Filament center tap (again)

[fdesalvo]

So, I've finally got a PT with a filament center tap and I've seen a few methods for grounding these things.  Normally, I would just create an artificial CT by grounding a pair of 100 Ohm resistors off the pilot lamp, but now I have this new lil' doodad.  What's the best method for lowest filament hum? 

Should I disregard the CT and use the artificial CT method with the 100 Ohm resistors, or should I just ground the CT?  I've also heard that placing the CT on a power tube's cathode will inject enough DC to float the AC, reducing hum, but I don't beleive it will work on an AB763 power suply since the tube's aren't cathode biased.  What say ye?

[sluckey]

Connect the CT to chassis ground, maybe one of the PT bolts.

[fdesalvo]

Good enough - and thanks.  Nothing like overthinking things.

[Fresh_Start]

When I built a Super Reverb clone, the PT had a center tap for the heater winding so I used it.

Elevating the "ground" reference for the heaters can make a significant difference in single-ended amps in terms of lowering the noise floor.  I think a push-pull poweramp eliminates most of that noise anyway, but it's easy enough to try.  In the AB763 circuit, there's a 220K resistor in parallel with the first filter cap(s).  If you lifted the ground connection of that 220K and inserted a 22K between it and ground, you'd create a voltage divider with about 9% of your B+ voltage at the junction of those two resistors.  Should be about 40-45 volts for an AB763, assuming roughly 470 volts B+. 

Cheers,

Chip