Dynaco A-470 (1959–c.1970s)

The "model" here is a transformer, not an amplifier. The A-470 was the output transformer used in the Stereo 70, and it's a good candidate for the single component most responsible for that amplifier's legendary status.

The A-470 is a push-pull output transformer with a 4,300-ohm primary and screen taps at 33% of the primary winding. That 33% figure wasn't arbitrary. It was found experimentally to give the greatest linearity from EL34 tubes while keeping good load regulation. Besides the usual 8- and 16-ohm taps, there's a 4-ohm output tap, added specifically for the 4-ohm speakers (notably AR models) that were beginning to appear at the time. It's rated at 35 watts, with tapped primaries rather than the tertiary winding used on the larger A-440. Dynaco's own advertising copy from the period claimed the transformer handled full rated power from 20 cps to 20 kHz, with response of ±1 dB from 6 cps to 60 kC. It was also said to be conservatively rated to handle double nominal power from 30 cps to 15 kC. The company also claimed permissible feedback of 30 dB.

Dynaco's 1961 transformer catalog listed the A-470 alongside the A-410 (15 watts, EL84/6V6/6AQ5, $14.95), A-420 (30 watts, 5881/EL34/KT66, $19.95), A-430 (60 watts, KT88/EL34, $29.95), and A-440 (120 watts, KT88/6550, $39.95). The A-470 was priced at $24.95, second only to the A-440 in cost.

The A-470's design traces back to Hafler's earliest work. His first output transformer, the A-430 used in the MK II amplifier, was designed for EL34 tubes with the same 4,300-ohm primary and 33% screen taps. The A-470 carries those same specifications at a somewhat lower power rating. In the Stereo 70, Dynaco ran the output tubes at a lower B+ (around 410 VDC versus the MK II's ~450 VDC) to get the lower power output. Even today, when people modify Stereo 70s, the A-470 is often the one part they leave alone, or the part they specifically seek out. One detailed technical analysis put it this way: the A-470 provides "marginal loading conditions" for pentode operation of EL34s, but in ultralinear mode it delivers a flat 35 watts RMS across the audio bandwidth at the lowest distortion levels of any configuration.

Surviving information on the A-470 itself, as opposed to the Stereo 70 it's most associated with, is thin. It appears in period Dynaco parts lists and transformer catalogs, and it's discussed extensively in technical analyses of the Stereo 70. There's no standalone owner's manual or dedicated review that we've been able to locate. What we know comes from the Stereo 70 documentation, Dynaco's transformer advertising, and the technical literature that's grown up around the amplifier.

Type: Push-pull output transformer, tapped primaries
Primary impedance: 4,300 ohms
Screen taps: 33% of primary winding
Output taps: 4, 8, and 16 ohms
Power rating: 35 watts
Original price: $24.95 (1961 catalog)
Associated amplifier: Dynaco Stereo 70

Today they turn up most often still bolted into a Stereo 70. Loose examples exist, but they're not common. The "cloth" lead version is the one to look for if you're chasing an early example.