Question from tube y newbie

If your amp allows individual biasing of each output tube and does not 'require' matched tubes, it's perfectly OK to replace just one. However, as Marakanetz said, overall distortion will be lower with a matched set. Whether that difference in distortion levels is noticeable by you is another question.

So would it be safe to say that, as long as I can bring all the tubes into correct bias, a new single output tube should be just as good as a matched pair or quad?
The age difference is still an issue, however.

Equally biased tubes i.e. matched tubes certainly produce less distortions especially on the extreme passages of music.

If you can bias each tube separately, you should be able to replace just the single tube without causing any harm, and the sound quality should not be materially impacted (depending on how critically you are evaluating the sound). One big advantage to having matched tubes is that they should age similarly over time. That being said, whether you should use matched tubes is (1) circuit dependent, so follow the manufacturer's recommendation on this issue, and (2) dependent on how resolving your system is and whether you can hear the difference using matched tubes may offer, again a circuit dependent issue.

Anytime you put in a new output tube, turn the bias all the way down before you turn on the amp, then gradually raise it as you check against the applicable meter reading. Because pairs of output tubes often interact with each other (thus the reason for using matched pairs or quads), turn down the bias on all the tubes and gradually bring the bias up incrementally in a "round-robin" fashion (i.e, a little for tube 1, then a little for tube 2, then a little for tube 3, then a little for tube 4, then start over again with tube 1).

So, no definite "should" on this issue (its amplifier dependent) and a qualified "alright".
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