A (Z)OTL with 300Bs: Part 1-The Makeover

Rebuilding an amplifier for reasons of better reliability, better aesthetics, or bringing it up to the latest standard can be very rewarding. And if it is a classical, technological grandstand, even more so. Thomas Perazella embarked on such an adventure with a famous David Berning ZOTL amplifier. In Part 1 of this article, Tom presents the amplifier and discusses its unique circuitry. In Part 2, he will proceed to dismantle and rebuild the amp for the new owner and place it in a new chassis

- Thomas Perazella

It has been intriguing to watch the renewed interest in the 300B triode for audio circuits. A descendant of the 300A originally produced by Western Electric, it has in recent years taken on an almost mystical reputation with certain audiophiles. There are many interesting stories of the 300B on the Internet.

One of my friends, David Berning, has been building very high-quality vacuum tube audio amplifiers for many years. He has used many different tube types, but the fundamentally outstanding feature of his designs is his patented ZOTL technology. One of the factors that is imperative in getting good performance from a vacuum tube amplifier is the process of converting the high impedance of the tube to the low impedance of the speaker in a manner that preserves the frequency response, phase, and low distortion of the active device. The most common method of getting this impedance match is the use of a transformer. We will take a closer look at transformers and the factors such as leakage inductance, interwinding capacitance, hysteresis, saturation, and others that make it very difficult and costly to build a wide bandwidth transformer free of gremlins.

Transformer Basics

Over the years, audioXpress has had several excellent articles describing transformers and their applications in audio. The most recent article series is listed in Resources and it references a number of additional articles. They contain a wealth of knowledge on transformer design, materials, construction, and compromises.

Two of the most important aspects of transformers in audio use are for electrical isolation and impedance conversion. Let's look at isolation first.

Isolation