How do I tell the different primary wires in a power tranny?

[eschertron]

Do you mean from the power switch lug where the black wire is hooked? It carries the mains voltage (around 225 Vac). Should I read between it and each of the 'purple' wires?

Right. For our purposes the brown and blue wires are both 'hot' and should be treated with respect. So with the power switch on, there is 225vac across the black wire to the PT (it is black, as pictured?) and the selector switch, where the one purple wire connects.

keeping the common probe of your meter on the black wire, each purple wire will read out a different voltage, and will allow you to ID each tap's purpose. The voltages will all read low (assuming the tap connected is 240vac), but they will all be relative to one another.

[Jazz P Bass]

What he said.

Here is a better layout drawing.

[Jazz P Bass]

I found the export transformer wiring on a Custom Vibrolux schematic.
Part# 049180.

Custom_Vibrolux_Export Transformer 049180.zip

If the transformers are the same: (sans wire colors)

In this case:
For 240Vac operation:
The Black wire goes to the mains (CP8)
Wht/Blk is unterminated.

For 230Vac operation:
The Black wire is unterminated.
The Wht/Black wire goes to the mains (CP8).

Aside: What are you trying accomplish?
Lower Vdc, B+?

Is this the same Vibrolux that you had power issues with in 2009?

[slidincharlie (Carlo P)]

keeping the common probe of your meter on the black wire, each purple wire will read out a different voltage, and will allow you to ID each tap's purpose. The voltages will all read low (assuming the tap connected is 240vac), but they will all be relative to one another.

I got it at last!
With the 240V primary tied to the selector, I have the following readings (I add the colour of each primary as an aid to other amp owners in a similar trouble):
white: 93 V
orange/black: 111 V
white/black: 118 V
yellow/black: 205 V
red/black: 242 V.
The '240V' primary is green/black.
To make things a bit more complicated: wall sockets in Italy are not polarized, thus it does not matter how you insert the plug. For this reason, depending on random plugging, the brown and blue wires are randomly hot or neutral... when I saw Jazz P Bass's drawing (thank you!) I was wondering if I had a problem with the power cable hookup!

[slidincharlie (Carlo P)]

Aside: What are you trying accomplish?
Lower Vdc, B+?
Is this the same Vibrolux that you had power issues with in 2009?

Wow, what a huge memory! Yes, that's it.
I just thought that it might be more correct to use the voltage selection closest to actual mains voltage.
But I need to go back to that old thread and to my block notes to take the correct decision.
I don't want to hijack this thread now... I will probably open a new one.
Thanks everybody for the help - precious as always.

[eschertron]

so good at last! If your wall voltage is closer to 220VAC than to 240VAC - especially if it hovers near 225vac normally - then the yellow/black wire (220vac nominal) would be a good choice. Of course, now that the voltages are 'dicovered', hooking them all up to the selector switch is probably what you intend to do anyway