A Year Spent on Circuit Design and Voicing:
A Monaural Power Amplifier Built Around the 211 Transmitting Tube
Sound That Transcends the Usual Boundaries of Tube Amps — High Signal-to-Noise Performance as the Defining Trait

Otomon is a tube-amplifier brand based in Sagamihara City, Kanagawa Prefecture. We've featured this brand's ultra-heavyweight amplifiers built around large transmitting tubes before, and this time we want to take a deep dive into another amplifier that shows the same dedication: an A-class monaural single-ended power amp built around the large transmitting tube 211.

The advantage of a single-ended design using large transmitting tubes such as the 211 or 845 is that it lets the amplifier draw high output power while making the most of the honest, highly transparent sound that directly-heated triodes are known for. This unit, too, achieves 25W of output from a single-ended 211 stage.

The first and driver stages each use one German-made C3g pentode, with a 5R4GW used for rectification. The top chassis houses a large oil capacitor rated at 1000V withstand voltage. The driver stage is designed to pass a large current, which is how the design achieves the 211's maximum output. By separating the power-supply section from the signal path, residual noise is held to 0.2mV — delivering, according to the maker, a high signal-to-noise sound that goes beyond what's typically expected of a tube amplifier. A small amount of negative feedback (NFB) is applied, but a switch is provided to disable feedback entirely, so listeners can also enjoy the straight, unadorned tone of the 211 on its own.

Listening Impressions

For the listening test, B&W's 803 D3 speakers were used, and the amplifier produced a sound with plenty of stability and composure. Its character is spacious, low-center-of-gravity, and well-planted — a settled, secure sound.