Basic concept of tube amp output transformers

MOST output transformer manufacturers amphasize their multi-layer winding technique, and claimed that is the trade secret of good sound quality.

The actual case is that IRON CORE plays the more important role. I estimate the iron core gives 70% sound quality while the winding only gives 30%. How come ?

A tube output transformer actually is two indenpendent coils winding on ONE iron core. The primary connected to tube and the secondary connected to speaker. ( ohm matching)

The primary coil passes energy to the iron core as magnetic flux form (step 1), and the core passes energy to secondary coil back to electical energy (step 2).

The problem is that some iron core does not response linear. That means the magnetic flux in core is not pure sinusoidal even the primary source is sinusoidal and leads to distortion on secondary side.

Futher more, the iron core will convert certain percentage of magnetic energy to heat, this is also known as hysteresis iron loss. Significant hysteresis iron loss will obviously reduce high frequency response.

An appropiate set up with an oscilloscope can see the iron core hysteresis loop, the SMALLER the loop area the BETTER the iron quality as well as SOUND quality.

See picture below, (a) is a typical hysteresis loop and (b) is distorted primary magnetizing current wave form.

The actual example from a power transformer see picture

Hysteresis loop

Magnetizing current wave form distortion (lower curve)

Hysteresis loop from an US tube radio output transformer.

Hysteresis loop from a Telefunken tube radio output transformer.

Hysteresis loop from a Tamura F 2013 output transformer, the loop almost is a straight line with minimum loop area to prove excellent sound quality.

Thank you for looking !

                                                                                          Written by    Terry  Tam   I.Eng  MIEEIE  MIET  (U.K.)