Have you ever heard of a 'globar' resistor? They have been around
since the early days of radio and were used, among other things,
to protect vacuum tube heater elements from burning up due to high
inrush current when first turned on. Globars have a negative temperature
coefficient (NTC) of resistance so
that, opposite of standard carbon and metal film type resistors,
they exhibit a higher resistance when cold than when hot. Mac and
Barney discuss their use in this episode of "Mac's Radio Service
Shop." You might be more familiar with the name 'thermistor' for
such devices, but globars are unique elements in that their construction
from non-inductive ceramic material makes them useful at high power
levels and high frequencies. Globar appears to now be owned by Kanthal
Globar). Interestingly, Keysight Technologies
(formerly Agilent Technologies, formerly
Hewlett Packard) has an old
Educator's Corner document where the student plots the resistance
of a globar resistor against its temperature.
Mac's Radio Service Shop: Barney Talks A.C.-D.C.
By John T. Frye
Mac peered through the open door of the service department and
saw the lanky form of Barney, his apprentice, silhouetted against
the bright September morning sunlight that was pouring through the
front windows of the service shop.
"Oh, Mr. Gallagher," Mac called, "as soon as all of the pretty
little high school girls have passed on their way to school, would
it be too much to ask you to come back here and help me get out
a batch of these a.c.-d.c. sets?"
Barney heaved a big sigh as he reluctantly turned away from the
"Only a man without the least shred of poetry in his soul could
say a thing like that," he muttered bitterly. "To ask me to turn
my thoughts away from - well, from what I was thinking about - to
a consideration of a.c.-d.c. receivers is to suggest a transition
from the sublime to the ridiculous."
"Spoken like a veteran service technician!" Mac applauded. "Cussing
the a.c.-d.c. sets is the badge of the radio man who has really
'arrived.' It serves the same purpose as the complaint of the kid
who has just returned from his first year away at college to the
effect that the old home town is dead and ought to be buried. Sort
of shows you are sophisticated, you know."
Barney looked sharply at his employer. "You trying to pull my
leg?" he demanded suspiciously.
"Perish the thought!" Mac said blandly.
"Do you like a.c.-d.c. sets?" Barney wanted to know.
"Well, I like to eat; and so-called a.c.-d.c. receivers furnish
me with a very sizable portion of my bread and butter."
"Why do you say 'so-called' a.c.-d.c. sets ?"
"It was a poor choice of words. They will work on either 110
volts of alternating or direct current; but not many of them have
an opportunity to function on d.c. The main idea in the original
design of the circuit was to get rid of the power transformer and
so reduce the bulk, weight, and cost, all in one fell swoop. It
happens, though, that subtracting parts is not good sales psychology.
The salesman likes to be able to tell the customer about the 'extra'
features that have been added. That is why, instead of saying 'This
receiver has no transformer,' he says, 'Now here is a receiver that
will work on either alternating or direct current:'"
"Of course," Mac mused, "here in the United States, the likelihood
that the ordinary individual would ever have occasion to plug his
set into a 110 volt d.c. main is about as remote as that of his
using his electric razor to take the fuzz off peaches; but the fact
remains that it could be done."
"I notice that the sets we get in that use 25Z5's, etc., seem
to have less tube trouble than the ones using either the 35Z5 or
the 35W4 strings. Why is that? Can't we make tubes as good now as
we used to make them?"
"I'm glad you noticed that," Mac said. "It is encouraging to
know there are rare occasions when an object does not have to be
edible or wear skirts to attract your attention. In the first place,
you must know that it is the heavy first surge of current that flows
through a cold filament that does the most damage. When the filament
is cold, its resistance is only a fraction of what it is when the
tube is at it proper operating temperature. The heavy current that
flows when the set is first turned on and the magnetic fields accompanying
such currents cause the loops of the filament to writhe inside the
cathode sleeve and produce fractures of the filament wire."
"Yeah, but the 25Z5 draws 300 mils of filament current, while
the 35Z5 and the 35W4 only take 150 mils," Barney pointed out. "It
looks like the 25Z5 filament would be the one doing the most wriggling."
"Quite true, but remember the heavy-current type always uses
a resistor in series with the filament string to make up for the
difference between the total tube voltages and the line voltage.
Sometimes this resistor is in the line cord; sometimes it is in
the form of a ballast tube inside the set; but it always serves
to remove the shock of that first current surge from the tubes.
When the set is first turned on, most of the voltage appears across
this resistor; then, as the tube filaments gradually warm up and
increase their resistance, the voltage division gradually shifts
to the proper proportion between the line resistor and the filament
"Which is better: the line cord resistor or the ballast tube?"
"Each has its advantages. The line cord resistor serves to put
the heat dissipated by the voltage dropping resistor outside the
cabinet, and this increases the life of the filter condensers and
other units adversely affected by heat. On the other hand, the line
cord will not take too much abuse without the resistance element
breaking. What is more, we always have the amateur 'fixer' who decides
he does not need all of that long line cord on his receiver and
proceeds to cut off two or three feet of it; and then he wonders
why his tubes burn out so quickly. While the ballast tube releases
all of its heat inside the cabinet, it is usually a better deal
if the set is to be carried about much or plugged in and out quite
"Would you say the fact that no series resistor is used with the
150 mil type of tube accounts for their shorter life ?"
"Not altogether. These tubes are an improvement over the former
types in the particular respect that they have more efficient filaments.
Since they draw only half as much current, and since the wattage
consumption is equal to the product of the voltage and the current,
a string of 150-mil tubes will twirl the watt-hour-meter only half
as fast as a 300-mil string. On the other hand, the filaments of
such tubes are smaller, or at least more fragile, and they are subject
to filament failures more often than the older tubes.
"But you are the fellow who is knocking the a.c.-d.c. receivers,"
Mac broke off. "What else is wrong with them outside of the fact
that they help us sell a lot of tubes?"
"Well, their filter condensers do not last very long, it seems
"That's true in a lot of cases," Mac agreed, "but you can usually
spot the reason. Quite often it is poor design. The filter condenser
will be jammed up against a hot resistor, rectifier, or output tube,
and the moisture will be literally baked out of it because the temperature
rating of the condenser manufacturer is greatly exceeded. Another
"common fault is to mount a filter condenser in a spring clip with
too-tight jaws. As the wax of the condenser container is softened
by heat, these spring jaws sometimes pinch the condenser nearly
in half, causing various kinds of damage to the foil, oxide coating,
and connecting tabs."
"What's the cure?"
"Always mount a replacement condenser in as cool a spot as you
can find. This usually means at the bottom of the receiver chassis
and as far away from the heat-radiating elements as possible. Make
sure the condenser is mounted securely in place, but also make sure
that it is not gripped by any clamp that will distort its form.
See that the set has all of the ventilation you can give it without
actually baring any portions of the 'hot' chassis to possible contact
with the owner's hands."
"Well," Barney thoughtfully conceded, "when everything is taken
into account, perhaps the a.c.-d.c. set does a pretty good job after
all. Such sets are comparatively cheap to purchase, and most of
them see lots of action without receiving too gentle usage. I suppose,
though, that they are on their way out now that television is here."
"Never think it!" Mac said as he picked up a service sheet he
had been reading. "Here is some dope on a new TV receiver using
an improved voltage-doubling, transformerless type of power supply
with the receiver tube filaments connected in series strings. The
voltage-doubling circuit yields all of the "B" voltage needed, and
a new type of resistor, called the "Globar," is used in series with
each string of filaments to remove the curse of the high initial
surge of current.
"A globar resistor has a negative resistance characteristic that
is just the opposite of that of a tube filament. Its resistance
is highest when it is cold. As it warms up, its resistance will
decrease to less than one-fifth of its cold value. That means that
the current through the filaments of a string of tubes in series
with such a resistor will remain practically constant during the
complete warm-up cycle. Such a system is actually easier on the
filaments than heating them with a transformer."
After a little pause, Mac summed up what he had been thinking:
"I think the root of the whole matter is that the a.c.-d.c. circuit
was first employed in an attempt to make a cheap receiver, and gradually
the term 'a.c.-d.c.' came to be used as a synonym for cheap construction
and pinch-penny engineering. This really does an injustice to the
transformerless type of circuit, because with modern tubes, selenium
rectifiers, Globar resistors, etc., you can do just about anything
with this type of circuit that you can do with one employing a transformer,
and you can usually do it on a smaller chassis, with less weight,
and at a lower cost. Communication receiver engineers have proved
"Say no more!" Barney interrupted with an upraised hand. "Let
the defense rest. Its case is won. From now on, I would no more
think of saying anything against an a.c.-d.c. set than I would of
criticizing Margie's appearance in a sun suit; and that is just
as near perfection as you will find anywhere!"
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of no other than John T. Frye,
creator of the Carl and Jerry series that ran in
Electronics for many years. Mac's Radio Service Shop began life in
Radio & Television News magazine (which
itself started as simply Radio News), and then changed its name to Mac's Service
Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his eager,
if not somewhat naive, technician assistant. 'Lessons' are taught in story format with dialogs
between Mac and Barney.
Posted August 20, 2015