Toroidal transformer on tube amp

@dafox 

From BAT’s website for one model:

VK-90t also means there are no tubes to bias; no fuses to replace; no pentodes imitating triodes; and no direct current at the tube output stage. VK-90t is the sound of the future—and musical truth.

I don’t know if the no-DC is a feature of the circuit, or if they have a circuit that adjusts to prevent DC? I am sure there are some tube hacks that can weigh in.

EI core transformers have inherent air gaps where energy in the magnetic field is stored, toroid transformers do not. If you get any DC bias on a toroid transformer, the inductance will change by a large amount. That’s bad. It will also stay magnetized after the DC bias is removed continuing to effect the inductance. The EI core transformer has more stable properties. I believe the EI is also more linear over a wider range due to the air gap, but I may be mistaken on that. I had the same question many years ago. I get the theory, but have not spent a lot of time thinking about it. I am sure others could weigh in in more detail.