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"Matching" of dual triode tubes

naimnut

naimnut

Deep in the Mines of Soul
I'd like to understand how matching affects the sound & performance of dual triodes. 6SN7s are of particular interest, but I suppose it could apply to any dual triode. There are several aspects to this question - probably closely related, but maybe not:

1. If a tube is not closely matched - let's say for the sake of discussion that the two triodes are within about 20% of each other, under what conditions will the difference be audible? How will it impact the sound? Frequency response? Extension at the extremes? Imaging? Noise?
2. In an exchange of messages on a different forum someone told me that the tube matching wouldn't really make a difference - that is, the tubes he was selling were "close enough for all practical purposes". Is this just an attempt to pass off poorly matched tubes? Or is it true?
 
Depends entirely on the circuit and context, but yes, it can certainly have an audible impact. I bought a valve tester (Orange Amps) recently and went through my pretty substantial collection of Mullard CV4024 (ECC81) that I use in my preamp trying to find three perfectly matched valves (same value on both triodes and across three valves) I eventually found three that did actually match perfectly and the preamp has never sounded better to my ears. By saying that I’d not get too obsessed with it, but I am certainly one of those who believes one needs a balance control with valve audio and vintage speakers as often the channel matching is just not within modern solid-state and ultra-precise automated assembly standards even if I far prefer the sound in other ways. I’d take a tiny, tiny mismatch over a sound quality I enjoyed less every time!
 
I'll beat Arkless to this one, even though I can only repeat the comment and not details of the reasoning.

Matching of dual triode preamp valves is of no benefit.
 
Indeed it's generally not that important as the negative feedback sets the gain (sets most things really!) not the amplification factor (mu) of the valves themselves and so even if the two channels shared a double triode with 20% difference between sections it would result in a channel imbalance of maybe 1% only.
A few genuinely zero feedback amps may be effected of course but even in many cases that no global feedback is claimed then cathode degeneration is often still used to provide local feedback.
Some varieties of phase splitter can be helped by the use of matched sections.
 
Commercially available testers generally test a LONG way from audio amplifier running conditions too...…………

Testing certainly won't screen out duds that are dud for anything but gain, even if it manages that.
 
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