Hammond organ OT question

[bigsbybender]

Hi guys.

I was rigged up a Hammond E100 Organ (AO63 amp) to run through a Leslie speaker.  It worked fine but I had a very strong hum that I've been chasing down. I opened up the amp for and realized that the OT secondary is Center-Tapped.  I find this odd....
 I do know that Hammond ran their signal level outputs balanced for long runs in churches or theatres so their small signal OTs have a center tap but this speaker-level OT just looks odd to me.

I guess I'd like to know why???? Advantages?

Schematic clipping below.


j.

[triode]

I am not 100% sure why Hammond did it on that amp, but it is not that uncommon. Audio Research Corp does almost all of their amps this way. IIRC, even very recent amps of theirs have this configuration. I was chatting with the designer of MQ iron (who also worked at Acrosound/Peerless) about this subject one day. He noted that in nearly every OT with 4/8/16 or 4/8 ohm taps that the 4ohm tap is almost exactly in the "center". So it does not really matter if you use the "0" as ground, or the "4" as ground. (e.g. if 4 is chassis ground, and you hook up between 0 and 4, you get exactly the same (although inverted) as you would if you hooked up between 4 and 0).

Could we say that an ungrounded tranny is "balanced"? Sure. Could we say that if 4 ohms is grounded, and I use the two other leads (center tap as you put it) that it is balanced? Sure. If I ground 0 and take the 4 ohm tap, is that unbalanced? Well, only in that signal does not "come from" the ground side.

Advantages? Well, we could argue some small things, but sound wise they will not make any great deal of difference.

[bigsbybender]

Thanks, Triode.

Another thing I should add is that the 15" speaker is rated 4 Ohm and the 8" is 8 ohm, so I wasn't sure if that configuration had anything to do with it.  The speakers have been replaced with dummy resistors, since I'm running the sound through a Leslie speaker now. I'm still trying to isolate the hum. It sounds 120hz, but a cap job and cleanup of the grounding scheme haven't helped much.


j.