SET amp recommendation

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If the 12 ohm load of the zeroformers remains relatively flat a 300b amp would love this load.

As long as you can live with the loss of power. I believe these calculations are correct.

from 12 to 4 ohms is 3:1 for the impedance ratio

take the square root of 3 to get the voltage ratio = 1.7

If you have an 8W 300B amp you have 8V at the 8 Ohm tap (8V squared divided by 8 ohms = 8W)

divide that 8V by 1.7 to get 4.6V which will get you a little over 5W max.

maybe, but that's not a lot to drive a speaker with a recommended power of 50W.

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Herman,
This much I know, my speaker is a 14 ohm load(94db efficient) and it is a splendid match with my 8 watt amplifier, sound levels usally 75-85db(in a 14x26x8ft room)If his speaker is truly 91db efficient and he listens at modest levels(his stated preferenc)i.e. around 60-75db range the speakers are only drawing fractions of a "watt" of power.
He says his room is small, With a good quality 300b amp he will be fine.

It`s my understanding/experience that tube amps like a higher ohm load that`s relatively flat.
Charles

12-12-11: Herman
As long as you can live with the loss of power. I believe these calculations are correct....

Not that it makes much difference, but the power loss will be a bit less than what you calculated. Since the output impedance of a SET amp is significant, it will be able to put more voltage into 12 ohms than into 8 ohms. Output impedances that I recall seeing for 300B SET amps range from around 2 ohms to around 5 ohms, corresponding to damping factors of 4 to less than 2.

Taking as an example an output impedance of 4 ohms (damping factor 2), I calculate that the 8W capability into 8 ohms would be reduced to 6.75W into the 4 ohm load through the autoformer's 3:1 impedance transformation.

Best regards,
-- Al

Thanks Al,
I knew there was some definite advantage for tube amps driving higher ohm loads. Higher voltage output, I`ll remember that now.
Charles

Hi Charles,

An additional reason stems from the fact that if speaker impedance is higher, amplifier output impedance becomes a smaller fraction of speaker impedance. That will decrease the magnitude of frequency response variations that will result from interaction of amplifier output impedance with variations of speaker impedance that occur as a function of frequency.

Also, the very low damping factor of these kinds of amplifiers will increase in proportion to speaker impedance.

Finally, higher load impedance means that less current (although more voltage) is required to deliver a given amount of power.

All of those are reasons why, as you said, "tube amps like a higher ohm load that`s relatively flat."

Best regards,
-- Al