The SET amp question.

Phd,

I would guess that 102 is closer to the correct rating. 110 is in the territory of extraordinarily large, horn loaded (including the woofer) systems.

102 is still quite efficient, and you could use almost any kind of SET amp. Given your price constraint, it would be a bit difficult to find a decent 211 or 845 amp. Those amps have very high voltages on the plate and require very substantial, and expensive power supplies, and special wiring to handle such voltages. Personally, I would be a bit afraid of "cheap" 211 and 845 amps.

While I personally like some of the higher powered SETs (e.g. the Wytech 211 amp), I don't think they quite deliver the exquisite detail and texture of the lower-powered SETs (2a3 and 45s are personal favorites). Of course, if one needs more power one may have to sacrifice in one area to get satisfactory performance in another. There are always tradeoffs. Someone above mentioned how SETs don't perform quite as well with denser, more complex music. To me, that is evidence of an amp being overdriven just a bit, not something characteristic of SET performance. A SET operating within reasonable power bounds is better than most other kinds of amps at very naturally laying out all the texture, detail and harmonic relationships of complex music.

A friend of mine with 106 db/w speakers has no problem with single-ended 45s, but then again, he does not crank his system up very loud (good, high efficiency systems actually perform very well at low volume even though they are capable of wailing with the best). You might get away with using a 45 or 2a3 SET. My own system is 99 db/w efficient and I use a parallel-SET 2a3 amp (two tubes per channel), and I have enough power, except perhaps for certain works with large choruses.

While SETs are typically simple in design and easy to construct, they are not necessarily "cheap," because they place difficult demands on the output transformer which cannot be cheaply made for SET use. This is more the case with high-powered SETs, again suggesting that you should look at the lower-powered variety (300B, 2a3, 45). I have not personally done much listening to pentode tube used as SET, so I don't know if those are good alternatives.

Also, don't overlook good triode pushpull amps. They may not deliver all of the BIG, spacious sound, beautiful texture, and ambient decay (hall sound) of the best SETs (differences can be quite subtle), but, they have a lot of virtues of their own (punchy dynamics, tight bass).

Well that's a pretty good size space but 102 dB is pretty darn efficient. Its not quite double the size of my room (14 x 23) but I do have 8.5' ceilings. Assumming you've got 7.5' its less than 50% larger, but lets consider it twice as large for now. I found 9wpc 300b amps fine for moderate volume singer songwriter, soft rock, acoustic jazz stuff but the amps ran out of steam with more heavily layered material at somewhat higher volumes; with 89 dB Merlins (flat 6 ohm impedance). IIRC, each 3 dB increase in sensitivity is equivalent to doubling the power. So 89-92-95-98-101 That's about 16x more effective power, so the 300bs should be just fine and maybe even the 2A3s. There are lots of 300b amps out there and the cost of decent power tubes has dropped.

Swampwalkers calculations are close, but it is actually 20x so 2a3s and 45s are even more reliably "in the ballpark." It really is a matter of taste. 2a3s tend to be more lean (less upper bass/lower midrange) than 300bs, but more open on top and detailed sounding than 300bs (to me 300bs have a kind of blub, blub, blub bass response). 45s are sort of in the middle between 2a3s and 300b, in terms of leanness, and have a MUCH tighter bass response. They sound punchier in the bass than either actually, aside from the much lower overall output.

The overall sound of any amp can be altered somewhat by the choice of output tubes and the choice of the other tubes, so there is a range of adjustability. Also, with some 45 amps, you could replace the 45s with 2a3s, and perhaps adjust the bias for a little more current; this would mean running a 2a3 conservatively which would mean longer tube life. A friend of mine takes a 2a3 amp and biases the current down a bit and runs 45s hot. He likes the sound of 45s this way, but the tubes don't last very long (he has MANY spare 45s). There are lots of possibilities.

I am using a Yamamoto A-08s with my Zu Def 2s. With the original NU 45s, the amplifier struggled with large scale orchestral and choral works. The Emission Lab solid plate 45s that I now use give it a lot more oomph.

SETs get their capabilities from 3 things: Class A, triode, and reduced distortion at low power levels in the output transformer.

Push Pull amplifiers actually *increase* distortion as the power level falls to very low levels on account of the somewhat different behavior of their output transformers. Anytime distortion is increased, the masking effect of the human ear means that detail is obscured.

IOW its all about the transformer (its no problem to build a push-pull triode class A amplifier). But- take away the output transformer (get rid of it) and the argument against P-P vanishes.

At only 102db, unless you are just fine listening at relatively low levels, a 45 will not really produce enough power to be lifelike, although 45s otherwise sound quite impressive. It happens that in general, the smaller you make an SET, the better they sound (wider bandwidth). 10 years ago the 300b was king; 5 years ago the 2A3 had taken over but the 45 was already on its ascendancy...