Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fdesalvo on May 22, 2012, 06:11:03 pm
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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?
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Connect the CT to chassis ground, maybe one of the PT bolts.
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Good enough - and thanks. Nothing like overthinking things. :BangHead:
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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