Frequently Asked Questions (part 2)

All the FAQ sections can be downloaded from the Web at this URL:

Part 2 - General Questions on valve/tube radios.

Original FAQ editor: Hank van Cleef, Gerard Tel (; now hosted on Trevor Gale's site (mail: can be sent using this link)
. Frequently Asked Questions (Part 2)
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
3.0 Dec. 10, 99 HTML version of this part editted, now part 2. Also some of the set-dating / age information moved to Part 3.

Part 2 - General questions about vacuum tube radios and phonos.

Original FAQ editor: Hank van Cleef. Email

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. This is especially the case where either overload / fault currents have been passed due to some other (repaired) fault, or if other extreme thermal temperatures were around during the life of the set.
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.

A brief mention from the original FAQ about some valve/tube types:
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.

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