I often get asked about Toroid output transformers, which make no sense at all due to the fact that a very small DC imbalance in output tube current will saturate them. I design and wind my own output transformers using EI lams and copper wire. Below is a quote from Tim de Paravicini, with whom I completely agree in this and many other things. He's one of the few designers I respect in this field.
Tim de Paravicini is blunt about it: output transformers determine the
performance of an amplifier. He describes himself as ‘very anti-toroidal’, and
goes so far as to judge amplifiers by their weight – a clue to the mass of their
transformers. ‘I lift these lightweight amps from China with 845s and 2A3s and
I know they’ll have no bass. There are no short-cuts. If those amplifiers don’t
weigh 70lbs, they aren’t gonna work.’ It explains not just the heft of EAR-Yoshino
models, but also the amps he conceived for Quad. Then ask Tim to describe his
hand-wound, in-house transformers and he sounds like a chef asked to name the
spices in his secret brisket dry-rub: ‘Basic EI construction, with special layering
and winding, with copper wire – not silver. The layering and how you arrange the
windings are the key. Most manufacturers just don’t get it.’