- #DO YOU HAVE TO BIAS GUITAR AMP PRE AMP TUBES INSTALL#
- #DO YOU HAVE TO BIAS GUITAR AMP PRE AMP TUBES DRIVER#
SO if the guy said he needed to install and bias a new 6SL7, I would be suspicious Certainly one should check that the circuit is working - ie no shorted bypass cap - but I would never describe that as biasing the tube. All preamp tubes do not have to be "biased" as a verb. All preamp tubes have to have some sort of bias to operate properly - bias as a noun. I think the argument here is what the shop meant. That said, they should keep you informed, prior to hitting you with a big bill, so that you can make the decision as to whether or not to go ahead, or take it somewhere else. With any tech/shop you are really looking to them for guidance as to what work "needs" doing, they are supposed to be the experts after all. If it ends up costing you maybe $150-$200 and it sounds great then that wouldn't necessarily be out of the norm (maybe more if you're particularly bothered about hums/noises or if you needed other major parts). Ideally, you want them to itemise the work done so that you can query it with an independant party in the event that you're not happy with the sound of the amp. Pots, jacks and tube sockets should be cleaned/replaced as necessary.
#DO YOU HAVE TO BIAS GUITAR AMP PRE AMP TUBES DRIVER#
So if you really want it to sound as good as it could I wouldn't expect it to be that cheap as I'd expect the filter caps to be replaced, preamp & output cathode bypass caps, the power tubes and at least the driver tube.
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.
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