25L6GT Push-Pull Power Amplifier Part 1

Output: 8W — Ingeniously Designed Power Supply Circuit Delivers Excellent Results, Designed and Built by Soya Susumu

This push-pull amplifier employs the beam power tube 25L6GT, one of the less commonly featured transformerless tubes in this magazine. A 12FQ7 cathode follower stage directly drives the 25L6GT output tubes. The input and phase inverter stages, built around a 12DT8, offer ample gain, allowing for overall negative feedback.

In addition, cathode feedback is applied to the output stage by utilizing the NF winding of the output transformer. Since the circuit is composed of easily obtainable parts, it’s highly recommended for builders to replicate and experiment with.

Introduction

In recent years, it seems that obtaining vacuum tubes has become increasingly difficult. Original versions of classic tubes such as the 300B, 2A3, and KT88 are now extremely hard to find—and even when they do appear on the market, their prices are prohibitively high.

By contrast, transformerless output tubes remain surprisingly affordable. Perhaps this is because their heater voltages are relatively high, making them less convenient to use, or simply because they are not widely known. In fact, for some types, ample stock still exists, making them something of a “hidden treasure trove.”

Among these transformerless output tubes, this article features the GE 25L6GT. The data sheet also lists a 50L6GT version with a 50V heater. The late Isamu Asano noted in his book ‘The Enchantment of Vacuum Tube Amplifiers, Volume 2’ that some transformerless tubes were actually used in commercially manufactured audio amplifiers in the past.