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Old 4th August 2012, 01:42 AM   #11
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Join Date: Nov 2007
Location: Near Dallas Texas USA
Did you try the 100 ohm resistors from each side of the heater supply to ground? Measure each side of the heater supply to ground. Reading of 50 to 100VAC are not uncommon. This large voltage is caused by capacitance within the transformer between the heater winding and one end of the high voltage winding. The 100 ohm resistors (called a virtual center tap) short this voltage to ground.

One way to eliminate the heater supply as the cause is to disconnect both sides from the transformer and connect a battery or other DC power supply.
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Old 4th August 2012, 01:18 PM   #12
xtaljo is offline xtaljo  United Kingdom
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Join Date: Apr 2012
Thanks everyone. Hum cured!!!

I was re-arranging the components at the input stage and this improved things somewhat. Then I noticed I had wired an unnecessary/duplicate ground connection to the bus bar causing a ground loop. One snip (amp off) and (amp on) all was well! Just a slight residual hum if you put your ear within a couple of inches of the speaker which is normal.

I'm so grateful. There is so much useful information in this thread I'm going to save it for future projects. Sorry it was partly my error.

The Mullard 3+3s sound great now, just as I hoped they would.

Thanks again guys.
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