Fender Twin/Dual Showman small change of output bias effects sound?

I just gave my vintage 1969 Dual Showman Reverb Pro (without master level control) to a tech and he said he tweaked the output bias of the amp by a little bit.

The amp now doesn't sound the same, it seems brighter and cleaner. It's lost the "magic". He reckons I might be imagining it, but I've had the amp serviced 3 times and this is the first time I've noticed a negative change. Also I compared recordings with the same setting and the sound has clearly changed.

Is it normal for a small bias tweak to negatively affect the sound like this?

And if it's just a very slight bias tweak, is there any reason why I can't just turn the screw slightly without bringing it back to the tech (it would be a huge hassle, and I don't have time to wait)?

Thanks in advance.

If it's biased too hot it may burn up the amp.  While sounding amazing.  AC/DC do exactly that on the road and kill a Marshall or two a night. For real.  Tubes also get used one day only.  Your tech may be able to bias it slightly hotter than it is and get some back, maybe with risk to the life of the amp which you'd have to agree to.  You should not change bias yourself without knowing exactly how. 

Did the tech only adjust the bias? Did they change anything else?

Gus said:

Did the tech only adjust the bias? Did they change anything else?

Click to expand...

By reading your post I would ask the same questions Gus asked.

I guess you took it to the tech for some repair work. What was changed?

If the bias was tweaked that means that probably the output tubes were changed also.  As a standard practice most people bias the push-pull amps to 70% of the maximum plate dissipation of the tube, in your amp they're 6L6GC.
That doesn't mean that 70%  is the right value to use, it's a compromise between Tube Life and the amp sounding "cold". Higher you may enjoy the sound but the tubes wont last , lower and the tubes will last but some people say the amp doesn't sound good.

Like was advised don't mess around with the Bias pot yourself.
If the sound changed, a lot of reasons could contribute to that, I will not put bias in the first place, I'm sure if you amp tech tweaked the Bias setting it was because it was needed.

Tell us more info first...

yes a slight change in bias can really change the sound,

your old bias setting used to a allow more idle current to the tubes,

maybe you had 45 ma per tube, now it might be 35 ma,  you can adjust the amp back yourself,

there is a pot for this which you can turn without taking the chassis out of the enclosure, it is located near the pwr transformer if this is the amp i am thinking of,

turn it slowly until you get your sound back, how do you know if you have gone too far? the tubes might begin to show red spots on the plates,  you can also just put your hand next to them and see how much heat they radiate, another sign of too hot a bias is that you will smell the sockets or tube base  material. Some tubes do this even when biased normal, Svetlana EL34's come to mind,

use to have a knob on the bias pot for my twin reverb, i could bias the amp according to the room we were in, bright room with no carpet means hot bias, dead room with no echo means colder bias for more punch,