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Rec.antiques.radio+phono Frequently Asked Questions (Part 4) Revision Date Notes 1.1 Oct 24, 94 Was part 2, now part 3. New material and revisions. 1.2 Dec. 5, 94 Added references to RCA Receiving Tube Manual, corrections and new material. 2.0 Nov. 19, 95 Move from part 3 to part 4 Part 4 - General questions about vacuum tube radios and phonos. ------------------------------------------------------------------------------ FAQ editor: Hank van Cleef. Email email@example.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. I've got a <name of radio>. What's it worth? A. This is the most frequently-asked question in this newsgroup. It is also the most unanswerable question. You can count on a small home entertainment set's being worth $5 or $10 if it is complete but not working, and maybe twice that if it is in good condition and working. Some consoles may be worth $40 or $50, and some high-end "boatanchor" communications receivers may be worth $100 or more if they are restorable. There are a few radios that are reputed to be worth considerably more, but one very significant variable is geographic location (in the US), another is whether the radio is shippable out of an area with a weak market. You can get all sorts of opinions, but in actuality, the only real way to determine a radio's value is to try to sell it and see what you are offered. There are simply too many variables to be able to place any reliable monetary value on antique electronic equipment of any sort. You will soon discover that what is being advertised over here for $500 is available over there for more like $5.00. Good clean electronic equipment restored to good working condition is worth more money, but generally much less than the costs of restoration, if one includes any value for skilled labor in doing the restoration. 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. "Antique Radio Classified" is a buy-and-sell sheet, probably the most accessible true market information available for inspection. 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. This type of set is known as an "All-American Five." 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." Typical schematics for All-American Five radios are given in the RCA RC series and GE Receiving Tube manuals available in reprint from Antique Electronic Supply. Actual production radios of this design had a variety of subtle variations, but the typical circuits in the tube manuals should help you find your way around one of these sets. 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. McMurdo Silver, E.H. Scott (Scott Radio Laboratories in Chicago) and Radio Craftsmen are fairly well know high-end receivers. Many of these last were sold as chassis only for custom installation. 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 SX42, 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," "Regenerative," "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. (1/95) Roy Morgan forwarded me a description of the super-regen by Dan Knierim for inclusion---here it is. >P.S. What's the diff between a super-regen and a regen detector? >I basically understand the regen circuit (gain stage near oscillation >behaving as high Q filter) but I don't recall what the principle of >the super-regen circuit is. And I'm definitely not an RF kinda >guy these days. A super-regenerative detector is a gain stage with positive feedback greater than unity (so that it will oscillate), but with an RC circuit in the plate or grid supply, so that the increased current during oscillation will lower the gain over a period of time proportional to the RC time constant, and finally kill the oscillation. Of course, once the oscillation quits, the current draw goes down, the RC circuit recharges, the gain goes back up, and the oscillation starts again. The frequency of this blocking oscillation is set (by picking the RC time constant) to be well above audible frequencies, but far below the RF oscillation frequency. So how does it detect? Any RF input signal at the frequency of the main oscillation (not the blocking oscillation) will help the main oscillation restart when the stage is coming out of the blocking mode. If the RF input increases, the main oscillation will restart faster, the stage will spend a higher percentage of its time in the oscillating mode, and the average plate current will be higher (where the average is taken over several cycles of the blocking oscillation). Thus the detected audio output is just the plate current run through a low-pass-filter. The average plate current as a function of RF input amplitude is not very linear; in fact it has a 1 / natural logarithm nature to it due to the exponentially rising nature of an oscillator starting up. This makes the audio quality from a super-regenerative detector low, but also acts somewhat like AVC. The pk-pk audio output amplitude is more proportional to the pk-pk RF input amplitude *ratio*. The steep slope of a logarithm near zero also implies a high sensitivity with very small input signals, which is one of the super-regens claims to fame. Some of its many drawbacks are: it makes a racket when not tuned to an input signal (in other words, it also has a high sensitivity to very small amounts of noise, in the absence of an input signal above the noise floor); it is tricky to keep running right; and it radiates like crazy if not preceded with a separate RF input stage. By the way, don't sneeze at regen sets just because they don't have a lot of tubes. I recently read a posting in another group that talked about a 1920's one-tube setup that blew smoke around some fancy radios. Edwin Armstrong, who contributed the straight regen, the super-regen, and FM, was a real genius. 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. (9-95) A few readers have exercised your FAQ editor on the topic of "high fidelity" in the AM band, generally citing the fact that broadcast transmitters built after 1930 were capable of modulating at frequencies above 10Khz. The evidence is clear that notwithstanding transmitter capabilities, there were very few program sources available to broadcasters that were capable of getting modulation above 5Khz to a transmitter. Telephone lines used to transmit network programs had this bandpass limit, as did standard home entertainment and jukebox phonograph records. Transcription recordings were made at 33-1/3 rpm, but were not the "microgroove" technology introduced in 1948. The existence of "high fidelity" receivers in the thirties (either TRF or using wide IF) is well-documented, but all evidence is that these were sold for use with the experimental wide bandwidth stations, particularly in the Northeast US. The vast majority of programming matched the limited frequency response of most receivers. The exception to this would be live music, played either in a studio or in a local concert hall where a telephone link was not required, until the advent of Armstrong's FM links between New York and New England in 1939. Microgroove phonograph records with wide bandpass capability, and magnetic recording, capable of operating beyond 20Khz, were introduced in the late 1940's, allowing stations to use prepared program sources that had a wider bandpass capability. Q. When was magnetic recording introduced? I keep hearing about "tapes" that were made in the 1930's. A. You can rest assured that anything involved with home entertainment was not recorded on magnetic media until the 1947-8 period, and not regularly used for broadcast purposes until around 1952. While magnetic recording, using a magnetic wire, was invented by a Dane, Poulsen, in 1898, the need for a bias to overcome hysteresis distortion was not recognized until the 1930's. Magnetic recording was used for military purposes during WWII, which the Germans being the leaders through much of the period. Wire technology became commercially available in 1946, using a magnetic steel alloy (fortunately, corrosion resistant) wire. Formulations for placing magnetic materials on tape reliably were not available until around 1948, and reel-to-reel tape only became common around 1951, replacing wire. The method for getting response above 10Khz. in early magnetic recorders was simple: move the medium quickly. Webster-Chicago wire recorders move the wire at about 25 inches per second. Early tape units operated at 15 IPS. Worth noting that magnetic recording is not discussed at all in the Radiotron Designer's Handbook, 4th edition (1952). 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. Modern components? But if I put modern components like mylar capacitors in the set, it won't be "original" any more. A. There is a wide range of opinion about use of modern resistors, capacitors, and wire in an old radio. Some feel that disguising modern components in the shells of old wax paper capacitors is important. There are (at least so far as your FAQ editor knows) no clear-cut guidelines on the "looks" of components installed under a radio chassis. Consensus seems to agree that all items that are visible when the chassis is bolted in place should "look like the original radio did." 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. (New 12-94) In the following discussion, there are references to the example circuits shown in the RCA Receiving Tube Manual RC-19, dated 1959. This manual is available in reprint from Antique Electronic Supply. Examples 19-1 through 19-4 in particular show examples of four standard circuits that were used, either identically or with minor modifications, in the majority of the smaller "collectible" radios built from the mid-1930's on. 1. The five or six-tube AC-DC radio with 150 ma. tube heaters wired in series. Example circuit 19-4 shows one of these radios, using 7-pin miniature tubes. This design is colloquially called the "All-American Five" by some of us. The design was first built in 1939, using octal tubes (i.e., 35Z5 and 50L6 in place of 35W4 and 50C5), so it is also called by some a "35Z5 radio" or a "50L6 radio." I list this design first, not only because it dominated home entertainment radio production for over 20 years, but because it is a very simple superheterodyne circuit. If you study this circuit and know what every component's function is, and study an example radio of this design, you'll be prepared to trouble-shoot and repair most post-1935 radios. 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. 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 tube set shown in the example is one of three common sets, having either octal, loctal, or 7-pin mechanical design, but electrically equivalent. Some sets, particularly in the early postwar period, were built with mixtures of tube mechanical types, because of tube shortages and availability, and some sets used more than one configuration during their production runs. The six-tube version had an RF preamplifier, and was more sensitive than the five-tube. Example circuit 19-3 shows the same basic design with an RF preamplifier stage, with tuned output (three-section tuning capacitor). Many of the six-tube versions used resistance coupling between the RF preamplifier and the converter stage (see Diagram no. 3, p. 339, in RC-19, for a resistance-coupled pentode circuit). The six-tube version was often called a "35L6 radio" because a 35L6, 35A5, or 35C5 was used, allowing connection of one more 12-volt heater in the series heater string. 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. Several low-end "boatanchor" communications sets used this circuit, adding multiple tuning coils and provisions for a beat-frequency oscillator. Notable examples are the Hallicrafters S-38, S-41, S-119, S-120, and Ecophone EC-1 series; and the National NC-46 and SW-54. The tube complements are: a. First version, built primarily 1938-40. (note: this design is similar to the 19-4 example, but is its immediate prececessor, so has a few substantial differences, noted below). 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. The major difference between this design and that shown in example 19-4 is the use of a 12A8, which uses a slightly different oscillator circuit than the 12SA7, 14Q7, or 12BE6. The other top-cap tubes are very similar to the single-ended octal tubes which followed, varying primarily in mechanical construction. 12J8 and 12K8 were sometimes used as converters as well. RC-19 unfortunately omits any circuits for these converter tubes. This version uses a series resistor in the heater circuit because the heater voltages do not add up to "near 120"). The proper place for this resistor, electrically, is between the rectifier heater and the power amplifier heater. b. Second version, built 1939-ca. 1960 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 from b., above,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 built 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. In summary, all of the designs identified in items 1 and 2 above either used the circuit shown in RC-19 example 19-4, or fairly simple variations of the design. There are very few radios with these tube complements that vary markedly from the design, which was established around 1932, and licensed to builders through Hazeltine and RCA patent licenses. In general, the sets that deviate markedly from the standard circuit are a few Philcos and Zeniths, and some off-brand sets that may have been marketed through chain stores with chain store brand names. 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. Manual RC-19 shows an example of an FM tuner in example 19-9. Many AM-FM sets "merged" AM capability into the FM tuner design by using a bandswitch in the RF and converter stages, and by connecting IF transformer coils for 455KC and 10.7 Mc. in series, the idea being that the desired frequency will cause one or the other to resonate (high impedance) and the other will appear as a low DC resistance. The bandswich would also select which IF fed the AM detector, and which detector's output was used to feed the audio section. Example 19-9 also shows both the limiter-discriminator and the ratio detector designs commonly used in FM-capable sets. This ends the "most common" AC-DC section. Now we will consider history, and some of the other designs. Example 19-1 in RC-19 shows a later battery-operated portable, using 7-pin miniature tubes. This design was built after about 1934, originally using 5-6 pin tubes in ST-12 bulbs; later, octal or loctal tubes. This circuit also is the basis for most later battery-operated "farm" sets, some of which were built as floor consoles. Close study of the circuit will show its resemblance to the 19-4 example. A very significant difference is the use of filament tubes, and the method of using a back-bias resistor (R10 in the example) to develop grid bias voltage for the output tube. Note also that a different local oscillator circuit is used for the 1R5. This circuit was often used in the "All American Five" design as well, and is not unique to the battery design. Resistance values in example 19-1 have been chosen for operating with a 67.5 volt B battery; otherwise, the circuit is suitable for operating with a 90 volt B battery. Example 19-2 shows a typical three-way portable. The term "three-way" may seem confusing, when the radio can be operated either from the power line or from batteries. However, the fact that it could operated from 110 volts DC as well as from AC lines was considered noteworthy when DC domestic service was common; thus "AC or DC or internal battery" are the "three ways." Note that a modern ricebox radio operating on an internal battery or with an AC adapter is not "three way" as it will not operate from a DC line. Once again, this is the Hazeltine-RCA standard circuit used in examples 19-1 through 19-5, with specific provisions for the three way feature. Example 19-2 also shows use of a double-tuned RF preamplifier. Notable are the use of series connection of the receiver filaments, provision of a rectifier, and a changeover switch. In practise, many manufacturers provided a dummy line-cord outlet inside the receiver. Plugging the line cord into this outlet would mechanically actuate the changeover switch, placing the receiver on battery operation. When studying this circuit, note in particular the order in which the tube filaments are wired, and the use of an 1800-ohm resistor (R14) in the 3V4 filament circuit to provide a shunt-feed balance current. The order of connection of series-wired heaters and filaments is significant in series-string sets. In this case, the 3V4 is connected to the high end to provide grid bias for operating, and the shunt resistor provides some of the plate and screen currents for the tube. The rectifier circuit shown is typical, although three way portables may use a 35Z5 or a selenium rectifier. DC output from the rectifier is around 120 volts, depending on the rectifier used, which requires a large dropping resistor to feed the receiver filaments. Note the use of two large electrolytic filter capacitors, C28 and C29, connected to either end of the 3V4 filament. Small filament tubes require "clean" DC power, thus these two capacitors filter out both residual ripple from the half-wave rectifier and audio-frequency variations caused by varying power draw of the power tube. This circuit arrangement is critical. If any filament opens, one or both of those capacitors will charge up to the rectifier output voltage. Also, the design assumes that the rectifier is part of the voltage-dropping string, and 1.5V filament tubes are limited in their ability to handle out-of-tolerance filament voltage. The circuit shown in figure 19-3 for an AC-operated receiver is the same as that in figure 19-4, with several upscale features, and resistance values selected for operation at 250 volts B+ rather than 120. Note that the circuits for the 6BE6 converter, 6BA6 IF, and 6AV6 detector-audio stages have the same configuration as those shown for those three stages in figure 19-4. An additional 6BA6 RF preamplifier is provided for higher gain and better selectivity. A pair of 6AQ5 tubes provides push-pull output. The second 6AV6 placed ahead of the lower 6AQ5 grid circuit inverts the audio signal for grid drive, with "approximately unity gain," determined by the tapped grid leak (470K/8200 ohms) in the top 6AQ5 circuit. This particular circuit is a classic example of older home entertainment engineering, and there is much to criticize in its selection over the use of a twin-triode balanced paraphase using a 12AX7 or a 6SN7. Why was it chosen? Habit, probably---it was a good choice for 1932. The main feature of this set which differs from AC-DC configuration is, of course, the use of a power transformer and a 5Y3 full-wave rectifier. The configuration of the rectifier circuit was one of the earliest and most durable circuits in the history of tube-type home entertainment radio. This later configuration uses a 5Y3 instead of an 80, has larger filter capacitors (20 mfd rather than 8 or 10 mfd), and a resistor in place of an inductance between the two filter sections. Older radios most often used a speaker field coil between the two filter sections, partly because Alnico magnets were not available until the late thirties, and partly because inductance at this point compensates for using smaller capacitance values to get good filtering. Note the configuration of the screen circuit for the 6BE6 and two 6BA6's. All three screens are connected together. This is poor design, and likely to cause parasitic oscillations. The circuit in figure 19-4 also shows the screens connected together, but in this instance, there are only two screen, in stages that operate in opposite phase, so any coupling between the two stages has a negative feedback effect. Older radios: 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. 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 filament 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 5-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. The various oscillator-mixer tubes used can affect a radio's ability to perform, particularly on shortwave bands. Historically, the first such tube was the 7-pin 2A7/6A7, followed by the octal-based 6A8, all using the same pentagrid construction and circuit. These operated well on AM broadcast, but had severe problems dealing with higher frequencies. While they were commonly used (particularly the 6A8) into the late forties, they generally give very poor performance on shortwave bands above 10-15 Mc (40 meters). The 6L7 was developed as a mixer to be driven by a separate local oscillator to overcome some of the limitations of the 6A8. The separate-section 6J8 and 6K8 were developed to provide better high-frequency performance without need for a separate local oscillator. These tubes can operate well up to about 25 mc. The loctal versions (7J7, which is the same as a 6J8, and the 7S7, which is a higher-gain 7J7) would operate over 30 mc. (10 meters.). The final version was another layout of the 6-grid "pentagrid" design, the 6SA7. The 6SA7 would operate, with the inner section as an oscillator, up to about 27 mc. The 6SB7Y octal, 6BE6 7-pin miniature, and 7Q7 loctal all would operate satisfactorily up the commercial FM frequencies. A common method for getting better high-frequency performance was to use a separate local oscillator with a 6L7, 6SA7, or 6BE6. Glow-discharge voltage regulator tubes were commonly used in high-end communications designs to regulate B+ to the local oscillator, giving improved stability to the circuit. For serious shortwave listening, you should avoid a set with a 6A7 or 6A8, and consider one with a separate local oscillator (typically a 6C5, 6J5, or 6C4) and a voltage regulator tube.