The importance of vacuum tube pairing
Why do the tubes need match, and how to match vacuum tubes?
Vacuum tubes can be matched for use in balanced circuits, such as pushpull output or fully-differential preamplifier use. Single-section tubes can be matched to each other, and dual section tubes can have their sections matched as well as establishing a match between tubes
For power output tubes, some amplifiers have DC and AC balance controls. These controls can null out some of the variance in tube parameters. In parallel (SEP) and push-pull parallel power tube operation, close matching is essential for maximum performance. There are generally no separate bias and drive controls when tubes are run in parallel, and if the tubes don’t share the AC and DC loads symmetrically, power output and distortion will suffer greatly.
Differential audio and instrumentation amplifiers achieve best performance when matched tubes are used. Quite often, miniature dual triodes are employed. These can have their sections matched, or all the tubes in a signal chain can be matched. The circuit topology dictates which is more important, but full matching for this type of equipment guarantees best performance.
To match tubes by emission, use the Fixed Bias Mode, and compare the resulting plate currents measured during the test. The closer the values, the better the match. In push-pull power output stages, the emission match should be within 5% if there is no DC balance control or separate bias adjustment for each tube. Even with these adjustments, closely matching the emission of each tube greatly improves performance and long-term reliability.
To match tubes by transconductance, first make sure that the emission is within 10% for the tubes. Then compare transconductance readings between tubes for a 5% or better match.
For preamplifier tubes, emission and transconductance matching can also improve performance. Particularly in “long tail pair” phase splitter operation, the tubes or tube sections should be closely matched. If the phase splitter is operating as symmetrically as possible, distortion will be reduced, and drive to the output stage will increase.
Sometimes there are driver tubes after the phase splitter but before the output tubes. These tubes should also be matched, so that drive to the outputs remains as symmetrical as possible throughout the signal path.