Yamamoto Acoustic Crafts A-03S KT88/6550 direct-coupled single stereo power amplifier

The A-03S is a single stereo power amplifier using KT88/6550, the most popular and easily available pentode vacuum tube, in the output stage.
The A-03S was designed with the goal of achieving 100% performance and sound from the KT-88 and 6550 large vacuum tubes, which have almost identical ratings, as a single, full-fledged amplifier.
The standard output tube included is a Tungsol 6550.

The A-03S has a high output power of 12W/12W by separating the output tube plate circuit from the screen grid circuit.
The circuit configuration uses a direct-coupled system that is renowned for its sound quality and circuit stability. A direct-heating type pentode tube is used in the first stage, and by directly connecting it to the output tube, a highly responsive and dynamic sound is obtained. The 1D5G in the first stage uses a constant-current ignition system, which stabilizes the output tube bias voltage and greatly improves the stabilization of the output tube current by using a constant current including the cathode bias resistance. This unit comes with an ammeter to check the output tube current so that users can easily replace various KT-88s and 6550s from around the world and enjoy comparative listening.
The power supply section of the A-03S uses a power transformer, choke coils, and output transformer with an Orient core cut-core transformer, and the output transformer uses Teflon insulation paper with excellent electrical characteristics to achieve excellent sound quality.
In addition, polypropylene film capacitors with excellent transient characteristics and distortion rate are used for most of the filter capacitors in the power supply to achieve a sound with excellent transparency. The charge and discharge times are much better than those of ordinary chemical capacitors, and the distortion rate of the capacitors themselves is also incomparably superior. Even for a capacitor in the power supply section, its sound quality is directly reflected in the reproduced sound. The use of this film capacitor contributes greatly to the clear and excellent characteristics of this unit.
The output stage uses a 5U4G rectifier tube, with two luxuriously connected in parallel to efficiently supply the large current of the KT-88/6550.

Many other top-quality components are also used. Dale non-inductive wire-wound resistors, speaker terminals machined from pure copper, 3-pin power input terminals, and a gorgeous champagne gold chassis and transformer cover are just some of the features that have not been skimped on at all. A solid cherry wood wooden frame and aluminum construction for total non-magnetic material eliminates magnetic distortion.
The circuit board is made of 2.0mm-thick glass epoxy board to ensure satisfactory sound quality.