output transformer, model 4U63 ..... It says Output impedances 0-3-5-7-15-400 ohms. My question is, are these figures cumulative? In other words, is the output impedance between “0” and “5” 5ohms or 8?

Impedances shown in your table are all between 0/common terminal and one of the other taps; the number indicating the recommended load impedance.

You use the closest one.

What is the custom, does somebody know?

No "custom" involved but what the spec says, and yes, we know.

I used “0” and “7” for an 8ohm speaker

correct.

now but I was wondering if I should use “0” and “5”

no, that fits a 5 ohm speaker.

or maybe “7” and “15” (15-7=8).

No,

**those impedance numbers do not apply any more if you do not use the common tap**.

And to make it more clear to you: transformers, even Output ones, really have windings which put out defined

**voltage**, but which

__translate__ to

**impedance** values.

Sadly the translation is not linear.

Now you can add/substract

**voltage**, no big deal, because there is a linear relation between them.

Say you have two 4V taps, adding them gives you 8V

Or you have 8V and 12V taps (always referred to 0/common), then you have 4 V between the 8V and the 12V ones.

There it

__does __apply 12V-8V=4V

But you can not do same with impedance.

Feel curious about what will really happen?

You must first translate impedance back to voltage, add-substract as needed and then re-convert to impedance.

What DF96 did , by the way, he didn´t pull his answer out of the blue.