Dynaco ST-70

A stereo tube power amplifier that became the best-selling tube amp in history, known for its musicality and accessibility.

Overview

The Dynaco ST-70 is a stereo tube power amplifier produced by Dynaco . It was offered in kit form or factory-wired, making high-fidelity audio affordable for a generation of enthusiasts. It pairs with the PAS-3 preamplifier and uses a push-pull ultralinear circuit configuration. The ST-70 is celebrated for its warm, musical sound and remains a favorite for restoration and modification.

Specifications

Power output
Power tubes	four 6CA7/EL34
Output tubes	EL34
Tube complement	4x EL34, 2x 7199, 1x 5AR4/GZ34
Rectifier tube	one 5AR4/GZ34
Preamp/driver tubes	two 7199
Circuit configuration	push-pull ultralinear

Design

The ST-70 was available in kit or factory-wired form, featuring a simple, reliable design with excellent transformers. It originally used a "quad-cap" multi-section capacitor in the power supply and .1uf interstage coupling caps on the circuit board. The 7199 tube was specially designed for low hum, low leakage noise, and minimal microphonism, making it ideal for high-fidelity audio.

Context

Dynaco, founded in 1955 by David Hafler and Ed Laurent, built the ST-70 around a vision of affordable, high-quality audio. It became the best-selling tube amplifier in history and served as the foundation for the Mark IV monoblock. The ST-70 was the gateway to tube audio for many and remains a legendary model in the company’s lineup of tube amplifiers.

Market

A used ST-70 was sold for $69.50 in 1965. Original power supply capacitors (quad-cap) are prone to drying out, and replacement of the original circuit board and components is common. The 7199 tube is no longer in production, with U.S. manufacturing ending in the 1980s and Russian Sovtek production ceasing around 2007. Replacing the power cord with a 3-prong can introduce ground loop hum if not done correctly. Countless units remain in service, supported by an active restoration community.