Pablo,
I do not have direct experience biasing the ATM-3 but from what Ive read it should be an easy processes. This paragraph was from an online review I found:
Bias adjustments are pretty cool: a small rectangle faceplate on the front bearing the logo is removed by two screws, which hide the six bias screws. The screws holding the faceplate do double duty as standard head screwdrivers; simply switch the bias meter to the correct tube and adjust the bias screw until the needle within the meter centered within the correct range. Elegant and simple.
From the pictures I've seen on the Internet it looks like the meter on the front has a small window that indicates the proper bias setting, so biasing should be straightforward.
Remove the faceplate to expose the toggle switches and biasing screws. Each switch and biasing screw is associated with its own power tube. When you are biasing a tube you'll flip the toggle switch to activate the bias meter for that tube, make the adjustment, and then flip the switch back to its operating position. You'll follow this procedure for all of the tubes.
Note: When biasing an amp it is important that it is not processing a music signal. I follow the below procedure if biasing an amp for the first time, but subsequently its a much less involved process.
1. If you have never biased the amplifier, or suspect the tubes are really far out of adjustment, before turning on the amplifier you can rotate the bias screws counterclockwise this would be the proper direction for the amps with which I have experience so that when you bring up the amp the bias current will be minimized. Or, you can power up the amp and after 4 or 5 minutes move through the toggle switches to make sure none of the tubes are biased way too high... from looking at the picture of your amp, you probably wouldn't want to see the needle pegged to the right. Whichever procedure you follow, after a few minutes adjust the bias of each tube to the center of its normal operating range.
2. After that first check you can play music or just let the amp sit there and continue to warm up. After its been on for about 15 minutes, if you are playing music, stop the music source and if needed adjust the bias of each tube to the center of its operating range.
3. After about 30 minutes check the tubes again and adjust if needed. Recheck again after the amp has been on for an hour or two and youre done.
The above procedure is what I do if Ive never biased the amp and have no idea how well the tubes are set. Once Ive gone through this first biasing procedure, for a new amp Ill check the bias three or four times over a two week period until Im confident that it holds bias well, then I check the bias probably 3-4 times per year. Twice per year is actually probably enough, but I tend to check it more often. When performing these routine bias checks just play the amp and after a minimum of about 30 minutes (I usually adjust bias after an hour or two of playing) make any needed adjustments. Just remember that when setting the bias the amp should not be processing any signal.
Hope this helps.
