Tube amps and speaker ohms

Anytime you are investing in tube amplifier technology, that investment will be best served by a speaker that is at least 8 ohms or more.

Tube amps, with rare exception, have reduced performance into four ohms.

Given that you have satisfied that part of the issue, the next is efficiency. 40 watts with a speaker that is only 86 db is not going to be a lot of power unless you happen to listen nearfield, regardless of the type of amp.

SETs in particular have certain advantages of low level detail over conventional push-pull, but to tap that advantage you will need a speaker that is a good 10 db more efficient if your amp is 40 watts! Otherwise although it might work 'OK', it certainly will not be meeting its design goals.

Hi Pauly, here's what's up with four ohms: Yes, you can use the 4 ohm tap, but at a price. On most output transformers that have a 4 ohm tap, measuring the response of the amplifier on that tap as opposed to the other taps will reveal slightly lower power and quite a bit less bandwidth (both due to increased turns ratio and distributed capacitance in the windings). Sometimes there is also additional hysteresis loss. So tubes amps can make lower distortion on higher impedance taps as well.

So, all other things being equal, most tube amps will sound better on 8 or even 16 ohms as opposed to 4.

The point of the topic was to ask the GENERALIZED question that tube amps prefer 8 ohm speakers(though even 8 ohm speakers can dip to 4 ohms on huge orchestral challenges).

The answer to that is 'yes!', regardless of the tube amplifier technology.

The 4 ohm taps on almost all output transformers (excepting those that the 4 ohm tap is the *highest* tap). perform to a lower level than the other taps. The loss in bandwidth and increase in distortion is readily measurable and audible.

IOW: "Anytime you are investing in tube amplifier technology, that investment will be best served by a speaker that is 8 ohms or more, all other things being equal."

Normally the tube is a self harmonic generator ( that does not exist on the original signal/recorded signal )

Raul, I would have thought you would have educated yourself better before making such an obviously false statement! You are in a lot more trouble with transistors in this area then you ever would be with tubes! Look how many tube amps can actually operate with low distortion with zero feedback, and then count how many transistor amps that do the same thing. The zero feedback transistor amps are *sorely* outnumbered!

If we run feedback on our amps, which are OTL, we can get the THD down to 0.001% at full power, but who'd want to (just like frying an egg on the sidewalk)? It is true that you reduce a lot of distortion that the ear does not care about, but you increase the distortion that the ear *DOES* care about!

Ultimately it is the rules of human hearing that are the most important factors in audio...

Hi Gregm, you might ask Nelson Pass or Charlie Hanson of Ayre, both of whom make zero feedback designs.

FWIW, negative feedback is a *destabilizing* feature in amplifier design, not a stabilizing one. This is a common misconception about feedback.

Direct-coupled transistor amplifiers tend to use feedback as part of their means to control DC Offset at the output- IMO an inappropriate use of feedback, as it requires a greater amount of it to do the job. It may be the traditional way to do it, but in the world of high end audio, 'traditional' is a weak excuse for a solution to a problem.