[35Z5]

I can't visualise the output transformer being a source of hum originates from insufficient filtering of power supply. A few transformers did have hum buck windings but if P/S is filtered sufficiently it isn't needed.

[fenbach]

has me baffled too, tom. the first electrolytic filter cap is 100uf. I tried doubling that. didn't help one bit. I don't see any hum on the scope after the 2nd or 3rd filter cap. but swapping out the transformer effectively eliminated the hum. go figure. for what it's worth, the replacement transformer is an old R/S audio transformer with various taps, for both input wattage and output impedance. I got the best sound quality with 0.62 watts in and 8 ohms out.