Extra tap on primary side of power transformer. What is it?

[atmars]

I've got a '40's ish amp that has a plug receptacle with three prongs (where the power cord meets the chassis, the original power cord is lost). The outside prongs are common and 117v. I have the schematic - I can't tell for sure - i think the center tap says 86v? What is this tap for?

[tedmich]

quite a mystery to me, the only insight being the voltage range for wall AC is sometimes given as 86-132VAC but why you'd need a primary tap at "brown out" voltages is still a mystery...

3 prong plugs are pretty rare on equipment this old, I'd beef up the grounding scheme and maybe put a MOV across the L/N lines

[Enzo]

What is the product? Is it a stand alone unit, or was it meant to be part of a larger system? That 86v tap might make more sense in some context.

I can think up lots of things, but who knows how valid they would be. A low mains tap. A lower voltage feed to some remote power indicator. A lower voltage feed to some sub system or accessory. A lower voltage input for use with a mobile powr source.

[atmars]

What is the product?

Film projector amp.

Makes sense that the tap would run some other part of the mechanism. Others I've seen don't have the extra tap, but all have the power supply for the amp fed from the projector, not the wall socket. i.e socket>projector>amp. So maybe the tap feeds something else on board.