Without changing transformers or squeezing in one just for filaments, wouldn't one be limited to a 2.5v 300B?
Heh-heh! No, I have planned ahead - the 2.5v winding is quad-filar, two independent bifilar windings. I can ask the supplier to make a bifilar winding of two independent single windings, which will supply 5v at 1.25 amps. A very small change to an existing design. Hum might be 4-6dB greater, so we'll have to try it and see - like I said, I have some ideas but they need to be tried out.
The good news is that we can do the experiments with a 2.5v 300B, and not mess with the power transformer before we know whether we have a good basic design.
There are a few more things in the pipeline before we get to this, though.
