The Pilot AA-904 was a Williamson-type power amplifier using KT-66s in an ultra-linear circuit with 24dB of negative feedback. The amplifier had no controls and was designed for use with an external preamplifier or tuner.

Harmonic distortion was typical of Williamson-type designs, less than 0.3% at 25 watts and less than 0.1% at 10 watts and frequency response was quoted by the manufacturer as 15Hz - 50kHz ± 1dB at 15 watts.

Finished in brushed gold, the AA-904 was described by Pilot as 'a deluxe model' and retailed for $89.50. A later model (AA-904A) with much the same specifications was selling for $99.50 in 1957.

Today these monoblocs are much sought after, with working examples fetching between $400 (fair to average condition) to over $650 for a mint or restored example.

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