The transformer has a 4 ohm, an 8 ohm, and a 16 ohm tap. Only two of them are on the outisde of the chassis, and they are wired in such a way that one tap is the main one (usually 8 ohms) and the other tap is meant to be hooked up in addition to the first one, so both are run to cabinets at the same time, and this other one is usually the 16 ohm tap. You can drill a hole and add a jack so you get the 4 ohm tap. You can also unsolder the 8 ohm and sub the 4 ohm, though the labelling on the back of the amp will now be incorrect. I don't recall which wire color is which, but if you investigate a bit, you should be able to get a better idea.
Greg
