A Field Guide to Tubes

The basic technology behind the modern vacuum tube has been around since 1906. Yet few musicians seem to have even an elementary knowledge of this rather low-tech device. In reality, the various functions and characteristics of tubes are easily understood. What’s more, once armed with this knowledge, you’ll be better equipped to troubleshoot amplifier problems and to improve your guitar tone.

Tubes (called valves in other parts of the world) can be divided into three families: preamp, poweramp and rectifier. Each family comprises a range of tube models possessing different performance specifications. Every model of tube has its own tone characteristics, and even two identical tubes generated by the same manufacture will produce marginally different sounds.

Of the three varieties of tubes, preamp tubes are the smallest, and they are the first tubes that a guitar signal encounters in an amplifier. It’s here, in the preamp stage, that the lion’s share of tone shaping takes place and where the desired amount of gain is applied to the signal. The common preamp tube is the 12AX7, also known as the ECC83 outside of the U.S. Technically speaking, the 12AX7 is suppose to be “hotter”-that is, possess more gain-than the ECC83, although from my experience, the difference between them is slight. The model 7025 preamp tube has more gain and is brighter than the 12AX7, but it is rarely found in the U.S. It’s worth noting that most preamp tubes are interchangeable.

Power amp tubes, also called output tubes, watch free videos boost the signal as it leaves the preamp. They are the largest variety of tube and, once installed, must be biased in order to optimize their performance within the amplifier. When driven hard toward their maximum potential, power amp tubes produce output distortion, a warm, organic sounding and very desirable tonality whose characteristics are determined by the power tube’s distinctive attributes. While some models of power amp tubes are interchangeable, many substitutions can’t be performed without amplifier modifications, and a few substitutions are impractical due to amplifier designs.

One of the most common and popular choices of output tube is the EL34. Widely associated with British amp makers, including Marshall, Orange and Hiwatt, the EL34 sounds great when driven hard, maintaining warmth, crunch and great dynamics, with evenness on both the low and top ends. The EL84 is another great-sounding tube with a rich British history. Although smaller than the EL34, the EL84 is anything but weak. Employed within Vox AC30, making the 30-watt amp sound, at full volume, like it’s pushing 50 to 75 watts, while retaining sensitivity, warmth and a sweet distortion.

Other popular output tubes include 6L6 and 5881, which can be found in Fender, Mesa Boogie, Peavey and more recent models of Marshall amps. The 6L6 tube is powerful and maintains clarity and definition while delivering a big low end and harder crunch than EL34. The 6V6 power amp tube has been a very popular choice in low-power combos. It’s a well-rounded performer in all ranges of the frequency spectrum and distorts relatively quickly, delivering a desirable sound. The 6550-probably most famous for its use in the Ampeg SVT bass amp-is seldom used, but it is a powerful beast, with a slug like Mike Tyson. Marshall amps that employ EL34 tubes can be professionally set up to run on 6550s, whereby they produce a mammoth bottom end.

Rectifier tubes, which include the GZ34, 5AR4 and 5U4, convert AC current to DC current. They are, however, a thing of the past, since most modern amps employ a solid-state rectifier in place of a rectifier tube. Even so, a tube-rectifier amp produces a type of compression that creates a “sag” in a signal’s tone; solid-state amps have a stiff response by comparison. Tube rectification isn’t necessarily superior to solid-state rectification; it’s really a matter of personal taste.

It’s important to remember that tube have a finite life span, Each time you fire one up, you wear it down a litter further. Eventually, you’ll need to replace all of your amp’s tubes in matched pairs, guartets or sixlets (depending on your amp’s requirements) and have any new power amp tubes properly biased by a professional. If you gig and rehearse heavily-say, four nights a week or more-I recommend that you replace your tubes once a year. If you play less often, keep your ears open for diminished tone and response from your amp, and replace your tubes at earliest sign of trouble. Where tubes are concerned, sooner is better than latter.


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