Unusual Tubes

Most aficionados of hollow-state electronics—at least most of the people reading this column—are familiar with the tubes most commonly used in classic or contemporary audio amplifiers. In this article, we explore some of the less common tube types used for audio applications.

- Richard Honeycutt

To begin, most of our readers are probably familiar with preamp triodes e.g., the 12_ 7 family, which includes the 12AU7, the 12AX7, the 12AY7, and the 12AT7); preamp pentodes e.g., the 6AU6); power triodes e.g., the 2A2, the 811, and the 300B); and power pentodes and beam power tubes such as the 6AQ5, the 6BQ5, the EL34, and the EL84 Photo 1). I have discussed these and other common tubes in past audioXpress articles.

Some tube audio fans are also acquainted with some of the antique tubes used in pre-war” meaning 1930s and earlier) radios, including the type 01, 35, 56, and 76 triodes; type 80 and 88 rectifiers; and others Photo 2).

A few of our readers might remember the tubes made specifically for TV use, some of which can be re-purposed for audio amps including horizontal deflection output tubes such as the 6BQ6GTB and low-power audio output tubes such as the 6K6, the 6F6, the 6W6, and the 6Y6 Photo 3). And some of you might even recall the 7-pin loctal tubes used in auto radios and a few guitar amps in the 1940s, such as the 7F7 high-mu twin triode and the 7A5 beam power tube Photo 4).

However, there are a number of lesser known tubes that were made for early radio receivers, telephony, and military uses. Let’s investigate some of these.

Photo 2: The 01 triode, like many antique tubes, had a 4-pin base.

Western Electric

Most of our readers probably know that Lee de Forest invented the Audion vacuum triode in the early years of the 20th century. The inventor and most of his contemporaries considered the Audion to be simply a sensitive radio detector. In 1912, about five years after inventing the Audion, de Forest began investigating the basis of its sensitivity as a radio detector: specifically its ability to amplify electrical signals.