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(Trevor Gale) by using this link.
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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.
You can E-mail the host of this FAQ
(Trevor Gale) by using this link.
on the Dutch Internet service provider XS4ALL.
Click on this link to go back to the FAQ index.