As one person pointed out, some tubes never do test out 100%. This just is how the tester is calibrated. For instance, the high voltage rectifier 1B3, 1G3, 1K3, and all other similar tubes, tends to test out at no better than 400. Often 200 is quite acceptable for this tube. For most tubes, 100% is ideal. Then you have the brand new ones that test out above 100%. So what does this mean? It just means that they are putting out more than the other tubes that run at 100%. It's all arbitrary. The numbers on an emission scale are just that, a scale. With a transconductance tester, they have a bit more meaning, but not much. A tube tester is simply an aid. Never should you replace a tube simply because it doesn't test out well against another tube or against the scale. If it obviously checks out weak, and a better tube improves performance, then go ahead and replace it. If you notice no difference between a new tube and a weak tube, then replacing the weak tube is only wasting a tube. However, in high power situations, a weak tube does not have much life left, so it may need replacement soon. If performance is acceptable, wait until it dies. In other sections like pre-amplifier and detector sections, the weak tube could last you for years. In low power amplifiers, a weak tube in the output section may last you for years as well. So again, if you like performance, leave it alone until it fails.

One reason I change tubes (on occasion) is for balance in push-pull situations, and for tonal characteristics in both push-pull output and non-push-pull output. Using balanced tubes in a push-pull circuit tends to improve tonal characteristics, and certain tube designs, though using unmatched tubes provides different audio characteristics. Sometimes distortion occurs and sometimes a different tonal characteristic occurs that may or may not be pleasing. Different internal construction of tubes with the same number type will lend itself to slight differences in tonal characteristics. Some tubes of a certain number may have more crisp treble, or more sharp mid-range, or more or less bass. These may be reasons for changing audio tubes. However, unless you have a musical ear (not necessarily trained by someone, but one of acute accuracy and pickiness), for everyday use by the average person, such tube selecting is seldom necessary.

Thomas