My attention was always called to the transformers which are made of c-core  form. They are also called wound cores. Production of this type cores is more complicated than the manufacture of standard stamped transformer laminations. Tape wound cores are made from thin strips with special magnetic parameters, which is wound in a specific form, generally elongated shape. To keep the strip precisely to shape manufacturers use a fixing resin which upon hardening  retains its shape permanently. After formation of the shape of the core is carried cut to give two halves which is marked by the long side. Marking is very important because the two halves must be assembled exactly in their original configuration in which they were before being cut. Then it proceeds to polishing of cut surfaces  in order to achieve good matching of the two halves.  The image  below shows  two halves from  typical C-core.

From quality of the cut surface depends two fundamental parameters. The No-Load current(Primary Winding Idle Current at No-Load condition) and magnetic permeability. In case of bad cut surfaces it can be observe decrease of the magnetic permeability and increase the idle current.

Very simple test of the quality of the transformer core is to check with ammeter connected in series of the idle current when no load is connected on the secondary winding.I’m of the opinion that high no-load idle current is not so good for the all core system.

Тhe main problem in the C-cores is the quality of the cut surfaces.

So, my desire always been to build criteria for simple quality test of standard cores. In order to find a core that could give excellent sound performance.

I want to note that is not talking about professional test and control of parameters.

Professional test of cores and transformers can be done with advanced inductance analyzers like the Wayne Kerr 3255B and 3255BQ.

3255B and 3255BQ are highly versatile and accurate inductance analyzers able to characterise devices in a clear and simple manner. The three  models in the range are the 3255BL (200kHz), 3255B (500 kHz) and the 3255BQ (1 MHz).

At the design stage of component development it is very important to analyse how components performs under different operating conditions. This includes operation over a range of frequencies, AC drive levels or DC bias currents.

The AC drive level can be set between 1 mV to 10 V. DC bias current can be set from 1 mA to 1 A internally (optional) with external DC bias units available increasing the maximum current to 125 A (3255B and 3255BQ). This device is expensive for hobby . It may therefore use other alternative low cost methods for measuring  parameters of  cores and coils that gives sufficient precision for hobby applications. The Wayne Kerr devices have measurement precision about 0.1%. The method with variable transformer and ammeter and voltmeter can give precision about 5%. This method that I will explain later is not professional way to check magnetic parameters but is  alternative to make simple and quick test of transformers cores. To go ahead and jump to next level is to buy device like Wayne Kerr that could give reliable certificate and warranty of quality products. For example if you could produce 10Henry inductor on 1A DC Bias condition on 100Hz, in hobby application there no problem if the inductance have some tolerance in range of 5%( from 9.5H – 10.5H). But this for another application could be serious problem like military production.