12-02-2016, 11:18 AM
I'm in agreement Gary about the additional negative feedback due to the lack of cathode bypass capacitor and the addition of extra HT decoupling to the triode section.
It might be interesting to just dab a suitable electrolytic across that cathode resistor with the volume at minimum - I assume that the volume setting makes no difference to the hum - and see (or, rather, hear!) what happens. If the hum level remains the same it would point to the smoothing as the culprit but if it increases, this is caused by the removal of the feedback and the resulting increased gain pointing to the origin being in the triode section.
Other possibilities are routing of the heater wiring - is this twisted or rely on the chassis for the return path? Temporarily moving the common connection to the ECL82 might help.
As you have two of these sets, side by side visual inspection of the physical layout might show why one hums worse than the other ...
It might be interesting to just dab a suitable electrolytic across that cathode resistor with the volume at minimum - I assume that the volume setting makes no difference to the hum - and see (or, rather, hear!) what happens. If the hum level remains the same it would point to the smoothing as the culprit but if it increases, this is caused by the removal of the feedback and the resulting increased gain pointing to the origin being in the triode section.
Other possibilities are routing of the heater wiring - is this twisted or rely on the chassis for the return path? Temporarily moving the common connection to the ECL82 might help.
As you have two of these sets, side by side visual inspection of the physical layout might show why one hums worse than the other ...
