Tube amp advice

There's a fair amount of misunderstanding about tube amps and what distinguishes a good one from everything else. To drive speakers that feature low impedences in the bass, a tube amp's wattage rating is largely irrelevant - what matters is the quality of its output transformers, which convert voltage to current, and how stiff (robust) its power supplies are. Really good output transformers are extremely expensive and heavy, and stiff power supplies are very heavy. Thus, there's an old adage, which is generally quite true, that the heavier and more expensive a tube amp is, the better it's going to sound on hard-to-drive speakers. To put things in perspective, 100 watt/channel tube amps tend to weigh 50 lbs. to 80 lbs. What is generally recognized by knowledgeable audiophiles as the most "powerful" regular production tube amp, the CAT monoblocks, put out 100 or 200 watts/channel (depending upon the model), but weigh 192 lbs. per chassis and cost $30k to $50k (depending upon the model). In short, most tube amps, including those that the magazines have concluded are "good", really can't drive tough-to-drive speakers like Wilsons (or anything else that drops below 4 Ohms in the bass).

An amp that was well built and is available for very reasonable prices used is the Sonic Frontiers Power 2 (it features balanced inputs, but is single-ended). If you can go up to $3k or $4k, the BAT VK-75 SE is a good amp that is fully differential balanced. Also for $4K is the VAC 70/70 (Mk. III or Signature), which is an amp I have owned for ten years. It is a tremendous amp that does it all (fully point-to-point wired and fully Class A biased, zero feedback, directly heated triode 300B output tubes, auto-biasing, sentry circuit to automatically shut down out-of-spec tubes, and totally dual-mono all the way down to separate power cords and on/off switches - the amp will drive 1 Ohm loads), but it does not have balanced inputs - I run a balanced cable to it from my balanced preamp, and use Neutrik convertors on its single-ended inputs.

My advice is to stay away from tube amps for low impedance speakers unless you are prepared to spend what it takes to get one that can do the job. Unfortunately, most tube amps manufactured these days cannot.

Good luck.

A final comment - one reason older tube amps are so desirable is that the output transformers in tube amps take forever - years - to fully break in. This is one large advantage five to ten year old amps have over new ones.

Although beside the point, another thing to remember is that the tubes are the circuit in a tube amp - if you retube the amp, you essentially have a new amp, this providing a gigantic advantage over solid-state amps for longevity (some high-profile solid-state amps become door stops if they ever lose an output transistor because the transistors have gone out of production - this won't happen to a tube amp that uses output tubes common in the tens of thousands of tubed guitar amps that are sold each year, e.g., EL-34's, 6L6, 6V6, 6550, etc., or that use a classic triode like the 300B).

A really good tube amp is like a Porsche 911 - a bit tricky to operate and conceptually not the most up-to-date, but capable of extreme performance and it will be around when most everything else is long gone.

Deltatrippers:

I intended to make a comment in response to your post, but forgot. I ran the original Revel Salons for three years and at least with respect to the original Salon, believe that the standard advice about powering the speaker at the time, which was that it sounded best with massive amplification, was dead wrong. The first and current versions of the Salons both use high-order crossovers and proprietary drivers that can handle a lot of power - one of my dealers had a customer that ran a pair of the original Salons with 1,200 watt monoblocks and they indeed can go extremely loud, cleanly. But this misses a crucial point, which is that the Salons are also very much a finesse speaker that comes a lot closer than many state-of-the-art speakers do at accurately reproducing the sound of unamplified acoustic instruments (i.e., most of the music in the classical and jazz genres). If you put a typical high-powered amp on Salons, the global feedback used to stabilize the circuits in such amps is clearly audible. If you search the threads, you'll see a lot of complaints about the speaker sounding lifeless and two-dimensional - that's because they tend to be demo'd with high-powered solid-state amps, often Levinson 300 series amps, which all use global feedback.

In short, the original Salons actually sound best, if the realistic reproduction of musical instruments is your goal (as opposed to shaking the neighborhood with home theater content), with lower powered, 100 to 150 watt amps that feature fewer output devices (i.e., fewer output tubes or transistors) and preferably, little or no global feedback - these speakers are very revealing and transparent like few others, and you really can hear what the upstream components are doing. Perhaps because the original version of the Salons can now be found used for $4,500 used, people think that they can be used with any old equipment - they can't. 100 high-quality tube or solid-state watts really lets them play music. If you want to reproduce dinosaurs stomping, and they can certainly do that very well, too, then run them with huge solid-state monoblocks, but if you want music, put a high-quality, zero-feedback tube amp on them - they can be really good speakers. As for the Salon 2, I don't have any meaningful time with them, but would imagine that they're even more demanding of quality amplification given their beryllium tweeters.

Deltatrippers:

These days, a dip to 3.7 Ohms in the bass is almost considered moderate given how many speakers drop below 3 Ohms (and even down to 2 or below). To answer your question, the problem with most tube amps driving speakers featuring large impedance dips in the bass is that the amp can't deliver enough current to control the woofers - bass response is flabby and ill-defined.

A real-world example is all of the people out there trying to drive WattPuppies with $4,000 tube amps and getting Sony rack-system bass. Perhaps the best example is the CAT JL-1's mentioned in one of my previous posts, which were the original CAT monoblocks from the late 90's and only rated at 100 watts per channel - they were designed to drive the very inefficient MBL 101B, a speaker which also features wild impedance swings. The output transformer alone in these 192 lb. amps weighs 55 lbs. The bass performance of those amps sounds like a 500 watt Krell. Here is an interesting thread that discusses the CAT amps at length:

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1099200096&openfrom&1&4

My best hi-fi buddy ran the CAT JL-1 Limited Edition amps (the LE's were $50,000, featuring incredible parts quality) for a few years and they are indeed the king of the jungle. With big symphonic music, a high quality DSD recording, and high-output speakers, they will knock you through the listening room wall on peaks.

I do not know how the Triton would perform with low impedance speakers, but it uses KT88 output tubes, which are difficult to make, the result being frequent tube failure. With KT-88's, it's all the more important to have a knowledgeable, reputable supplier.