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Servicing Test Instruments: Peter and the Pilot Light
October 1952 Radio-Electronics

October 1952 Radio-Electronics

October 1952 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Incandescent pilot lights are definitely a thing of the past; LEDs are the ubiquitous choice these days for obvious reasons (low cost, low current, long life). Nonetheless, there is still lots of equipment in service that do use them. In 1952 when this article appeared in Radio-Electronics magazine, incandescent (and neon) lamps were standard in test gear, radios, televisions, tape recorders, record players, kitchen appliances, cars, industrial machinery, and many other everyday things. Electronics repairmen needed to deal with them on a regular basis. Although not technically sophisticated, knowing a little something extra about the little bulbs could be quite handy. One tidbit if useful info in this article is how the color of the little glass bead that mounted the filament to the base was often a clue to the current draw of the lamp at nominal operating voltage. I have noticed the colored beads, but never knew it had any specific significance. Note the pun at the end. 

Servicing Test Instruments: Peter and the Pilot Light

Servicing Test Instruments: Peter and the Pilot Light, October 1952 Radio-Electronics - RF CafeBy Harry A. Nickerson

That's a wreck of a heck of a set," said Peter.

Peter, is my right-hand man (or rather boy). He dusts off the chassis, makes repairs strictly as directed, handles the screwdriver and soldering iron almost as well as a girl set-assembler in a factory. He's also a pump, that pulls up my radio wisdom or lack of it and sometimes stores it in his small reservoir of a brain.

"Look at the dial - there's not even a pilot light," he continued. "How would you take care of that?"

Sure enough, there was no pilot-light socket or bulb. The set was an old a.c.-d.c. using 0.3-amp tubes; the line cord had been completely removed.

"Well," I said, somewhat cautiously, "you take down that radio catalog and look up pilot lamps." He did, and this is the rather lengthy list:

"Now," said I, "In practice you seldom see any except 0.15 and possibly 0.25-ampere bayonet-base pilots that operate around 6 volts, but in the older sets there were mostly screw bases, and in one old set, a 4-tube a.c.-d.c. I repaired once, there was a plain 0.50-ampere pilot in series with the string of 0.3-amp. tubes."

"What is that No. 51 and 55 c.p. - certified public stuff?" queried Peter. I explained that years ago electric lamps were usually classified according to candle power.

Being in a generous mood, I explained further that the filament inside the glass bulb was supported near its base by a bit of colored glass called a bead. "You can usually depend on a brown bead to mean a 0.15-amp and a blue bead to mean a 0.25-amp filament, while a white bead very likely is a 0.50-amp type, although if you are replacing a pilot light, it is better to put in one of the same number as marked on the brass base, rather than depend on the bead."

Peter was not interested. "Let's get back to the set! I can see you putting in a 6-8-volt bulb on a 6-volt a.c. set where all the tubes are in parallel; you just put the pilot in shunt with the tube filaments; but how do you work this resistance line cord and pilot light business with a.c.-d.c.?"

"Well, Peter, I think I've explained how to calculate a line cord's resistance for you eleventeen times, but here goes again. You add up the total voltages of the tubes in series, subtract it from 117 (the line voltage) and divide what you have left by the amperage flowing through the string of tubes in series to find the resistance. Now if you want to put in a pilot, you put it in series with the tube filaments too, but to do it right you find the resistance of the pilot as shunted by a proper resistor and make the line resistance cord that much less ohms value. Suppose you have two 25-volt and two 6-volt tubes; they add up to 62 volts. Subtract 62 from 117 and you get 55 volts. That is the voltage drop. If you have 0.3-amp tubes, you divide the 55 by 0.3 and get about 180 ohms for your line cord. If you have a 6.8-volt, 0.15-amp pilot shunted by a 25-ohm resistor, as frequently is the case, their combined resistance in ohms will be found by figuring the resistance of the pilot as 6.8 divided by 0.15, or about 45 ohms; then since the resistance of two resistors in parallel is equal to their product divided by their sum,

(45 x 25) / (45 + 25) = 16 ohms

So you really ought to use a resistance line cord of 180 ohms less 16, or about 165 ohms value, with the pilot bulb and its shunt resistor also in the circuit."

"But," protested Pete, "if you say that the resistance of the pilot lamp is 45 ohms, and it burns 0.15 amp, why don't you just put a 45-ohm resistor across it? Then the lamp will get half the 0.3 amp current, and will light up O.K. without all the figuring. Why do they want to use a 25-ohm resistor?"

"There's a good reason," I told him. "Actually, if you did it your way, it would work out all right, at least for a while. But the pilot would be burning at full brightness, and would flare up every time you turned the set on. So they make the resistor take a little more than half the current. Then when the set is turned on, the light does not go up much higher than its rated voltage, and when the filaments warm up, it settles down to about 5 volts. That's bright enough, and the light lasts longer.

Besides, if it does burn out, the voltage across the 25-ohm resistor only goes up a volt or two, and the set works just as well as with the light in circuit. With 0.3-amp tubes, use about a 2-watt resistor in shunt with the pilot."

"I just noticed this old hunk o' junk doesn't have any glass dial or place for a pilot light to shine through," observed Peter.

"There are two ways at least you can fix that," I told him. "Hang a dial light from the top of the speaker, running two insulated wires through one of the holes on the edge of the cone frame, and let the light hang in the middle of the speaker cone. It will shine through the grille cloth and at least show that the set is turned on. Or drill a hole through the metal dial, put in a rubber grommet just big enough to let through a pilot light socket at the back of the dial, then push in a bayonet type No. 51 small round dialite, which will project a bit at the front of the dial."

"These jobs with the pilot hooked in with a ballast tube puzzle me. What do the numbers on ballast tubes mean?" asked Peter the inquisitive.

"They aren't uniform," I answered. "A lot of them are marked to kill three birds with one stone, but we have some General Electric ballasts that we have to look in the catalog to tell what they are. The three-bird kind might be marked K55B or BK55B. The K or BK means that a 0.15-amp filament lamp is to be used with the particular ballast tube; the 55 means the voltage drop (for a 0.3-amp tube string the hot resistance of the ballast would be around 180 ohms); and the B stands for the wiring of the resistances inside the tube; so you know what prongs of the ballast are to be connected to the tubes and which to the pilot. Most of the octal-base ballasts have the pilot connected across pins 7 and 8, and the resistance of the ballast in series with the tubes (besides that across 7 and 8) would be the resistance from pin 3 of the ballast to pin 8. The letter B stands for a single pilot, other letters stand for one or more pilots and their method of connection across pins of the ballast. I generally look it up in an old "Mye Technical Manual" which has a lot of good stuff in it that I never get around to memorizing. An L55B would be intended for use with a 0.25-amp; M55B probably with a 0.20-amp pilot. Sometimes a fourth wire in a line resistance cord provides a shunt for a pilot, but somehow or other I don't use that. Besides the low-voltage pilot lights, of course some old sets used small 110-volt pilots. At least one old-time Colonial had them, and some very old RCA's. You still see some 4-tube a.c.-d.c, sets with two 110-volt (about 7-watt) lights of very small size in series. Usually these are cheap Jap bulbs that would have burned out very soon, if only one had been put across the line; but by putting them in series, they burn a little dim but last a lot longer. There's a 4-watt candelabra-base 117-volt pilot, too."

"Any other uses for these nawsty little bulbs?" asked Peter.

"Plenty of shops use them as fuses in the center-tap of the bench power supply. When there's a short, somewhere they usually blow before something else does. Good thing for an experimenter, too, and almost necessary in the B-negative lead if you have a setup for working on battery radios. And laboratories use them where they need a nonlinear resistance. One of the original audio volume expanders used a couple of them across a special output transformer. With weak signals, they took a lot of current. As signals got stronger, they lit up a little and their resistance got higher, so more of the signal went through the voice coil. And some high-class audio oscillators stabilize their output by using a 117-volt pilot lamp in a cathode circuit, so it supplies negative feedback that rises with the output. And it seems to me I remember a Crosley that had a pilot lamp in series with a capacitor somewhere in an audio circuit as a rather successful tuning indicator - to show when the station was exactly tuned by the brightness -" "O.K., O.K." said Pete. "Boy, when you start letting your knowledge loose you can sure pilot on-"

Which was about the usual thanks I get from Peter.



Posted December 9, 2021

Amplifier Solutions Corporation (ASC) - RF Cafe

About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

1996 - 2024


Kirt Blattenberger,


RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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