Fixed bias means the negative bias voltage comes from the power supply and is fed to the grid. It stays constant, or "fixed", as opposed to cathode bias, which changes with the cathode current when signal hits the tube's input. Whether fixed bias is adjusted by changing resistors or turning a pot doesn't matter, it's still called fixed bias. Your new Ampeg is fixed bias too.
then I just used the wrong term because my point was that not all amps have adjustable bias. My older Ampegs are non adjustable although you can certainly change resistors to accomplish that.
And my fairly new Mesa Blue Angel has no adjustable bias either. In fact, there was a section of the owners' manual that specifically addressed this and said that they didn't believe in adjusting the bias ..... they set it where it should be and they preferred that you get tubes from them or get tubes they reccomend because they know what the design called for and didn't want any techs mucking it up.
It also said that for this amp the bias wasn't a critical issue and though I don't know this for sure, the tone of that section led me to think that perhaps all their amps are set up that way.
IT sure sounded like that's how they think amps should be.
Other amps, like Doctor Z's for instance, are nowadays self-biasing which does away with the need for it.
But I very clearly stated in my post above that rebiasing after a power tube swap is preferable .... just becaus I don't bother with it doesn't mean anyone else should make that their mantra. My main point was that not doing it doesn't mean death and destruction to your amp.
Sometimes when people see these things, they get the idea that if you don't rebias, it'll destroy your amp and in fact someone in this thread made a similar comment.
I'm not saying
don't do it ..... I'm simply saying that if you don't ..... it won't really destroy anything ..... your amp simply won't be at its' best.
As for why I don't do it ...... it's because I gig virtually every day and often double gigs .... hell, bike week coming up I already have 11 gigs and will probably squeeze in 3 or 4 more. I tend to grab whatever amp's by the door and when I get home I toss them in a corner. After over 40 years of it I just don't really care about equipment and the endless tweaking to get this or that sound I'm obsessing over anymore. I get comments on my sound often and I just do what I've always done since it seems to work well.
But my main point was that it's not an absolutely neccessary thing to do to avoid amp damage.
