variation, the local tube, had its own versions of these, 14 Q 7, 14 A 7, 14 B 7, 50 A 5, and 35 Y 4, respective functions.
By this time, the AA 5 acquired its designation, the All American 5 from WWII surplus tube dealers who sold to hobbyists. You sometimes find AA 5 radios using a mix of octal's and loktal's, or octal's and minis. An example of an AA 5 that used a mix of octal's, loktal's and minis is the Philco 81 - 122, using 7 A 8, 12 BA 6, 14 B 6, 50 L 6 and 35 Z 5. 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 AA 5 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 18 V at 100 mA heaters, so those used the same amount of power as the 12 V 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 18 BE 6 or 18 BA 6. They were: 18 FX 6 converter, 18 FW 6 IF amp, 18 FY 6 audio detector and signal amp, 32 ET 5 or 34 GD 5 audio power, and 36 AM 3 rectifier (which the RCA tube manual (RC 24) 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 50 V 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 compaction tube version was in development, but turned out it would have cost more to make than the miniature 7 pin tubes already out. The 56 R 9, a compaction triode and power pentode, is listed in the 1973 edition of GEs Essential Characteristics manual, page 212. With a heater current of 150 mA, this may have been to be an AA 5 compaction. 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 AA 5 type radios. The end of the AA 5 radio was around 1968 or so. By then, many were made in Japan, and Japanese AA 5 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 1 st Audio OUTPU 58 55 59 80 19339 / 44 75 42 1934 78 43 25 Z 5 300 mA heater 33 31935 First octal socket 1937 SA 7 6 SK 7 6 SQ 7 1939 12 A 8 12 K 7 12 Q 7 35 L 6 35 Z 4 150 mA heaters 35 A 5 35 Z 3 First Loktalsockets 12 SA 7 12 SK 7 12 SQ 7 50 L 6 35 Z 5 1940 First Miniature 1 T 5 sockets 1 LA 6 1 LG 5 1 LD 5 1 LA 4 117 Z 3 1 LH 4 12 BE 6 12 BA 6 12 AV 6 50 B 5 35 W 4 1946 12 AT 6 50 C 5 14 B 8 14 A 7 14 B 6 50 A 5 312 GA 6 12 EA 6 12 FM 6 mid 50 s 12 AD 6 12 AC 6 12 AJ 6 12 V B+ tubes for 12 AG 6 12 AF 6 12 FK 6 car radios, same 12 FA 6 12 BL 6 12 AE 6 pinouts as their 12 FT 6 AA 5 counterparts 18 FX 6 18 FW 6 18 FY 6 32 ET 5 36 AM 3 early 60 s 34 GD 5 100 mA heaters 56 R 9 150 mA compaction triode/ power pentode tube, early 70 s? References: 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 Radio tron Designers Handbook, RCA Corp, 1943. 38 c
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