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- From: vancleef@netcom.com (Hank van Cleef)
- Subject: Rec.antiques.radio+phono Radio General Questions(FAQ: 4/9)
- Message-ID: <antique-radio+phono-faq-4-845766911@netcom.com>
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- Date: Sat, 19 Oct 1996 23:15:24 GMT
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- Posted-By: auto-faq 3.1.1.2
- Archive-name: antiques/radio+phono/faq/part4
-
- 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 vancleef@netcom.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.
-
-