Article 1337 of rec.antiques.radio+phono: Path: news.umbc.edu!cs.umd.edu!zombie.ncsc.mil!news.duke.edu!news-feed-1.peachnet.edu!gatech!howland.reston.ans.net!news.sprintlink.net!bga.com!bga.com!nobody From: vancleef@bga.com (Henry van Cleef) Newsgroups: rec.antiques.radio+phono Subject: FAQ rec.antiques.radio+phone (part 3 of 5) Followup-To: rec.antiques.radio+phono Date: 27 Oct 1994 18:14:05 -0500 Organization: Bluebonnet Hollow State High Tech Works Lines: 598 Message-ID: <38pc7t$iu1@ivy.bga.com> NNTP-Posting-Host: ivy.bga.com Summary: Part 3 - General questions about vacuum tube radios and phonos. Rec.antiques.radio+phono Frequently Asked Questions (Part 3) Revision Date Notes 1.1 Oct 24, 94 Was part 2, now part 3. New material and revisions. Part 3 - General questions about vacuum tube radios and phonos. ------------------------------------------------------------------------------ FAQ editor: Hank van Cleef. Email vancleef@bga.com, vancleef@tmn.com This is a regular posting of frequently-asked questions (FAQ) about antique radios and electronic phonographs. It is intended to summarize some common questions on old home entertainment audio equipment and provide answers to these questions. Q. What is published to tell me what an old radio is worth? A. There are some guides that list prices. The most commonly mentioned is Bunis, Marty and Sue, "The Collector's Guide to Antique Radios." It is available from Antique Electronic Supply. There are several other books available from them for identifying old radios, some with price information. What a specific radio actually is worth may be quite different than what these guides list. In addition, the condition of the radio (both cosmetics and electronics) has to be considered. Q. I just got an old radio at a yard sale for $5. It is a Radio Wire Television Model J5. When was this radio built? Can I get it to work? Is this radio worth restoring? Can I get a schematic somewhere. A. Requests like this send everyone scrambling for their references, schematics manuals, etc. etc., and sometimes nobody responds. There is some very basic information that you could, and should, include, that would get you an answer instantly. If you included "this radio uses five tubes. They are 12SA7, 12SK7, 12SQ7, 50L6, and 35Z5." See below on "how to date radios by design features." Listing the tubes often says everything. The example used here is one of an endless long list of AC-DC table radios built after 1940 using this tube complement. Most people who repaired radios in the forties and fifties could draw the schematic for any of these radios from memory----it's a case of "seen one, seen 'em all." This particular radio has a grand total of 9 resistors (including volume control), a whopping 14 condensers (including the tuning condenser as one), three transformers, one oscillator coil, a loop antenna, a loudspeaker, and a panel lamp. Add the five tubes, and that amounts to the whopping sum total of 35 electrical components, and if you want to insist on including the chassis, five tube sockets, cabinet, panel lamp socket, and cabinet, we are still talking about 50 parts. No wonder they sold for $4.98 in 1940. If it has value, it is for its case and mechanical configuration. As a project radio to learn radio repair and restoration, an AC-DC 5 or 6 tube table set is probably ideal. Most of these sets need one tube (burned-out heater), new electrolytics and paper capacitors to get it "working like new." Q. I just looked at a Radio Wire Television model B45. It has 13 tubes and two loudspeakers. I couldn't see all the tubes but I saw a 6H6, two 6L6's, two 5Y3's, and a bunch of metal tubes with top caps. It has three bands, two shortwave, and a phono, and is in a custom-built plywood cabinet. What can anyone tell me about this set. The radio works, but not well. The owner wants $100 for it. Is it worth it? A. This is the type of radio you should be asking questions about. The radio itself is a "class act"---high fidelity, 1938 style. It's the same manufacturer listed in the question above, and shows that "brands" could range from absurdly cheap to top quality. It also is typical of the radios that justified service shops paying good money for Rider's manuals over the years. As a "collector" radio, it's a difficult one to put dollar value on. But as a museum piece, an example of what a high-end thirties radio was, it is a class act. For those who have Rider XVIII, look at Radio Wire page 18-8, and notice that only the schematic and a few notes are published, some ten years after the radio was made. (confession: I owned one of these from about 1948 until sometime in the sixties, and it was my first really hard-core restoration project. It also was my "hi-fi amplifier" for many years). If you want an example of high tech history, it's well worth the $100, and if you restore it, you'll find that quality is a lasting thing. But restoring a set like this can be a major project and take a good deal of skill. Other "high tech" radios that are more readily identifiable by brand name are the Farnsworth Capehart sets and the 2-chassis Magnavoxes. Q. I saw a little table radio with a very pretty plastic case, but the owner want hundreds of dollars for it. The case looks like marble, but the radio inside is just another of those 35Z5 and 50L6 five tube jobs. Why does the owner think its worth almost a thousand bucks? A. Well, you've stumbled on the collectors' hot item of the nineties, the "Catalin" case. The reason the owner thinks it is worth this much is that the collectors' market seems to be willing to pay these prices for a catalin case. Whether it will continue to do so is open to question. It is difficult, in a FAQ item, to explain the whimsies of the "collector" market, because these tend to change. Q. Well, if a low-tech radio is worth hundreds of dollars because of its case, and a high-end console with tremendous sensitivity and a powerful amplifier with good fidelity is worth a lot less, what's the correlation between price and value? A. There isn't any. Some radios, such as the Atwater Kent TRF sets and the RCA catacombs superhets are valuable because they are relatively rare today, and represent technological history. An old communications receiver, such as the Hallicrafters SX43, which was also sold as a home entertainment radio, has much more value to a ham than an old Magnavox radio-phono, so has value because of its technology. Novelty items, particularly if they are rare, seem to be high-ticket "collectibles" in any area. So you see dollar values attached to radios with reading lights built in, radios with cameras in them, catalin cases, the Sparton blue mirror sets, incredibly small portables, etc. Q. I keep hearing about "Neutrodyne," "TRF," and "Superheterodyne." What do these terms mean? A. The first home entertainment radios were crystal sets which used a single tuned antenna circuit and a crystal detector. When tubes were added for amplification, these were set up with tuned circuits that had to be individually tuned to the station being received. These are "TRF" sets, for "tuned radio frequency." Later on, manufacturers learned how to build TRF stages using either mechanical coupling between the tuning condensors or a single ganged condenser, and to provide adjustments to get them to track (i.e., all tune to the same frequency across the range of broadcast frequencies), so later TRF sets have one-knob tuning. The Neutrodyne refers to a method of "neutralizing," or compensating for, detuning effect of grid-plate capacitances by feeding back an opposing signal. These sets are TRF sets with neutralizing circuits in them---generally, another coil in the tuned circuit used to generate the neutralizing signal. The superheterodyne uses the physical principle that two oscillators running at different frequencies will produce "beat" frequencies equal to both the sum of and difference between the two frequencies. This can be heard when tuning musical instruments; the principle is the same for radio frequencies. The incoming RF signal is "mixed" with a local oscillator signal and fed to a fixed tuned stage that is sensitive to the difference frequency between the two signals. Use of one or more fixed-frequency tuned stages gives the set relatively constant sensitivity and selectivity, both of which are difficult to get in variable tuned stages. To illustrate what these words mean, take a common five-tube US table radio and a station at 1000 Khz ( 1 megacycle). An antenna coil and one section of the tuning condenser (capacitor) are tuned to resonate at 1000 Khz, "selecting" that frequency. A local oscillator is tuned by the other section of the tuning condenser to 1455 Khz. In a set with a 12SA7 tube, the 12SA7 is wired as an oscillator, with the oscillator signal appearing on the first grid (g1). The tuned RF signal is fed to the third grid (G3). The plate circuit is connected to a transformer tuned to 455 Khz, to respond to the difference between the frequencies being injected on G1 and G3. Signals at 455, 1000, 1455, and 1455 Khz all appear on the 12SA7 plate (the two fundamentals and the sum and difference), but the tuned "intermediate frequency" (IF) transformer selects only the 455 khz signal. This intermediate frequency is generally amplified by one or more tuned (455 khz) stages---in our example, a 12SK7 with double-tuned input and output IF transformers (i.e., both the plate and grid circuits are tuned to resonate at 455 Khz) is used, and the output of that stage is fed to the a diode detector. This may sound a bit complicated, and I've left out all the fine points of the design to focus on "what's supposed to happen."---a good engineering text discusses design details beyond this description. One point of terminology----the mixer stage (12SA7) was often called a "first detector" in early designs; thus, the 12SQ7 diode detector in our example is called the "second detector," a term that has persisted through the decades. One other common early design was the "regenerative" set. In these sets, an RF amplifier was designed as an oscillator, but provided with a control that could be adjusted so that the stage wouldn't go into oscillation. The positive feedback in the stage provided substantially more gain than a simple tuned circuit would provide. Misadjustment of the feedback control would make the stage oscillate, producing squeals in the output, and quite powerful RFI (radio frequency interference) as well. The "superregenerative" circuit is a refinement that prevents sustained oscillation, but was generally not used in home entertainment sets. Q. I have an old radio-phono. The radio works fine, but the phono doesn't make any sound in the loudspeaker at all. What's the deal? A. Your phono pickup probably uses a Rochelle salt crystal cartridge, and the salt crystal has failed. You will need a new cartridge. (faq editor note---I'm including this, and have a radio-phono with a dead cartridge. What's available?). Q. I just got an old radio that I think was made in 1939. But it has a jack on the back labelled "television." It only has a volume control/on-off switch and tuning control on the front. What's the deal with the jack? How can a radio receive television, and why is a 1939 radio labelled like this when TV broadcasting didn't really begin until after the war. A. You are looking at a marketing ploy. The jack on the back is an audio input jack, and if there is no switch for it, it is wired permanently to the top of the volume control (detector output), so has whatever signal the radio is receiving on it as well. Television was "just around the corner" in the 1937-39 period and there were some experimental stations broadcasting what is essentially NTSC video on Channel 1 (48-54 Mhz) after 1936. Putting these jacks on the radios was to convince the buying public that their new radio wouldn't be made obsolete by television "next year." Commercial television actually began in 1939, but WW II intervened, and the mass-marketing push for TV did not begin until 1946-7. Q. I have a console with 6L6's and a twelve-inch loudspeaker. Is this "high fidelity?" Just what can I expect to hear from my old radio for audio quality? A. Both AM broadcast radio and 78 RPM electrically-recorded disks were limited to a bandwith of 50 cycles to 5 kilocycles (Hz. for the moderns---I use the old term because it is what you will find in contemporary documentation). The human ear can actually hear from about 10 cps to about 21 or 22 kc. Until the advent of FM broadcasting 1940) and 33-1/3rd RPM "long-playing" records (1948) there were no commercially-available program sources for a wider bandwidth than the 50 cps-5 kc. standard, which a single 8-12 inch loudspeaker will reproduce quite well. High-end postwar consoles responded to the 15 cps-15 kc. bandwidth of FM and early long-play records, primarily by using a 15-inch woofer and one or more 5-inch high-frequency speakers, with a crossover network, still mounted unbaffled in the cabinet. With a restored older high-end radio, you will get the fidelity that was available from the program sources at the time. More pestiferous is the question of power supply hum. The AC-DC sets with half-wave rectifiers have "built-in" 60-cycle modulation of everything. Most of them used small (5-inch) speakers which did not reproduce this frequency well, although the hum is quite noticeable close to the radio. Transformer-operated sets with full-wave rectifiers had a much lower hum level, at 120 cps, although it can be heard as a very light background noise near the radio. Only a few later high-end consoles had sufficient filtering to make them as hum-free as one expects from audio equipment today. Significant hum when an old radio is tuned to a nearby station, at mid-volume, heard from ten or fifteen feet away, means that the set has electrical problems. A good floor console or cathedral radio, operating properly, should have good-quality audio for AM broadcast listening. Q. I have a nice old Philco cathedral radio that I have listened to for years. It only gets local stations, and even at maximum volume, is not particularly loud. Can I get it to work better than it does now? A. Probably. You have a sixty-year-old piece of electronic equipment that has probably had two or three tubes replaced, and maybe one bad capacitor, in those sixty years. In short, it's a candidate for an electronic overhaul. Some things that may have degraded over the years: a. Capacitors. Electrolytic capacitor problems generally make themselves known quite quickly. However, those little wax-impregnated "paper condensors" may all be leaking current and delivering less capacitance than needed for good performance. b. Resistors. These may have "drifted" to a much higher resistance gradually. c. Misalignment of tuned circuits. The "tweaks" on the tuning condenser and the IF transformers generally don't drift very far unless the coils have absorbed moisture. Altogether too often, the amateur restorer will tweak the set out of alignment by fiddling with these. Don't touch them unless you know exactly what you are doing and have the equipment needed to align the radio. d. Tired tubes. I put this last, although a lot of people look here first, and assume that a tube tester's readings will correlate with set performance. The best test for tube condition is to substitute a known good tube in each position and seeing if it changes anything. A sick pentagrid converter tube (6A7, 6A8, 6K8, 6SA7, etc.) may very well test normally under DC conditions in a tube tester yet fail to oscillate reliably in the set, particularly on shortwave. Q. You say "electronic overhaul." Will that restore my set to like-new performance? A. Generally, yes---actually, better than new. Modern resistors and capacitors are better circuit components than were available in the thirties and forties. Capacitors in particular are much smaller, and larger values can be used to advantage in some places, particularly in the filtering circuits. Q. I have a Philco battery-powered radio. It has a four-prong plug for the battery. Can I get a converter at Radio Shack and use it to make my radio work? A. No. The battery radios required 1.5 volts for the tube filaments and 67-1/2 or 90 volts for "B" (plate) voltage. The 3-way portables (AC-DC-battery) had built-in battery eliminators, and the tube filaments were generally wired in series, requiring a 6 or 9 volt "A" battery. You'll need to make a supply that can deliver 1.5 volts at about 400 ma. and 90 volts at about 50 ma. for your four-prong Philco. Both have to be good clean filtered DC. The power-pak-in-the-plug type power units sold by Radio Shack and others are made to deliver 6-9 volts at 100-200 ma. unfiltered DC. DATING OLD RADIOS BY THEIR TUBE COMPLEMENT The development of vacuum tubes, both electrically and mechanically, advanced at a rapid pace between about 1925 and 1950. The vast majority of radios sold for home entertainment between 1920 and the late 1950's were built to various standard circuits. In most cases, checking out what tubes are used in the radio will place it's date of manufacture within a few years, identify which of the standard circuits it used, and give a some indication of the quality of the set. Most radio repair technicians in the 1930-60 era did not need to look at schematics most of the time, even when the problem was not a burned-out vacuum tube heater or filament. The tube complement is not always an accurate guide, except insofar as the presence of a given tube indicates that the set was built after that tube was placed in production. You won't find any 1932 radios using tubes with octal bases or 6.3 volt filament heaters, and you won't find any prewar radios with 7-pin miniature tubes. But you may find a 1946 table radio built to a 1935 design. There are also a few other design features that are very obvious on casual inspection; I'll mention some of them as we go along. 1. The five or six-tube AC-DC radio with 150 ma. tube heaters wired in series. These sets do not have a power transformer, and could operate in places like mid-Manhattan, which had 110 volts DC as its primary electrical service. I place these sets at the top of the list, because more of them were made than all other designs put together. Most of these were built as table radios, although some were installed in small consoles and radio-phonograph combinations. Virtually all clock radios use this circuit. These are generally AM-broadcast-only. The six-tube version had an RF preamplifier, and was more sensitive than the five-tube. In the fifties, some of these radios were built with a selenium rectifier, omitting the rectifier tube. Also, a few manufacturers built a four-tube version that omitted any IF amplification. The tube complements are: a. First version, built primarily 1938-40. 12A8 RF-converter, 12K7 IF amplifier, 12Q7 detector-audio, 35L6 power output, and 35Z5 rectifier. The first three tubes had small top caps for the signal grid connections, with either metal or glass envelopes. The original glass tubes had a "G" suffix, indicating use of an ST-12 stepped bulb envelope. b. Second version, built 1939-ca. 1960 (Note from FAQ editor---the original FAQ specified 1940 as the year of introduction for single-ended octal tubes. Typically, they were introduced in "1940 models," but were built beginning in 1939). 12SA7 RF-converter, 12SK7 IF amplifier, 12SQ7 detector-audio, 50L6 power output, 35Z5 rectifier. This is almost the same radio, but using single-ended tubes in the first three stages and a power output tube with a 50-volt heater. The major difference is in use of a 12SA7 in place of the 12A8---these tubes are different internally. Note that the sum of the nominal heater voltages adds up to 122.8 volts, allowing operation without need for any series resistor in the heater circuit. c. Postwar version, 1945-mid '60's 12BE6 RF-converter, 12BA6 IF amplifier, 12AT6 detector-audio, 50B5 power output, 35W4 rectifier. The only difference here is the use of seven-pin miniature tubes. All are electrically identical to the octal versions above. Some sets were built using a mix of seven-pin miniature and octal tubes, however, the presence of seven-pin miniature tubes indicates that the set is postwar production. d. Loctal tube version, 1940-ca. 1960 14Q7 RF-converter, 14A7 IF, 14X7 detector-audio, 50C5 power output, 35Y4 rectifier. Once again, the same radio as version b., using loctal-base tubes in place of octal. Philco and GE were fond of using loctal tubes. Note that some radios used a 14B8 converter, which is the same configuration in a circuit as the 12A8. The six-tube configuration used the same tube type for both RF preamplifier and IF amplifier, and the 35 volt heater version of the output tube. In most cases the RF preamplifier is resistance-coupled to the RF-converter stage, and the radio used a two-stage tuning capacitor. Some later versions used movable slug tuning in place of a variable capacitor. This variation began around 1947, and became more common during the next decade. 2. Five or six tube AC-DC transformerless radios using 300 ma heaters wired in series. These radios were the precursors of the 150 ma. series heater radios. Some of these radios also included a tuning eye indicator, typically a 6E5. Total voltage drop of the series heater string was 68-74-82 volts requiring an external voltage dropping resistor of some sort. These radios often used "ballast" tubes or resistance wire in the line cord for this purpose. a. Version using large-base 5, 6, or 7-pin tubes, 1935-50. 6A7 RF-converter, 78 or 6D6 IF, 75 detector-audio, 43 power output, 25Z5 rectifier. Most of these sets were built before 1938, although a few manufacturers reintroduced them in the early postwar era. There are more variations on this design than on the 150 ma. heater designs described above. As noted, some sets had 6E5 tuning eye tubes. Sets with shortwave often had a 76 triode as a separate local oscillator for the 6A7. b. Version using top-cap octal tubes, 1936-1950's. 6A8 RF-converter, 6K7 IF, 6Q7 detector-audio, 25A6 or 25L6 audio, 25Z6 rectifier. This reflects the switch to octal tubes in 1936. The first three tubes had small top caps for signal grid connection. The 25A6 is an octal version of the 43; the 25L6 is a 25 volt heater beam power tube identical, except for heater, to the 35L6 and 50L6. The 25Z5 is a full-wave rectifier (two diode sections), and was usually connected with the two sections in parallel. However, some manufacturers, notably Philco, used the two sections to provide voltage doubling for B+. Radios with voltage doubler power supplies are AC-only, as a voltage doubler requires alternating current to "pump" the doubler circuit. c. Version using single-ended octal tubes, 1939-50's. 6SA7 RF-converter, 6SK7 IF, 6SQ7 detector-audio, 25L6 output, 25Z6 rectifier. Once again, this is a "switch," this time to single-ended octal tubes. Major circuit difference is in the 6SA7 circuit because of differences internally between the 6SA7 and 6A8. This version was generally not built as a "price leader" inexpensive table radio because of the availabity of 150 ma. tubes that didn't require a dropping resistor in the heater circuit. It was very often used as the basis for an upscale AC-DC radio. Some configurations that you may run across: 1. Shortwave receiver using an additional RF preamplifier, separate local oscillator, and second IF stage. The 6SK7 was used for the RF and IF stages, and a 6J5 as a local oscillator. 2. Push-pull audio output, using two 25L6 tubes and a 6J5 as a phase inverter. This may be combined with the RF-IF additions, above, and a tuning eye tube (6E5 usually). Note that use of rectified line voltage gives a relatively low B+, a major limitation in the transformerless design. The primary market for a "full house" receiver that had all of these features would have been the DC service metropolitan areas, particularly New York City, and that is the general area where most "odd-ball" configurations of transformerless sets can be found today. 3. Postwar AM-FM sets, 1945-up. These were made in two configurations: separate FM front end, and common front end (i.e, RF, IF, mixer, and IF amplifiers. There are many variations on both designs, using 7-pin miniature tubes, loctal tubes, or "hot" octal tubes. The 6SB7Y was a "hot" 6SA7-type tube capable of self-exciting oscillation at FM frequencies, and the 6SG7 a "hot" replacement for the 6SK7. The presence of 88-108 MC FM in a radio always means that it is a postwar set, as this band was not assigned to FM until April, 1945. This ends the "most common" AC-DC section. Now we will consider history, and some of the other designs. Home entertainment radio began in 1920. KDKA in Pittsburgh generally has gotten credit for being the first commercial broadcast station. The two major receiving tubes available at the time with the UX201 and the UV199, as they were called at the time. The UX201, later revised and called 01A was a low mu triode. The V99, as the UV199 came to be termed, was derived from a telephone amplifier triode, developed during WWI. Several manufacturers built sets, but the most predominant in the collector market is the Atwater Kent neutrodyne TRF set using 01A's driving headphones. A standard inexpensive set used regenerative feedback to achieve gain. These were prone to oscillate, squawk, and whistle, and created no end of radio frequency interference, and rapidly lost favor, particularly in high-density metropolitan areas. The first commercially significant superheterodyne receiver was the RCA "catacombs" receiver of 1924. This set used V99's, a 42 KC IF frequency, and a headphone-driving-a-horn "loudspeaker." Both the A-K and the RCA sets required three DC voltage supplies. The A supply (5 volts DC for 01A, 3.3 volts DC for V99) heated the filaments. The B supply, typically 90 volts, provided plate voltage. The C supply, ranging between 9 and 15 volts, and connected as a negative supply, was used to bias the tube grids. RF gain was controlled by a rheostat which controlled the filament voltage. These three voltages were supplied by lead-acid storage batteries, with a Tungar bulb charger for charging the batteries when the radio was not being used. All of the RF stages, and the catacombs superhet local oscillator, were tuned by separate dial knobs. If this sounds like the definition of a kloodge, it was. I had examples of both an O1A Atwater Kent and an RCA "portable" (ran on dry batteries) catacombs set, complete with lead-acid batteries and Tungar charger, at the end of WWII. These sets sold by the thousands, but were obsolete by 1929, and most of them were discarded when their storage batteries wore out. Worth noting that "Philco" is a contraction of "Philadelphia Storage Battery Company." It is also worth noting here that RCA, or "Radio Corporation of America," was not a separate company until 1929, but a patent pool and sales company owned by General Electric, Westinghouse, and AT&T. The phonograph fans will, no doubt, describe how the Victor Talking Machine Company and Radio Corporation of America became RCA Victor. Automatic volume control methods were developed around 1925. AVC, which is synonymous with the term "Automatic Gain Control" (AGC), allowed sets to operate at much higher input sensitivity, and to reduce that sensitivity to prevent overloading in the presence of a strong signal. Methods of tracking RF stages and a local oscillator operating at some difference frequency were also developed in the mid-late 1920's. The final developments needed to build a mains-powered single knob tuning "modern" superheterodyne radio were filaments capable of working on AC without developing hum, a suitable high-voltage rectifier, and a tube with high plate resistance. The first two appeared around 1928 in the form of the 26 and 71A tubes and the 80 rectifier. While these were not the actual "first" devices, they appear in almost all of the early mains-powered radios. The third came about a year later in the form of the UY224 tetrode, later known as the 24A. The 24 also had another recent innovation, the indirectly-heated cathode, which allowed the cathode element of each tube to "float" at a different voltage from the heater supply DC reference. Problems with secondary emission from the 24 were "cured," more or less, by processing the plate material to reduce this emission. This produced the 24A. However, a more permanent fix was to include a third grid to "suppress" the reverse current resulting when plate voltage was lower than screen voltage. The 57 and 58 pentodes were the result. Both have 2.5 volt indirectly-heated cathodes. However, the 58 has a characteristic known as "variable-mu." Actually, with pentodes, one considers transconductance, and what "variable-mu" actually does is to reduce the transconductance as the tube is more heavily biased. The feature is desirable in circuits with AVC. These pentodes showed up around 1931. The pentode power amplifier was also introduced around the same time, with the 47 replacing the 45 in many designed of the 1932-34 era. The last significant development in tube design for AM Broadcast radios was the development of a single tube with two control grids to serve as a self-exciting local oscillator and mixer amplifier. The 2A7, quickly replaced by the 6-volt-heater equivalent 6A7, was the predominant design, and the 6A7 was used very commonly until after 1940. The 6L7 also was introduced fairly early. This is a mixer that is not designed to operate as a self-oscillator, and was used, particularly in communications sets, with a separate local oscillator, until the 1950's. Availability of a single tube for the superheterodyne oscillator-mixer function was essentially the death-knell for TRF designs. Another contemporary development which entered production in 1933 was the 2E5 "tuning eye" tube, which varied a shadow area on a visible target as an inverse function of the control grid voltage. TRF sets were built into the 1950's, but are not very common. They tend to be either very cheap radios for use in metropolitan areas with strong signals or in high end sets where the broad bandpass allowed "high fidelity" (though the AM stations actually only transmit a signal that has 5KC as the 3db half-power point in the modulation). Availability of components for a vibrator power supply made automobile sets operating from 6 volts DC practical. There was a wholesale switch from 2.5 volt heaters to 6.3 volt heaters in 1934. The 2.5 volt heater series of tubes quickly became obsolete. The switch to 6.3 volt 300 ma. filaments was parallelled by development of a two-diode rectifier and an output tube with 25-volt 300 ma. heaters, making series string wiring of the heater circuit practical. These are the 300 ma. heater transformerless sets described above, which date from about 1934. Octal-based tubes enter the picture in 1936. Many of the original designs were built in self-shielding steel envelopes. Metal octal tubes were built with a flat "button" glass seal, which allowed much shorter electrode lead connections. Early glass octal tubes continued to use the older "press" design, with relatively long leads. RF and AF tubes in the original octal series had small top caps for connection to their control grids. It was not until about 1939 that single-ended tubes entered production. Development of a button seal that could be used with glass envelopes allowed manufacture of metal-based "loctal" tubes. These entered production in 1939. At the same time, a cylindrical bulb for glass tubes also entered production, allowing closer spacing between tubes. Experimental FM became a commercial broadcast enterprise in 1940. The original FM band began at 42 megacycles, and production of home entertainment receivers to receive that band began in 1941. The band originally overlapped the experimental television band (later channel 1, 48-54 megacycles). The FM band was reallocated to 88-108 megacycles in the spring of 1945, thus a set with 88-108 capability is postwar. Another "strictly postwar" feature is the 7-pin miniature tube. The 9-pin miniature followed around 1949. A few tubes were "survivors" through the 1928-50 period. The standout among these is the 80 rectifier, which was still being used in new production in the mid-1950's. The 5Y3GT which replaced it is nothing but an octal-based version of the 80. The 2A3 and 45 power triodes, as well as the less-common 6A3 were all used from the early 1930's until well into the 1950's, and there remains today, something of a cult that believes that these triodes are the only audio power tubes worth considering. All of these tubes use filaments cathodes, and the most practical circuits for using them required a separate filament winding, elevated to the 40-60 volts needed to bias these tubes near cutoff. Beam power tetrodes were introduced as octal tubes, although the 807 (very rarely seen in the home entertainment market) continued to use the older large 7-pin base. The principal beam power tetrodes were the 6L6, 6V6, and 25/35/50L6. The 6L6 in a push-pull circuit required more current than a 125 ma. 80 could provide, and presence of a pair of 6L6's with a bigger rectifier means a "high-end" set. Push-pull 6V6's could be supplied by an 80 and provide very adequate audio power of good fidelity to the open-mounted loudspeakers used in virtually all home entertainment equipment until the mid-1950's. Generally, a push-pull power output stage, using any pair of triodes, beam tetrodes, or pentodes, means a quality set with other desireable features, low hum, and good sensitivity. I will note in passing that several "super-6L6" tubes were developed in the 1950's for early "high fidelity" amplifiers. Notable among these are the 5881, KT66, and 7591---the last having several different characteristics, as well as different base connections. -- *********************************************************** Hank van Cleef vancleef@bga.com vancleef@tmn.com ***********************************************************