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RE: Help with KT150 questions

Posted by Alex Kitic (A) on April 23, 2020 at 11:07:33

In Reply to: RE: Help with KT150 questions posted by daveyf on April 22, 2020 at 13:31:18:

If one tube was shown as down in transconductance in respect to the other three, this means it is not matched. If it is one half of a matched pair, obviously the pair is not matched.

"The tubes seem to work fine in the amp" - which is a subjective statement and is most probably quite correct because the main purpose of the amp is to amplify music, and if you are satisfied with the sound quality, that is sufficient.

From a technical standpoint, the tube that is not matching will cause higher distortion in that channel (I am assuming push-pull pairs, where one pair is matched while the other one is not). This would probably be evident on measurements. Still, the distortion as measured - might not be noticeable when listening.

Thus, since the amplifier is self-biasing, there is not way for you to either correct somehow the problem (by manual bias, although that is not the whole story because adjusting the current does not adjust the transconductance i.e. the other parameters of the tube in the circuit - only the current draw, which is beneficial for the output transformer which will see less or no DC unbalance) - or even be aware that the problem exists.

Getting back to the different transconductance for the particular tube of a presumably matched quad that came with the amp as supplied by the manufacturer - if it was tested different at any relevant voltage (let's say at 300V and not 500V) - than it is different and not matched to the rest.
How did that happen? Very probably, the tube has lost more transconductance when aging in respect to the other tubes.
Another explanation is that the tubes were never really matched, either due to a mistake in labeling or putting in the wrong box... or you can try to imagine any humanly possible mistake.

BTW, you never mentioned the manufacturer of the amp. When it comes to KT150, there is only one manufacturer - but it is a new production tube and quality is nowhere near as equalized or constant as it used to be when tubes were in mass production with serious and less serious manufacturers.

I hope this is both easily understandable and clearly readable.

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