Tube amp recommendation

I'd say "provide a budget", but I'll just take the liberty of suggesting a used Sonic Frontiers Power 2 if you can afford it ($1,500-$2,000). It holds bias really well, can drive most speakers and features very high build quality, especially for the money. If you can't afford one, a used Sonic Frontiers Power 1 can be had for $800-$1,000. At 50 watts/channel, a Power 1 won't drive 86 db. efficient speakers to loud volume in big rooms, but it will do fine with most 88 db+ efficient speakers in average rooms and has all the qualities of its "bigger brothers". These amps use 6550 tubes, which are fairly cheap to replace ($80/pair) and you'll get a couple of thousand hours out of the tubes anyway.

A word to the wise - you need to be very, very careful in buying a tube amp, as the vast majority of them cannot control the woofers of typical speakers. Only very expensive tube amps (which are very expensive because they use high quality output transformers and beefy power supplies) can get it done. "Entry" or "mid-level" tube amps are almost always very compromised designs and you are better off going with solid-state if you are on a budget - at that price point, it is extremely difficult to buy a decent tube amp, while there are a number of good sounding solid-state amps available which will offer better overall sound.

Ralph: I did not write that there is a correlation between the "size of the woofer and the ability for the amp to control it". What I was trying to convey, but did not do so clearly, is that the vast majority of tube amps do not have the high quality output transformers or beefy power supplies needed to control the woofers of the average audiophile speaker, which these days tends to present a 4 ohm load or less on average in the bass. To fill in the blanks for this gentleman, it takes twice the current to drive a 4 ohm speaker than it does to drive an 8 Ohm speaker, all things being equal. In rough terms, the output transformers convert voltage into current, and in my experience, only the very good ones (i.e., very expensive ones) do this with the efficiency required to drive low-impedence speakers with authority ("authority" meaning tight, accurate bass). The role of power supplies is to house and supply the voltage that is processed by the output transformers, and big power supplies are needed to allow a tube amp to properly drive low impedence (i.e., 4 Ohms or less) speakers because such speakers require a lot of current. There are very well known 750 watt/channel monoblocks that cannot control the woofers of typical 4 ohm speakers because they use mediocre output transformers, while there are also 8 watt/channel tube amps featuring extremely high quality output transformers and huge-ass power supplies that can very effectively control the woofers of low impedence speakers.

In suggesting a speaker of 8 Ohms or more, what you are suggesting is not inconsistent with what I wrote. Of course, most of your designs are output transformer-less, which is why you advocate an 8 Ohm speaker. Again for the author of this thread, an "output transfrmer-less" tube amp is, as the name suggests, a tube amp that has no output transformer. The advantages of such an amp are greater purity and immediacy, with the downside being that, generally speaking, they do not do well with low impedence speakers (maybe your very expensive products can, but they would present an exception to the rule). The problem is, other than horns, the majority of audiophile speakers feature an average impedence well below 8 Ohms, which is why I provided the above advice, which I stand by. Please don't get me wrong - I know you build top-quality products and have myself owned a zero-feedback, triode power amp for many years now (VAC Renaissance amps) - but with a typical 4 Ohm speaker, which is most of them these days, I believe it takes a really fine tube amp, i.e., something with top-notch transformers and lots of capacitance, to provide tight bass.

A couple of quick corrections to my most recent post, which I fired off in a hurry in order to make a train.

First, I did not mean to insinuate that Atmasphere (a/k/a Ralph) advocated an 8 Ohm speaker because his output transformerless ("OTL") amps do best on such speakers - Ralph has integrity and advocated such a speaker, as I read him, because such speakers are easier for any amp to drive, whether tube or transistor in design, all things being equal. As for OTL amps, they generally do require a speaker with a benign impedence (it is hard to generalize, but 6 Ohms at a minimum, 8 Ohms being better) and that is what I was trying to convey, albeit inarticulately.

Second, my last sentence concluded "with a typical 4 Ohm speaker, which is most of them these days, I believe it takes a really fine tube amp, i.e., something with top-notch transformers and lots of capacitance, to provide tight bass." I meant to write "a typical speaker that averages 4 Ohms in the bass". The impedance of most audiophile speakers varies up and down considerably depending upon the frequency, but the problem for tube amps tends to be speakers featuring an average low impedance in the bass (those also featuring steep phase angles in the bass add to the agony), as most tube amps can't deliver the requisite current (because of crappy transformers, puny power supplies, and oftentimes both).

I want to second Ralph's comments about higher impedence speakers lending themselves to better sound quality, if only because they can be driven by low or no feedback amplifiers. Absolutes are a dangerous thing in audio, but my experience is that negative feedback, which is present in virtually all high-powered solid-state amps (and too many tube amps - hello Audio Research), is the single most deleterious design feature in high-end amplification. So why do amp designers use it? Because most speaker designers emphasize linear frequency response, which means they have to dampen down the efficiency of driver cones so that a given cone puts out even volume throughout the driver's bandwidth. This means a lot of 86 db. efficient speakers (and a "laid-back", as opposed to "live-band" sound), which require an amp that can put out some serious watts. As soon as you start to get north of 125-150 watts/channel, you need negative feedback in order to have a stable amplification circuit. The result is the lifeless, uninvolving sound that you get with big solid-state amps and 86 db. efficient speakers. Compared to that, Ralph's OTL zero-feedback triode amps paired with good horns or with a very high-end, easy-to-drive speaker like the Escalante Fremont provides extremely dynamic, open and vibrant sound that is much closer to live.