Tube amp for Sound Labs

Thanks for the great responses! You have given me the the kind of thoughtful information I have come to expect from fellow Audiogon members.

The last version of the SL *before* the 'torriod' option was introduced was an easy load for our amps- even the M-60 did quite well on them. When the torroid option was introduced, along with it came the low impedance in the midrange that Albert refers to. No tube amp made could drive it. Since ESLs and tubes are traditional matches, after a few years I contacted SL and suggested that they rethink the crossover so that the impedance would not be so severe.

Dr. West responded with an improvement, introduced about 4-5 years ago that made the speaker easier to drive. In more recent times the newer diaphragm materials have resulted in greater efficiency and reliability, so these days the speaker has become a reasonable load again- our dealer in Chicago demos them with a set of our MA-1s quite convincingly.

In most rooms it seems that you will need about 200 watts to make things go. The speaker is based on the power paradigm so you will want an amplifier that makes constant power, not voltage, with respect to load (for example 200 watts into 4, 8 or 16 ohms), else the system will not make the bass that the speaker is capable of. IOW you need a tube amplifier. It seems that the best choices for the speaker are the CAT, Wolcott and our own MA-2 (about 75% of our MA-2 production is running on Sound Labs).

Albert did have some tube issues with our amps, but he was using NOS American tubes which were proven unreliable in our amps even way back then (Albert has not had MA-2s for about 10-11 years). Its curious that the American tubes seem so much less reliable than the Russian and Chinese tubes, but they were built to a different spec, and our amps were made for what was in production, part of our 20 Year design rule. Despite that our newer amplifiers are apparently more accommodating of the NOS Americans, but we still recommend the imports.

I've owned two pair of Sound Labs (A-1 and U-1) and over the years ran Air Tight ATM-3, Atma-Sphere MA2, VTL 750, Wolcott Presence 220, Tube Research GT 400 Viva Aurora (both 572 version and the 845 Version) and half a dozen others including some solid state designs.

All of the tube amps I've listed ran the Sound Labs satisfactorily but the Atma-Sphere MA-2 and VTL 750 have superior power delivery, particularly if you're in a large space or play loud. The Wolcott has wonderful control and is VERY reliable.

In terms of tube losses, the most reliable I've owned is the VTL 750, second would be tied between Air Tight and Wolcott and most tube failures came from Atma-Sphere and Tube Research and worst of all the Viva, which I should not have expected to drive an impedance curve like the Sound Lab presents.

Depending on which model Sound Lab you own, the Atma-Sphere may or may not like the load. There are at least four different impedance curves measured from Sound Labs, depending on model, year and design. Very early models rise to nearly 60 ohms in the deep bass and 2.7 ohms in the high frequencies. Oddly enough my old Atma-Sphere MA-2 loved the high impedance and drove the hell out of this difficult design.

There was a time when the midrange in Sound Lab dropped to a very low number (1 or 2 ohms?) and Atma-Sphere amps did not do well with this model.

The VTL 750 will drive any Sound Lab but the sound varies enormously depending on which impedance curve you're dealing with.

The Air Tight can sound thin and lacking in dynamics on these loads and generally does best with NOS Mullard XF-2 EL-34's in the output stage (or) if budget does not allow that, the Tesla JJ E-34L. As you push the power envelope of the Air Tight the high frequencies can get brittle due to stress on the power supply. Again, depending on which model and back plate design of the Sound Lab in question.

The Wolcott Presence 220 has a feedback control to vary the damping which helps a bit on Sound Labs. In my experiments, the Wolcott drives the earlier and middle era design Sound Labs best set to about 10:00 O'clock position on the feedback and selector set to the up position.

The Viva sounds wonderful on low contrast, low dynamic music but quickly runs out of head room at the first sign of difficult program material.

The Audio Research Ref 600 Mk 2 would probably sound wonderful on Sound Labs, I have some listening experience with that amp as well. However, failures of that model are so frequent as to rule them out unless you are a technician and can do repair yourself. Shipping will kill you on these they are so heavy.

Good luck with your project.

There are some good choices and some bad choices, because Sound Labs expect an amplifier with a good power supply design and capable of delivering a consistent number of watts independent of impedance rather than pure wattage ratings. There are more choices in monoblocks, but some stereo tube amps capable of driving them exist. Since I am a dealer for Sound Lab and some brands of tube amplifiers it would be inappropriate for me to self promote, but a few names mentioned by Sound Lab and others, in alphabetical order, are Air Tight, Atma-Sphere, Audio Research (big),
Convergent, Jadis, VTL, Wavac, and Wolcott. Some sound much better than others, and some are much more reliable than others.

The Wolcott amps are a proven winner with Soundlabs. There may be better, but for the price, unlikely. They are very reliable, and do not rely on esoteric parts, or a stressed design. When an output tube goes bad, it will tell you which one. Just replace it ($15) and with autobiasing, you are good to go! For high current large amps, monoblocks make sense. They also facilitate placement close to the speaker, which means a short lenght of hook-up wire can be used instead of megabuck cables. Since they use balanced IC's, these can be as long as you need without problems.