[6L6PPP]

I see output transformers with 4-8-16 ohm taps. But I wondered what range of impedances will these different output impedance transformers actually drive with good power output? Specifically, to begin, how would the 8 ohm output transformer on my NS-30 or NS-10 do with a pair of 16 ohm impedance speakers with a 10.2 ohm minimum impedance?

I have two pair of 5.25" Vifa BC14SG49-08 which have a nominal 8 ohm impedance and a 5.6 ohm minimum impedance. I would like to use these drivers for a MTM paired with my favorite Vifa D27TG45-06 tweeter. Paralleling these would give a 2.8 minimum which I know my 8 ohm taps don't like. But wiring them in series would give the 16/10.2 ohm figure.

Now compound this with the tweeter being a 6 ohm nominal impedance with a 4.7 ohm minimum .... as noboby seems to make a good high impedance soft dome anymore. How would the 8 ohm output transformers handle all this ???

[Niteshade]

The best thing to do: Use the impedance tap that works/sounds the best. I have a pair of 8 ohm speakers that sound best with the amp switched to 4 ohms. They most likely have a dip around 5 ohms, like yours.

Many 16 ohm speakers will sound good with an 8 ohm output tap. You may not get the same volume out of them as with a 16 ohm tap. Since they dip down around 10 ohms, there should not be any problems.

[Ericus Rex]

I remember a discussion on the Music Reference Circle about how 'light-loading' the tubes (I think I remember it called this; using a lower ohmed tap than the speakers, i.e. 4 ohm amp tap with 8 ohm speakers) will result in your tubes lasting longer as they aren't working nearly as hard.  Is this the case with your amps as well, Blair?  The trade-off is that you will distort and clip at lower volumes so this setup may not work with all speakers.  If you can get adequate volumes with light-loading then do it!

[Niteshade]

I had a thread about this a year or so ago on here. What I said: "In regards to speaker impedance of your amplifier, use the output tap which sounds the best." Sometimes it's the 8 ohm with an 8 ohm speaker and sometimes the 4 ohm will sound better. There are so many variables to consider- what sounds like light loading may not actually be the case all the time. An amp's output transformers have to be taken into consideration as well. Some people might use a primary that has a low impedance.  Using the 4 ohm tap will actually produce less distortion and provide more volume.

In a nutshell: Use what works best for you.