Is a Tube Amp Capable of High Current?

Tube amps (except OTL types) use an output matching transformer. This transformer presents (relatively) fixed impedance (usually settable to 4, 8 or 16 ohms) to the speakers. The Matching Theorem (see any good EE circuits textbook) says that a source (i.e. amp) with fixed output impedance supplies maximum power when the load (i.e. speaker) has the same impedance.

Thus tube amps normally don't do well with demanding loads with impedances that vary across the audio spectrum. For example, when the speaker is attached to the 4 ohm tap on the amp, it gets maximum power when its impedance is also 4 ohms. Most speaker impedances vary across the audio spectrum, so power delivered to the speaker also varies with frequency. This effect is partly responsible for some of the tube sound.

Another way to look at it is that the inherent impedance of the transformer limits the current.

We can turn the matching theorum around, and ask what source (amp) output impedance delivers the most power to a given load (speaker). In this case, the lower the source impedance, the more power you get into a given load.

Transistor amps normaly don't have output transformers (there are a few exceptions). They tend to have very low output impedances. Many, (especially those built with massive power supplies and running class A) have output impedance much less than an ohm and can deliver large amounts of current into low impedance loads.

So if you need lots of current, solid state is the way to go. You can get there with tubes, but you need a lot of them, and massive (expensive) output transformers to go with them. They still supply maximum power only at the selected output impedance, though.

I don't think a 6 Ohm load will require that much current. 84 db's will require power though. A high power tube amp should fit the bill. Though a high power tube amp usually comes with a high bill. I'm not familiar with your speakers but a high power VTL should fit the numbers. Of course fitting the numbers doesn't mean it will float your boat sonicaly.

I second Angela100's suggestion of Wolcott amps (disclaimer - I'm a Wolcott owner & dealer). No, they won't double their power output going from 8 ohms to 4 ohms, and again into 2 ohms, like a beefy high-current solid state amp will. But, they will deliver their rated power into a 1.7 ohm load, which is pretty darn good for a tube amp. Especially for a tube amp that uses EL34 tubes, which are traditionally noted for their midrange performance, rather than their stability into difficult loads. But Wolcotts are the amplifier of choice of many Sound Lab owners, and Sound Labs are a difficult speaker to drive.

A feature of the Wolcotts that is quite welcome with demanding loudspeakers is the adjustable output impedance control, which allows the user to fine-tune the amplifier/speaker interface.

I'd be happy to answer any specific questions you might have.

There are a number of tube amps that will be able to handle that sensitivity. I can't comment on specific matches, but I can tell you that there are several (VTL and smaller) that might work. Feel free to email me if you want any specifics on my own experiences with other speakers. I have been on a real quest to match my current and planned inefficient speakers with "tube-like" SS. I can tell you that it will be much easier and cheaper to find high current tube amps than tube-like SS...

BTW nice answer Ghostrider!

Best of luck,
Jacob

We represent the Wyetech Labs SET Tube amps and found them to drive very difficult speaker loads better than much higher powered tube amps. the Topaz stereo amp which is rated at 25 watts can out drive my 175 watt Airtight monos. a very well designed power supply is the key!