Orange OR15 hum

[ElectronAvalanche]

Dear All,

so I recently acquired this nice little Orange OR15 amp which sounds quite nice I must say. The only problem with the amp is that it has a nasty 50hz heater hum. I redressed and twisted the heater wires and also installed an elevated heater mod, which helped some, but the amp still has some hum going on.

So looking at the amp (schematic attached), there is a single ground point which connects to the chassis. The elevated heater mod delivers 55V DC to the heaters and is grounded to the same ground point. For the elevated heater mod, I cut the PCB trace between the junction of the two 500Ohm heater symmetry resistors R54/R55 and ground. I grounded the elevated heater mod (100uF / 450V cap, in paralle with 100k to ground and with 470k in series to B+ after the 500Ohm series resistor feeding the screens) on the PCB at the single ground point where originally the R54/R55 junction would connect.

I can dial out the heater hum with the heater balance pot nicely, but only for a specific gain pot setting and master setting. I.e. if the gain is high and the master 0, I get hum which I can dial out with turning the heater sym pot. At lower gain pot settings and higher master settings I get hum again and would need to turn the heater sym pot to some other setting. So there seems to be a perfect heater symmetry setting for a specific gain/master pot setting, but for other settings, there is hum.

I think about taking the elevated heater mod ground directly to chassis. Or maybe take it out again.

Has anybody ever experienced something like this: elevated heater mod + heater sym pot does only work for specific gain pot settings? The odd thing with this amp (and probabyl the nice thing about the creamy crunch) is the dual gang gain pot which spans the two gain sections. Could there be some sort of ground loop which comes into play here with the heater symmetry ground?

Any help highly appreciated.

Best Regards,

Electron