Identifying and testing Transformers.

This page is a result of a suggestion sent in by Gordon who asked for a page on the subject of transformer lead identification. He suggests that if the leads are too faded by the dust of time that the transformer end bells may be removed to expose insulation that has not been exposed to the dust and other ravages of time. Be careful not to lose any of the insulation which may come loose from the end bells. The individual laminations are insulated from each other and must remain so if the transformer is to continue to function. The long screws that go through the laminations are often insulated from them by a paper or plastic tube. The tube may stay with the laminations or it may come out with the screw.

If the transformer is an open frame type there will be no end bells to protect a portion of the wire insulation. Using a pair of wire strippers will sometimes expose a bit of the original insulation color on the newly cut end.

Power Transformers.

North America uses 120 volt 60 Hz power and the transformer lead colors are almost universal. I know that the line voltage in Europe is 240 volts and I wouldn't be the least bit surprised to learn that transformer lead colors are different. Since I don't know I can't say what the lead colors on European power transformers might be. If you reside in that part of the world you had best find information that applies to the transformer's country of origin.

In power transformers the lead colors are as follows.

Black = 120 volt primary.
Green = 6.3 or 12.6 volt secondary, high current.
Green and Yellow striped = 6.3 or 12.6 center tap. May not be present.
Yellow = 5 volt secondary, high current for 5Y3 or 5U4 filaments.
Red = High voltage secondary.
Red and yellow striped = High voltage secondary center tap.

You should confirm each winding by an ohmmeter test. Secondary windings that have a high current rating, from 2 to 16 amps will be heavier gauge wire.

You may find one wire that seems to be odd man out. It may be white, green, or even bare. This lead connects to an electrostatic shield between the primary and secondary. Tests should confirm that it is not shorted to any of the windings. When the transformer is installed in a home brew project this wire should be grounded.

Variations on the Theme.

Primary leads may be brown. Some transformers have dual primaries that may be connected in series for 240 volt operation or parallel for 120 volt operation. The leads for these will be basic black with red, yellow, or white stripes. An ohmmeter will have to be employed to figure out what is what.

The transformer may have other leads such as blue and/or blue with yellow stripes. These may be a different voltage heater winding, or a winding that is lower voltage than the high voltage but still a hundred volts or more. These will have to be figured out by powered testing, see below.

Sometimes leads of unexpected colors may be taps on the high voltage winding for bias or lower B+ voltages. The ohmmeter test will show what leads are connected to what. When testing with an ohmmeter be sure to try all possible combinations no matter how unlikely they may seem. The transformer may have a fault that could present a danger to the user. For example a primary to 6.3 volt secondary short. Such transformers may still be used but the leads to the offending winding should be cut off inside the transformer bell so no one can ever try to use them.

Powered Testing.

I wish to say in the strongest possible terms that you should avoid testing a power transformer by connecting it directly to the power line. You will have the transformer sitting on your bench with its leads hanging out and positioning themselves where ever they want to. The leads have bare ends and some of them will have lethal voltages on them. You have clip leads connected from the transformer leads to your meter and they will fall off and leads will short in such a way as to do the most dammage possible. It's one great big disaster waiting to happen. I know what you must be thinking, yes, I have done it but it is dangerous as hell and you should do as I say not as I have done in the past.

The best way to test a transformer under power is to juice it up from the secondary of a 12 volt filament transformer. You can use any low voltage transformer but the 12 volt one is most convenient because all of the voltages will be 1/10 of normal operation.

Before testing under power you should have done thorough ohmmeter tests and drawn a diagram of the transformer. If you haven't done so stop and do it right now.

Dual Voltage Primaries.

A dual voltage primary requires some special attention. Connect just one of the primaries to the 12 volt transformer to complete the tests below. After you have measured all the voltages connect the two primaries in series across the filament transformer. If they are in the correct phase each voltage from the secondaries will be half of what it was. If the phase is not correct the voltages will be very very small. If the phase is incorrect reverse one of the primaries so they are right. Now note how the wires are connected. Suppose that the wires of one primary are called A and B and the wires from the other primary are called C and D. Suppose they are connected as follows.

A to line
\ B to C
D to other line

To connect them properly in parallel connect as follows.

A to C and to one side of line.
B to D and to other side of line.

Single Primary Transformers. Connect the primary of the TUT (transformer under test) to the secondary of the filament transformer. Make sure none of the TUT's leads are touching each other and do your best to form them so they won't touch. Turn on the power and start making voltage measurements. If either transformer hums loudly check to see if either one or both or the connecting wires are getting warm. This indicates a short in the TUT and you might as well sell it to a junk dealer. If all is well write the voltages down on your diagram as soon as you measure them.

Add a zero to the end or move the decimal point to convert them to the voltage that will exist when the transformer is operated at line voltage.

Knowing the Maximum Current.

If the transformer had been marked with ratings you wouldn't have to do any of the above. So we may presume that there were no markings on it except for a meaningless number. It is possible to roughly estimate the current handling capability of a transformer's winding. This depends on measuring the resistance of the windings and some of them have resistances measured in milliohms. Elsewhere on this site is a project for constructing a low resistance measuring device and a program you can download that will help you estimate the maximum current.

That's about it. Good luck.

Output transformers.

In general output transformers don't look any different from power transformers. The only visible distinguishing feature is the thickness of the core laminations. They are much thinner in an output transformer as compared to a power transformer. Thinner laminations means the transformer works to a higher frequency. The reason for this is beyond the scope of this article.

Once again an ohmmeter check should be used to confirm that the leads are what you assume they are and the primary is not open.


In a single ended transformer the primary has two leads, red and blue. The red one goes to B+ and the blue one goes to the plate of the tube. Very rarely you will encounter an ultra linear single ended transformer. The screen grid tap will most likely be coded blue with a yellow stripe.

Conventional push pull transformers have three leads on the primary. Red, Blue, and Brown. The red one goes to B+, the blue one to one plate and the brown one to the other plate.

An ultra linear transformer will have two additional leads colored blue with a yellow stripe and brown with a yellow stripe. The blue-yellow lead goes to the screen grid of the tube that has the blue wire connected to its plate. The brown-yellow lead goes to the screen grid of the tube that has the brown lead connected to its plate.


The secondary is distinguished by a very low resistance, less than an ohm. There is a lot of variation in secondary colors. The standard for a multi-tapped secondary is Black, Brown, Green, and Yellow. The order in impedance from lowest to highest is usually brown - green - yellow with black as the common.

Powered Testing.

If you apply 12 volts to the secondary of a 10,000 to 8 ohm transformer, a transformer doesn't mind working backwards, the primary voltage will be 424 volts. If you apply 12 volts to the primary of the same transformer the secondary voltage will be 0.339 volts. The first way is as dangerous as testing a power transformer connected to the line and The second requires measurement of voltages less than one volt. If you have a DMM you have no problem with small voltages.

So connect the 12 volt transformer you bought to test power transformers and connect it to the primary of your unknown output transformer. See "Using the Line Frequency below. For push pull transformers use the entire primary. Measure both primary and secondary voltages.

Zp = Load x (Vp / Vs)2

Where Zp is the primary impedance also known as the plate to plate load impedance, Load is the assumed speaker impedance, Vp is the measured primary voltage, and Vs is the measured secondary voltage.

An example.

You connect an unknown output transformer to the 12 volt transformer.
The primary voltage measures 13.0 volts.
The secondary voltage reads 0.453 V.
Assume an 8 ohm load.
Zp = 8 ( 13.0 / 0.453 ) 2 = 6588 ohms.

This is most likely a 6600 ohm transformer. The 12 ohm error is only 0.18%. This was a made up problem. I can say with certainty that measurements on a real transformer won't come out this close.


When negative feedback is to be added to an amplifier the phasing of the output transformer is critical. If the output transformer is improperly phased the negative feedback will be positive feedback and the amplifier will become a power oscillator. You don't want to have the amplifier connected to a speaker if this happens. A speaker would most likely be destroyed within a few seconds and the amplifier output tubes within a minute or two. Verify the phase of the output transformer with an oscilloscope and count up the number of inversions so you get the phase correct. Unfortunately there doesn't seem to be any standard regarding the phasing of a transformer. Two very well known output transformer manufactures currently making transformers build then with opposite phasing.

Using line frequency for the test.

In output transformers size does matter. If you are testing a found transformer and it is roughly 2 inches or larger across it is likely to be a high fi transformer and it will have no problem responding to 60 Hz. On the other hand if the transformer is roughly 1 inch across such as the one from an all American 5 radio it will not do well with 60 Hz and the measurement will have a considerable error. To test such a transformer you need a function generator and set it to 1,000 Hz.

Keep smiling and have fun with tubes.

This page last updated March 29, 2014.

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