Re: how to bias an amp...
My biasing technique, since I don't have a oscilloscope, is to use a bias tool and a voltmeter.
First, put the bias tool in between the chassis and one of your current tubes, just to see how many mA are running through it. That'll at least give you an idea of where the new ones should be.
Next, take the chassis out, plug your guitar into it, and plug it into a speaker cab.
With the chassis propped on it's side, find the mini pot near the power tubes and use a mini screwdriver to turn it...not yet tho.
I lay my guitar on the ground and strum it with one hand, while carefully making adjustments with the bias pot. I keep my eye on the mA meter to make sure 6L6's are around 32mA and EL-34's around 38mA. As I make those slight adjustments, I listen carefully to the tone to make sure it sounds it's boldest, loudest, and healthiest.
I also listen on clean and OD channels, to make sure there's no mushiness anywhere.
As long as the mA are inside their parameters and the amp sounds as good as possible, I'm happy. Then, I move the bias tool around to make sure the other tubes are reading equal numbers, make a small tweak if necessary, then put the chassis back into the cabinet.
What you DON'T want to do, is allow the bias adjustment to be too hot or cold. It's very easy to turn those gray plates bright red, by turning the pot too fast, or something. Techs only suggest you take it to a professional because they know you might burn up your tubes. As long as you keep the bias pot somewhere in the middle of it's travel, use the same type of tubes the amp had before, and know how to adjust the bias to make your amp sound it's best, it's really not that difficult.
