Matched output tubes are simply that…..matched. Companies take hundreds or even thousands of single output tubes and find their match based on a few parameters with the two most considered being transconductance and current draw. Transconductance (labeled Gm) is the overall strength of the tube. Current draw (labeled mA) is the measurement of how many milliamperes each tube will draw. When two tubes that have the same parameters are found they are considered a matched pair. Keep in mind that some guitar amplifiers use a pair, quartet or even sextet of output tubes so every output tube in the amp will have to match. A pair of tubes are considered matched when the transconductance is within 10% Gm and and current draw is within 5 mA of each other according to tube distributors that do the matching.
Now, are matched output tubes really that necessary and should they really command a premium pricetag? I believe no to the first part and yes to the second part. Perfectly matched tubes are not really necessary for guitar amplifiers. After so many hours of use tubes tend to drift from their original specs and fall out of “match”. Back in the heyday of vacuum tube production guitar amp manufacturers were buying large quantities of these firebottles. Factory workers on an assembly line picked tubes from bulk packaging and installed them in a guitar amp, tested it and sent it to the shipping department for it’s next destination which was the dealer and finally to it’s owner. If you notice there wasnt any matching or fussing with output tubes at all. They were just picked, installed and fired up. Now mind you the quality of the tubes back then were better because of materials and manufacturing techniques so the tolerances were probably tighter. But without careful matching these tubes would not have been truly matched. No way, no how…..unless you got lucky. Yes to the second part of matched tubes costing more because there is more labor involved. You are paying for the time it takes to pick and match the tubes with very similar characteristics.
In my personal experience I have found that matched and unmatched output tubes do sound a little bit different. There is not a drastic difference from what I have heard in my experiments but it’s there if you listen closely and “feel” for it. The difference in sound to my ear is that the closely matched output tubes sound smooth & even with a tight sounding quality and have a feel that’s fast and quick to respond. The out-of-match output tubes have more texture and character and feel somewhat looser. A true blues player may like a set of output tubes that are out-of-match. One of the most interesting sounding pair of 6L6 tubes I heard were about 20Ma out of match a few thousand Gm apart and were of two different brands! The amp had a unique voice and felt great. Another point regarding current draw is setting the bias so that the output tube that is drawing more current isn’t running too hot and “redplating”.
Listen, all this stuff is so subjective and some players may not hear or feel the difference at all. Then another group of players will totally hear a difference and it will make their day. In the end the question is does it really matter? I always say when it comes to dealing with tube amp mods and tweaking “if it’s safe and the amp and player are not in harms way then it’s ok”. Experiment for yourself. If your amp uses a pair of output tubes then buy a pair plus a random single and have yourself a little tone party. Who knows you may just discover a cool new personal tone on the cheap!