[smitty]

I would say, get the schematic and go by it.
I do not have 640B sch, and 640 might not be the same.

And you have to measure the transformer's leads to see where the centertap is or where the field coil comes in.

Unless someone knows off hand.

it turns out that its a 620
so here's the section of the schematic that shows the transformer and mentions the center tap.

the components are listed as follows:

62 = output transformer
63 = voice coil and cone assembly
64 = field coil and pot assembly
71 = power transformer

do you mind walking me through identifying things?