A quick question about output transformers

[PositiveNegativeMan]

Is there a convention to the way the Primary and Secondary impedances are stated?

What I mean is an OT with a 5.5k primary and 16ohm secondary is easy.
But if the same transformer were to have a second tap added on the secondary for 8ohm, then if I were to use that then the primary isn't seeing 5.5k any more. Now I think I could calculate the voltage but...

I have quite a few OTs collected now that are labelled along the lines of 5.5k primary, 4, 8, 16 secondary.

So how do I know which tap gives the rated primary figure?
Back in the early dawn of the establishing of output transformer wisdom was unwritten lore set down which means that the given figure for the primary impedance is that which you get from using the highest value tap on the secondary? Or the lowest? Or was there no such convention ever established and I'm going to have to either guess or just test each one as and when I'm thinking of using it. Which I will probaly do anyway since manufacturers given values are often out anyway.

[Steve Conner]

I have quite a few OTs collected now that are labelled along the lines of 5.5k primary, 4, 8, 16 secondary.

So how do I know which tap gives the rated primary figure?

They all do.

If you connect a 4 ohm load to the 4 ohm tap, it gets transformed to 5.5k on the primary.
If you connect an 8 ohm load to the 8 ohm tap, it gets transformed to 5.5k on the primary.
If you connect a 16 ohm load to the 16 ohm tap, it gets transformed to, you guessed it, 5.5k on the primary.

That's the whole point of putting taps on the transformer: so it can do that!

[PositiveNegativeMan]

AAAAhhh!!!!
So the only time the primary will 'see' a different impedence is if you connect a speaker to a "wrong" tap

With that in mind do people often "cheat" if they don't have exactly the correct OT to hand? Plugging 4ohm speakers into 8ohm sockets to reflect a different load onto the primary???

[Steve Conner]

With that in mind do people often "cheat" if they don't have exactly the correct OT to hand?

Sure, a good quality OT will work fine with a factor of 2 mismatch either way. That is to say, if it were specified as 5k:8 ohm, you could use it as a 2.5k:4 ohm or 10k:16 ohm.

I have a 6.6k:16 ohm OT in one of my homebuilt amps. It only has a 16 ohm tap, but I just connect up 8 ohm speakers anyway, and it seems to get on fine as a 3.3k:8 OT. I put a pair of 6550s in, and they give me about 60w with either 8 or 16 ohm loads.

And my tube stereo amp sounds best with the speakers hooked to the 8 ohm taps, even if they're 4 ohms!

Generally you lose a bit of bass response if you run higher than the specified impedance, and lose treble response and efficiency if you run lower. Better quality OTs have more bass and treble to start with, so they can tolerate more of a loss from mismatching.