Good, scientific evidence to help clear up a myth on tube amps and biasing?

[pompeiisneaks]

I've been hearing conflicting information from several different sources on 'Do I need to bias my amp every time I swap tubes?'  I think it was Mesa that implied you should never need to, I've read Aiken Amps implying all amps may need it pretty regularly, even cathode biased ones as things 'drift' over time, and such, but someone, somewhere was adamant, maybe only for fenders, but that biasing is rarely if ever needed, unless someone biased an amp wrong before hand.  What's the consensus on that?  In my understanding, right now, you should really only worry about bias if an amp is used, but new to you, in case someone was crazy with it, but otherwise, if it's played fine with the current tubes, they've had a long life and all seems equal, swapping tubes isn't a mandatory bias set as well.  (I know how to, and will for mine of course, but for the average player is more the point of this question). 

Anyone have any good scientific information or stats on this I could use to understand the topic better?

~Phil

[SILVERGUN]

I know you mentioned Aiken, but not sure if you read this article:
http://www.aikenamps.com/the-last-word-on-biasing

And there's this, if his approach is more up your alley:
https://tone-lizard.com/biasing/

This book is an invaluable reference and full of scientific facts and figures:
https://www.ampbooks.com/mobile/books/power-amps/

[brewdude]

I think a fixed bias amp should be checked when changing power tubes.  Cathode bias is more forgiving. 

[pompeiisneaks]

I think a fixed bias amp should be checked when changing power tubes.  Cathode bias is more forgiving.

Thanks, I'm more going for the facts and data, I believe that process isn't 'bad' but I have also heard opposing opinions, I'd like to get more data on facts around this. 

~Phil

[pompeiisneaks]

I know you mentioned Aiken, but not sure if you read this article:
http://www.aikenamps.com/the-last-word-on-biasing

And there's this, if his approach is more up your alley:
https://tone-lizard.com/biasing/

This book is an invaluable reference and full of scientific facts and figures:
https://www.ampbooks.com/mobile/books/power-amps/

Thanks I'll check those, yes I mentioned Aiken because I'd read that article, I may reread it again to see, but again I don't recall if he quotes any real scientific data on it, or if it's just opinion as well.

~Phil

[jjasilli]

I A/B'd bias settings in a fixed bias amp for a customer while he played guitar.  For visual reference, I made marks on masking tape for coldest & hottest bias settings at idle.  Then I dialed in different settings while he played guitar through the amp.  There was a definite difference in tone with different bias settings.  I set it where he liked it (we actually both agreed on the best setting).

[pompeiisneaks]

jjasilli, I agree 100% that the bias setting impacts tone, this question is about the mandatory bias every time you change tubes type situation.

silvergun, As for that tone-lizard article, that's one I'd read a long time ago and is exactly the kind of thing I was looking for, it explains things with scientific explanations, that make sense.  Thanks!  I've got that third book at home, I may have to look for bias data in it too.

~Phil

[jjasilli]

I found that small changes in bias setting made an audible difference in tone, which was noticeable by two people at the same time (me and the amp owner). This implies that tone will change when bias changes over time as tubes age; or with new tubes.  CAVEAT:  none of the tone within the bias range was bad.  There did seem to be a sweet spot, but that's listening "in the living room".  I cannot guarantee it could be heard in a band situation.