what is an ideal tube amp

Regarding the statements that have been made about losing half the power vs. not losing any power, if the nominally 16 ohm speaker is connected to an 8 ohm tap, in most cases the truth lies somewhere in between. It depends on the output impedance of the particular amp, and in some cases perhaps also on the current capability of the particular amp.

The output impedance of the 8 ohm tap of most (but not all) tube amplifiers will be considerably less than 8 ohms, corresponding to a damping factor considerably greater than 1. As a consequence of that, and assuming that power delivery into 8 ohms is not limited by current capability, the amp will not be able to deliver as much power into 16 ohms as into 8 ohms, but the reduction will be smaller than in the case of a solid state amp having near zero output impedance. The reduction will be MUCH smaller if the amplifier has a particularly low damping factor/high output impedance, such as SET amps commonly have.

An actual increase in power delivery into a 16 ohm load connected to an 8 ohm tap, compared to an 8 ohm load connected to the same tap, would occur in the relatively uncommon situation where the amplifier has a damping factor significantly less than 1, corresponding to an output impedance significantly greater than 8 ohms, and/or if its 8 ohm power capability is limited by current capability.

A separate question relevant to connecting a 16 ohm load to an 8 ohm tap (assuming the amp is not an OTL and has output taps) would be whether or not a perceivable increase in distortion might occur in some cases. Perhaps Ralph will speak to that, as I don't know the answer.

Regards,
-- Al

If a tube amp with an output transformer has a 16 ohm tap, it will have slightly more power and certainly less distortion when driving 16 ohms. This is because the output transformer is more efficient. In fact it will also exhibit more bandwidth- often with greater bass extension.

With transistors you lose power, but you also lose distortion- causing the transistor amp to sound smoother and more musical. This is because there is a non-linear capacitive aspect to almost any semiconductor junction, which is magnified by current. This aspect contributes to odd-ordered harmonic distortion, which is unpleasant to the human ear. By reducing the current (i.e. going to a higher impedance load) odd-ordered harmonics are reduced. This is evident in the distortion curves of all solid state amplifiers, even class D.

With OTLs, depending on the size of the amp, you may gain some power- in particular with smaller OTLs. For example the S-30 makes 45 watts into 16 ohms. With larger OTLs this may not be true- for example with our MA-1 the output power is unchanged. What is more important is what happens to distortion and efficiency of the OTL! In all cases it will operate with less distortion, meaning that it will sound smoother and more detailed. The increased efficiency also means that you will get less heat with less power drawn from the wall, as more of the power generated by the amp will be dissipated in the load rather than the output section.

IMO there is every argument for 16 ohms **if your goal is sound quality**. There is no argument for 4 ohms regardless of the amp, unless your goal is **increased sound pressure where sound quality is less important**, *and* you have a solid state amplifier. IOW 16 ohms is a simple way of making the speaker seem more transparent, smoother and more musical, as opposed to 4 or 8 ohms.

Atma-sphere should go back to the older style amps. I like the way the older units look compared to the newer ones. The ones with the handles are what I am talking about. Very cool looking.

The new style stands a chance of fitting into an equipment rack.

If you prefer set and forget, if you don't want futzing, you might be wise to pursue a self-biasing tube amp. Personally I would consider the little Berning.
If you want to keep the budget small, there'a a guy on Audio Circle selling an Almorro 205. Or you can buy my Audio Mirror 20 watters.

And, of course,the Ongaku option appears occasionally.