Total primary inductance was actually measured at current flowing thru all four primaries connected in series. The current was then doubled in order to correct for the mmf when using one primary winding only.

5V/50Hz was superimposed upon a swept DC voltage. Due to the ac portion always being more than 15%, L was plotted versus pure Idc only.

Comparing this result with the L(I) dependency for the OPT TI-57613-C, it is clear that this standard un-gapped core is useful for pure well-balanced Class A operation only.

It is also interesting to note that the direct current for Lmax as measured for L(U), obviously is so small that it is even hard do detect. This seems to be the case for the OPT also, but for that type of special core an inductance increase was actually detected.

From the above result it is fair to say that useful inductance when using this transformer as an OPT is no more than 80H and this is when the bias unbalance is no more than approximately 1mA!