Negative-filtered power supply? (Hammond content)

[Philoffline]

This weekend I spent some time reconditioning a beat-up Hammond M3 that's in my band's new rehearsal space. Unstuck and lubed the mechanicals; added grounded plug and fuse holder; replaced (missing) field coil speaker with a massive wirewound resistor; and replaced scary old electrolytic caps.

I almost got the #1 cap disastrously wrong by grounding the negative end, like every guitar amp and Hammond pull I've ever seen. Not so! The negative connects to the PT center tap, which feeds the speaker field coil as well as a resistor string that supplies some oddball negative voltages.

So one cap seems to do double duty in two different power supplies. Besides saving the cost of another part, is there any advantage to this setup? Was it common practice long ago? (This organ is from 1955 or early '56.) Any insight is appreciated...

[DonMoose]

It's not doing double duty, though. It's filtering the 410VDC supply (+310 - -100), but for some reason there's an LC pole in the GND side instead of the more usual B+ side.

It IS odd, and I don't really understand it, but I think it makes a kind of sense.

Hope this helps!

[Philoffline]

Sort of helpful, thanks. The organ sounds great now so it's really just a matter of curiosity.

I am glad I used 500v ratings, though -- modern wall AC kicks all the voltages up a notch so it was more like 430 volts across a 55-year-old, 450v cap.

[UR12]

That unit used a field coil speaker. Instead of the normal permanent magnet on todays speakers that speaker had a field coil or a electromagnet on the speaker. The speakers field coil served double duty as a choke in the power supply and the magnet for the speaker.

[Philoffline]

It used to, anyway. The field coil speaker is missing from ours so I replaced it with a resistor per an old Hammond service bulletin. I used the bolt-on, heat-sink type and put it on the outside because it gets seriously warm.

It seems there are more complex ways to approach the issue, but this works fine.