How to measure on tone transformers
(Last updated: 30-Jun-2016 11:26 )

If you magnetize the core of a tone transformer with an ohms meter, the sound of the transformer may become bad.

This applies for all transformers without DC capabilities. That is either the Lundahl PPZ series for parafeed, or the miniature ones, in the white shielded housing, which by default have almost no DC capability.

Single Ended transformers by nature have DC capability. Also the Lundahl Push Pull tube transformers, and (yes) the Lundahl Mains transformers have enough DC capability to withstand an ohms meter.

Sometimes you want to check the connections of a signal transformer, and just take a digital multi meter and quickly test it. Well that can damage a tone transformer before you know it, and also without you being aware. With a digital multi meter, you find no indication of the damage you did, and even so the transformer is not out of function. The only problem is now, it distorts. So latest when the sound is bad, you find out, but you or your technician may not even remember this happened. Needless to say, you should just NEVER buy a used tone transformer from a stranger, like on Ebay. Chances they have been tested with an ohms meter are pretty high.

A tone transformer usually has high inductance, which means many windings. Like many hundreds. For a transformer core, it is the same when little current flows through many windings, or high current flows through little windings. Magnetism results from Current x Windings. So 1mA though 1000 windings is the same as 100mA trough 10 windings.

Now I have done some investigation about what most multi meters do. The results are shocking. No matter if you have a 8$ DMM from China, or a 1500$ instrument, they all work the same.

What are those meters doing on auto range? They are all programmed in some way. Also the 8$ ones. The problem is, they detect "low resistance" when you connect a transformer winding. Then, the logic response is, pumping 1mA DC current through the "Resistor" and see what happens. So they try to become a 1mA current source. This will work very nicely with a coil connected of course. So the meter charges the coil with 1mA. What happens when you disconnect the meter? The coil will now generate a very high voltage, attempting to keep the 1mA flowing. It can be several hundred volts. It is unclear what happens now, but you have a risk to damage the meter even by that.

But let's stay with the 1mA test. That's what all DMM on auto range begin with. If the result makes no sense, the meter chooses another range. It really must be said, there are many electronic devices that you can kill with 1mA. To my onions, this is a very wrong way to go, because the meter does have the possibility to begin with 10uA or 100uA range, but it's just not doing so. So it begins with 1mA. Then when the software sees this makes no sense, it says "Oops" and it switches to 100uA. Perhaps another "Oops" and it switches to 10uA, and then does the Ohms measurement for you. You don't see much of that. All you see is the digits move for a fraction of a second, and when it stops you read the value. Then, the tone transformer is already damaged. (or your meter)

I am not joking, this is really so. Just take two multi meters if you have that. This test will open up your eyes. Set one meter to 1mA Ampere meter, and use the other as an ohms meter, on auto range. Now, you take the ohms meter, and with that one, you measure the impedance of the 1mA Ampere meter. Watch what happens now! You will see it boost 1mA into any unknown device, to begin with. Immediately it is satisfied and display the "Ohms" value.

Why do they not begin with 10uA, then 100uA, and then 1mA? I do not know. Even if they do, it doesn't protect the coil, but the meter would go to 1mA anyway as a coil is a low resistance device. I found some multi meters to be safe on very high ohms range. At auto range they are ALL unsafe.

With an analog multi meter you are safe, as they can not change something to the choice you make, but then you need to choose the highest impedance range, and you need to check FIRST what the test current is, by using a second meter.

We recommend the following:

  • Never use a digital meter. Such a meter can damage when you try to measure resistance of a coil. Also, such meters use 1mA DC current, to measure unknown “resistors”. This is too much for “AC-only” Audio transformers.
  • Never measure resistance of an amorphous core “AC-only” Audio transformer.
  • An analog ohms meter can be used for non-amorphous core transformers, when set on 1 Meg Ohms or higher. You can not accurately measure the DC resistance this way, but you can measure if you have a open contact. Any setting below 1Meg ohms, do not use it.

An Exception is an amorphous core transformer with DC current capabilities (Air gapped transformers). These can be tested with a analog ohms meter, also on low resistance setting.


In case you magnetized the transformer, there are two repairs possible.

  1. Just use it, and after long enough time, magnetism may eventually reduce. Some call it burn in, and report improvement after long enough use.
  2. Connect the transformer to an oscillator. Begin with high 100Hz, zero voltage. Then increase the voltage to the transformer's maximum rating. Not more, and not less.
  3. Then lower the frequency, below the limit the transformer is made for. Do not go lower as you need. Like a factor two is fine. This will now saturate (magnetize) the core in the AC rhythm of the frequency. It is not needed to keep it here for a long time. So right after, you can VERY slowly increase the frequency to 100 Hz, and when you are at 100Hz, reduce the voltage slowly to zero, and you're done.
  4. No guarantee this repair works. It was recommended to me by a transformer specialist, as a method to repair a magnetized tone transformer.