How Vacuum Tubes Affect an Amp's Sound

With the evolution in electronics technology, the lowly vacuum tube (called a 'valve' overseas) is still the preferred choice for guitarists everywhere in search of that warm, rich tone that only tubes can deliver. However, with so many different kinds of tubes out on the market, choosing one can seem overwhelming, especially for a novice. In this article, we will cover a few basics about the various names and styles of tubes and the sound characteristics they deliver.

First, let's review a little about the nomenclature of vacuum tubes (how they are named). The most common tubes found in American amps are named using the "North American RETMA (Radio Electronic Television Manufacturers Association) System". There would be no point in going any deeper into this than the first number, which, in this system, represents the heater (cathode) voltage of the tube (to the nearest whole number); therefore, a 12AX7 tube requires a 12 volt DC current to operate, and a 6V6 tube requires half as much.

IMPORTANT - before replacing any tubes with different tube types, always have the amp checked for bias (with the exception of units equipped with self-biasing circuitry). An overbiased or underbiased amplifier will perform poorly and may even suffer permanent and irreparable damage.

As I mentioned in my previous article, the two main kinds of tubes are preamp tubes and power tubes. First, we will explore some of the differences in preamp tubes. The preamp is where most of the tone and distortion happens, and many American amps come with tubes of the 12AX7 variety. Generally speaking, set of 12AX7 vacuum tube generally produces a nice, crunchy distortion, while a 12AX7A can deliver the same sound characteristics with a gain increase of up to 15%. A set of 12AT7 tubes produces a clearer, sweeter sound, while 12AU7s give an even clearer sound than 12AT7s, with better, brighter high-end presence.

Power amp tubes, usually much larger than preamp tubes, are the components that create the volume, but also have some bearing on the tone of the amp as well. Power tubes can be many different varieties, but the basic 6L6 tubes are the most common, and they give off a real thumpy low-end with a great dynamic range. 6V6 tubes produce a sound that is thicker and smoother than 6L6s, and gives off a smoother distortion that does not sacrifice sound definition.

If the classic "Arena Rock" sound is what you're after, then the EL34 power tubes might be what you're looking for, which deliver a big, bad crunch when overdriven into distortion. Also, EL34s exhibits quicker distortion response, and emits more harmonic overtones in the midrange and treble frequencies than 6L6 or 6V6 type tubes, but the low end response can be a bit sloppy. A KT88 tube sounds similar to an EL34, but with a much bigger, better bottom end.

Lastly, 6550 power tubes are among the rarest, most sought-after, and most expensive out there. These highly-desirable tubes are more durable and longer-lasting than most power tubes, and stay clean-sounding even when maxed out. When driven to distortion, the produce a more solid sound and a tight, punchy low end response. The 6550 make great tubes for playing harder, more aggressive forms of heavy metal music.

Tube sound may also var by manufacturer, also. For example, two 12AX7 tubes made by two different companies may sound similar, but the internal configurations and manufacture process can have an effect on their sound characteristics that can cause them to exhibit different nuances in their comparative sounds. Even the country of origin can have an effect on a tube's sound. Some tubes come from Russia (Sovtek, Svetlana), China (Shuguang, Tung-Sol, Liuzhou), Czech Republic (AVVT, KR Enterprises), Slovakia (JJ Electronic), and the US (Richardson, Westrex, Groove Tubes). Furthermore, there are firms that sell old vintage tubes that have long since been discontinued.