6L6GC Push-Pull Altec-Type Stereo Power Amplifier Part 1 【DIY Amp Lab】

Simple Circuit Design Achieving Rich Bass and 31W Output, Designed and Built by Ueno Hiroshi

This 6L6GC push-pull power amplifier was designed with a focus on high output power. Its circuit follows the Altec-type configuration, employing the triode-pentode MT tube 6GH8 to secure sufficient gain in the front stage — the pentode section serves as the input stage, while the triode section functions as a phase splitter using the PK-split method.

To improve low-frequency response, an Orient EI-core output transformer was adopted. The design applies 18 dB of negative feedback, and the article details techniques for preventing low-frequency oscillation.

The resulting amplifier delivers balanced sound, with lush mid-to-high tones layered over an excellent low end. It achieves 31 watts in beam connection and 18 watts in ultralinear connection.

Pursuing High Output with a Simple Circuit

What we expect from a power amplifier depends on the TPO—time, place, and occasion. When you want to quietly enjoy music alone after work, a few-watt single-ended amplifier built with carefully selected parts may be ideal. On the other hand, when reproducing big band, brass band, or orchestral performances at a listening session, you’ll need a push-pull amplifier capable of delivering a stable output of 20 watts or more.