September 1956 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954  April 1985. All copyrights are hereby acknowledged.

Here is an interesting
article that was probably much needed in its day (1956 Popular Electronics
magazine) when many hobbyists were building and repairing their own electronic equipment.
Being able to measure and identify unknown transformer winding parameters is key
to both troubleshooting and determining whether a particular transformer is applicable
for your need. Even professional repair shops often scavenged transformers from
scrapped chassis for use in equipment brought in for service. The process can be
extended to multiple winding transformers. Now you, too, can confidently tackle
the unknown transformer and determine its construction.
Identifying Salvaged Transformers
Every Experimenter accumulates various and
sundry audio output transformers  the type that's used to match a power tube, or
tubes, to a loudspeaker. He salvages them from old radio receivers with the idea
that one day they'll be useful. Sooner or later our hero finds that he has neglected
to tag or otherwise identify the transformers he has stowed away.
The equipment needed to identify a transformer consists of: an a.c. vacuumtube
voltmeter, such as the Heathkit Model AV2; a small 6.3volt filament transformer,
such as the Triad F13X (any 1ampere unit will do as well); a 10,000ohm wirewound
potentiometer; and a power cord and plug.
Set up the circuit shown in the schematic diagram. If you are unable to tell
by inspection which two of the four or five leads coming out of your transformer
are the secondary connections, measure the d.c. resistances between the various
leads. The two leads (of a 4 or 5lead output transformer) that have the lowest
resistance are the secondary leads.
If you're checking a 4lead transformer, the two remaining leads are the primary.
If you're checking a 5lead transformer, the two remaining leads that have the highest
resistance are the two plate leads; the third lead is the centertap connection.
A 4lead output transformer is made to match a single tube to a speaker voice coil.
A 5lead transformer matches pushpull tubes to the voice coil.
After you have made the few simple connections, set the potentiometer to where
the full 10,000 ohms is in the circuit, and plug the line cord into the 117volt
a.c. socket. Connect your v.t.v.m. across the secondary of the output transformer.
Then, adjust the potentiometer until your meter reads exactly 1 volt.
As soon as this adjustment has been made, set your meter to a higher range (the
100volt range will usually be the right one), and immediately switch the meter
leads to the primary of the output transformer. Measure the voltage across the primary
and write it down. Multiply this number by itself, and then multiply the product
by the voicecoil impedance of the speaker you want to use. Your final figure is
the load impedance into which the plate of your power tube will work when you're
using this particular transformer and speaker.
Example I:
Suppose, after setting the voltage across the secondary
of our output transformer very carefully to 1 volt, we read the voltage across the
primary and find it to be exactly 39.5 volts. We write this down and multiply it
by itself (39.5 X 39.5) and obtain the product: 1560.25. The voicecoil impedance
of the speaker we want to use happens to be 3.2 ohms, so we multiply our 1560.25
by 3.2, and get 4992.8. This is close to 5000, so we'll call it 5000 ohms  the
right impedance to match the plate of a 6V6 or a 6AQ5 power tube.
Example II:
Suppose we have a 5lead output transformer and,
after carefully setting its secondary to 1 volt, we measure the voltage across the
two primary plate leads and find it to be 42 volts. We have an 8ohm speaker, so
we multiply 42 X 42 X 8 and get 14,112. Call it 14,000 ohms. This will match a pair
of 6F6's, or 6K6GT's, or 6AR5's in pushpull to the voice coil of our 8ohm speaker.
Proper plate loads at various grid bias and plate potentials for a number of
power tubes can be found in the RCA Receiving Tube Manual, or the tube section of
The Radio Amateur's Handbook published by the American Radio Relay League (A. R.
R. L.). Frank H. Tooker
How It Works
It is easy to identify the impedance ratio of an output transformer by means
of two useful ironcore transformer formulas:
(Ns/Np)^{2}= Zs/Zp and Ns/Np = Es/Ep
in which Ns represents the number of turns on the secondary of our output transformer;
Np, the number of turns on the primary; Zs, the impedance to be placed across the
secondary; Zp, the impedance reflected into the primary; Es, a voltage impressed
or induced across the secondary; and Ep, the voltage impressed or induced across
the primary.
In this circuit. a filament transformer is used to drive the secondary of the
unknown transformer. The voltage is carefully adjusted and measured by an a.c. vacuumtube
voltmeter. Once the secondary voltage is known, the v.t.v.m. is used to measure
the voltage induced in the primary. Through the formula above, the nominal primary
load impedance can be found.
Posted February 23, 2023 (updated from original
post on 11/2/2016)
