Need some help with a valve amp guys :)

[Chuck H]

Well... Actuall, I think you could. You would have to run the speakers independant and have the OT secondary - lead between them so they would act as individual windings. The speakers would have to be wired in reverse polarity from one another and youd have to set the primary impedance to a happy medium. Forget trying to add switchable impedances. But it should work.

Chuck

Edit: I really can't imagine when it would come up like the above scenario... But there it is.

[loudthud]

A 'center tapped' secondary on an output transformer may be used for the purpose of accommodating multiple values of speaker impedance.... a 4 ohm load would use half of the secondary, while an 8 ohm load would use the full secondary.

A voltage center tap is not an impedance center tap. E=SQR(P*R).

As an example:
100W@4ohms = 20V
100W@8ohms = 28.28V
100W@16ohms = 40V

The 4 ohm tap is the center tap of the 16 ohm winding.

You'll have to experiment to find out how the secondary is configured. Taps on schematics usually aren't drawn accurately.