do you have to bias guitar amp pre amp tubes

Hum/noise reduction can be time consuming too. The old can cap values are not often available, in any case I prefer to rig up discreet caps which can provide mounting headaches. I love the sound of old cathode biased Ampegs, but don't really enjoy working on them. Old cathode biased Ampegs typically didn't run their power tubes at very high plate dissipations, so these also would't really need rebiasing unless they switched 7591s to a different type like the GT/EH7591XYZ which requires a new cathode resistor.

So, if they have hit you with tens of dollars, specifically to rebias your preamp tubes, then that is NOT normal. Cathode resistors rarely fail so replacing/rebiasing is rarely necessary unless you are setting out to change the character of the amp. Not that many shops have a tech permanently situated in-house, so there might be a slight case of Chinese whispers going on - because 6SL7s in Ampegs, like all preamp tubes in guitar amps, are self biasing - either 'grid leak' biased or 'cathode' biased (the M12 schematics - no dates - I have seen both show cathode biased 6SL7s). Depends what they charged you, if it's just a nominal cost/handling charge I wouldn't lose sleep over it.

Do you have to bias guitar amp pre amp tubes