Air Tight ATM-211 tube monoblock power amplifier By Robert Deutsch   •   October, 2002 Single-ended triode amplifiers (SETs) have a considerable following, but even their most devoted fans admit that its maximum power output is not among an SET's strengths. You'd be lucky to get an SET that puts out 7Wpc, and some (like those using the 45 tube) are closer to 2Wpc. Highly sensitive speakers (eg, horns) will tend to offset the power limitation, and SETs usually sound more powerful than their measurements indicate, but the laws of physics still apply: 2W is 2W, regardless of the kind of amplifier that produces it, and an amplifier's manner of clipping and recovery from overload take us only part of the way toward achieving greater volume.

Since I've settled on the Avantgarde Uno horn hybrid (now in Series 3.0 configuration) as my reference loudspeaker, I've been exploring the world of SETs, and I now have experience with amplifiers based on the 300B, 2A3, and 45 tubes. There is something special about SETs: a kind of midrange magic, a harmonic rightness that tends to elude other amplifier designs. I can appreciate why their fans are willing to put up with their power limitations and other idiosyncrasies.

But are there ways of getting more power out of a SET? There are at least two methods that I know of. One is to use two or more power tubes in parallel. The other is to use a single high-output power tube of the type originally developed for radio transmitters. The Air Tight ATM-211 falls in this category.

Description and Design
No doubt about it: the Air Tight ATM-211 is one gorgeous piece of gear. Its three large transformers occupy about three quarters of the chassis, their covers finished with what looks like automotive metallic lacquer of the highest quality. The rest of the chassis is taken up by the tubes and a bias meter. The big 211 tube is an impressive-looking beast, casting a warm glow (and emitting a fair amount of heat) when fired up. A 12AX7 and a 12BH7A comprise the rest of the tube complement.

A front-panel switch turns on the meter, which provides monitoring of the 211 tube's bias voltage; the bias can be adjusted with a trim pot accessible through a hole in the top of the chassis. The first time I used the amplifiers, both meters indicated a slight under-bias (after suitable warmup), which was easily corrected with the trim pot; the bias did not drift during the review period. Inputs are single-ended RCAs; the speaker terminals are made by WBT.

The ATM-211 has a feature seldom seen in power amplifiers: a volume control/attenuator. Although purist audiophiles are likely to object to having another set of contacts and a potentiometer in the signal path, the idea of a volume control in the amplifier is that this lets you set your preamp's volume control in the range of its optimum performance—and absolute purists with only a single CD source could connect it directly to the amplifier, using the amp to control volume. For those with preamps that have no balance control, the ATM-211's volume control provides a way of correcting left/right imbalances caused by slight differences in gain between the two channels or asymmetric room acoustics.

The ATM-211's design is said to follow that of Air Tight's highly successful ATM-300 (300B tube, 8Wpc), the 211 output tube permitting higher power. The ATM-211 operates in class-A with no overall negative feedback, and uses a direct-coupled cathode follower-driver, and a DC-driven heater filament that is claimed to eliminate hum. There is a relay muting circuit, intended to protect the electrolytic capacitors for the output tubes. (A blue indicator light flashes during this warmup period.) Volume control is provided by a 100k ohm variable resistor providing a shunt to ground. The output transformer comes set for a nominal 8 ohm load, but can be factory-configured for 4 or 16 ohms.