Where Do Tubes Come From?

One of the biggest misconceptions people have about tubes is where they’re manufactured. Currently there are no tubes made in the U.S.; production stopped sometime in the 1980’s. China, Russia and Yugoslavia are the only countries where tubes are manufactured today.

The marketing of some American companies however leads you to believe that they not only match (see below) and package the tubes, but also manufacture them. Groove Tubes, one of the biggest offenders, purchases tubes in bulk from companies such as Svetlana, Sovtek (Russian companies), sells them under the GT label and then claims their sound is better than others and they charge twice as much. While GT does test and match the tubes, there are other American companies such as Ruby who don’t claim to manufacture their tubes and charge more reasonable prices. All right, enough of the self-righteous stuff, let’s talk about tubes.

Why Do We Match Tubes?

Matched tubes: A pair, quad or sextet of power tubes that draws an equal amount of current, measured in milliamps (mA). *Note: tubes are also matched for transconductance, but this article focuses on current draw.

Tubes are matched to maximize an amp’s power, tone and reliability. When tubes are not matched, bad tone, low output and/or short tube life can result. For example, say you have a pair of 6L6’s in a Super Reverb (normal current draw between 25mA & 35mA per tube); one tube is drawing 40mA and the other 12mA. This mismatch can cause weak output, crossover distortion (a non-musical distortion) and a loud hum. Additionally, the tube drawing 40mA will wear out more quickly than the one at 12mA. This type of mismatch is extreme, but possible.

If you have purchased a used tube amp and you’re not sure if a matched pair of tubes was installed, save yourself from an expensive repair in the future and bring it in for service.

What is Bias?

Now you want to install the new “matched” tubes you’ve purchased. Not so fast! In amps that have a fixed bias (e.g., most black and silver face Fenders, Marshall 50 watts or higher, Mesa/ Boogies, etc.), make sure to adjust the bias accordingly. If the bias is not adjusted properly, a few things can happen:

• It works fine: You were lucky enough to purchase tubes close to the existing ones (possible but very unlikely).
  
  
• Crossover distortion, bad tone and low output: The tubes are over biased, low current draw, and run very “cold”.
  
  
• Short tube life: The tubes are under biased, draw too much current, run very hot (indicated by tube plates that glow bright red). This can damage the tubes and ultimately the amp itself.

Tubes can be biased within a certain range. Within this range the response of an amp can be adjusted. For example, say you have a pair of 6L6’s in a Super Reverb, the safe bias range would be between 25mA and 35mA. At the high end, 35mA, the amp will distort more quickly and respond better than at 25mA, the low end, which will create a cleaner, more sterile tone.

If you’re interested in learning how to bias tube amps, there are several good books available (e.g., Tube Talk for Guitarist and Tech, Inside Tube Amps). Of course, a professional repair shop can also ensure the bias is adjusted appropriately.