Amiga Plus 1995 #2/Amiga Plus CD - 1995 - No. 2.iso/internet/faq/englisch/rec.antiques.radio&phono-radio

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Archive-name: antiques/radio+phono/faq/part3

Rec.antiques.radio+phono Frequently Asked Questions (Part 3)

Revision  Date            Notes

1.1    Oct 24, 94    Was part 2, now part 3. New material and
revisions.
1.2    Dec. 5, 94    Added references to RCA Receiving Tube Manual,
corrections and new material.

Part 3 - General questions about vacuum tube radios and phonos.
------------------------------------------------------------------------------
FAQ editor: Hank van Cleef.  Email [e-mail], [e-mail]

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.  Most people who
repaired radios in the forties and fifties could draw the schematic for
any of these radios from memory----it's a case of "seen one, seen 'em
all."  This particular radio has a grand total of 9 resistors (including
volume control), a whopping 14 condensers (including the tuning
condenser as one), three transformers, one oscillator coil, a loop
antenna, a loudspeaker, and a panel lamp.  Add the five tubes, and that
amounts to the whopping sum total of 35 electrical components, and if
you want to insist on including the chassis, five tube sockets, cabinet,
panel lamp socket, and cabinet, we are still talking about 50 parts.  No
wonder they sold for $4.98 in 1940.  If it has value, it is for its case
and mechanical configuration.  As a project radio to learn radio repair
and restoration, an AC-DC 5 or 6 tube table set is probably ideal.  Most
of these sets need one tube (burned-out heater), new electrolytics and
paper capacitors to get it "working like new."

Q. I just looked at a Radio Wire Television model B45.  It has 13 tubes
and two loudspeakers.  I couldn't see all the tubes but I saw a 6H6, two
6L6's, two 5Y3's, and a bunch of metal tubes with top caps.  It
has three bands, two shortwave, and a phono, and is in a custom-built
plywood cabinet.  What can anyone tell me about this set.  The radio
works, but not well.  The owner wants $100 for it.  Is it worth it?

A. This is the type of radio you should be asking questions about.  The
radio itself is a "class act"---high fidelity, 1938 style.  It's the
same manufacturer listed in the question above, and shows that
"brands" could range from absurdly cheap to top quality.  It also is
typical of the radios that justified service shops paying good money for
Rider's manuals over the years.

As a "collector" radio, it's a difficult one to put dollar value on.
But as a museum piece, an example of what a high-end thirties radio was,
it is a class act.  For those who have Rider XVIII, look at Radio Wire
page 18-8, and notice that only the schematic and a few notes are
published, some ten years after the radio was made.  (confession: I owned
one of these from about 1948 until sometime in the sixties, and it was
my first really hard-core restoration project.  It also was my "hi-fi
amplifier" for many years).  If you want an example of high tech
history, it's well worth the $100, and if you restore it, you'll find
that quality is a lasting thing.  But restoring a set like this can be a
major project and take a good deal of skill.

Other "high tech" radios that are more readily identifiable by brand
name are the Farnsworth Capehart sets and the 2-chassis Magnavoxes.

Q.  I saw a little table radio with a very pretty plastic case, but the
owner want hundreds of dollars for it.  The case looks like marble, but
the radio inside is just another of those 35Z5 and 50L6 five tube jobs.
Why does the owner think its worth almost a thousand bucks?

A.  Well, you've stumbled on the collectors' hot item of the nineties,
the "Catalin" case.  The reason the owner thinks it is worth this much
is that the collectors' market seems to be willing to pay these prices
for a catalin case.  Whether it will continue to do so is open to
question.  It is difficult, in a FAQ item, to explain the whimsies of
the "collector" market, because these tend to change.

Q.  Well, if a low-tech radio is worth hundreds of dollars because of
its case, and a high-end console with tremendous sensitivity and a
powerful amplifier with good fidelity is worth a lot less, what's the
correlation between price and value?

A.  There isn't any.  Some radios, such as the Atwater Kent TRF sets and
the RCA catacombs superhets are valuable because they are relatively
rare today, and represent technological history.  An old communications
receiver, such as the Hallicrafters 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.  (revised 12-94)
(This item originally indicated that bandwidth was limited to 50 cycles
to 5 Kc by program sources, which provoked considerable discussion in
the newsgroup).  Based on measurements on a 1949 Magnavox AM-FM-SW-phono
(3-speed) console with 4 speakers, it is unlikely that any of the
packaged high-end consoles had sufficient bandpass to be called "high
fidelity" in any modern term.  What you will get from one of these sets
is hum-free low-distortion audio that makes very pleasant listening.
A good floor console or cathedral radio, operating properly, should have
good-quality audio for AM broadcast listening.
(update 1-95)  Ghirardi "Radio Physics Course" (1932) mentions that most
radio stations of the time had much more "high-fidelity" capability in
their studio and transmitter electronics than was available either in
most radios or in most program sources, so typically transmitted a
narrower bandwidth for most programming.

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.

--
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Hank van Cleef  [e-mail]  [e-mail]
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