The way most tube suppliers do it,is they measure the current at a specific voltage,theoretically it should match the voltage of your Boogie.If the tube falls into a safe current draw for that voltage,they are sold to you as supposedly "matching" the current draw of your amp.There can be a fairly wide range of draws that are safe for a particular amp.So you have a set of tubes that sounds good,when you go to replace with what you think are tubes of the same current draw,they may be hotter or colder than your last set.Some suppliers will get you a set of tubes that will be closer to what you want,but you have to ask for a specific current draw.For example,a set of 6L6GC's can safely be set to draw 30-60ma's at 400v's.Thats a wide range for a tube supplier to use,making for less rejects on his part,but can make a big difference in your amps response if you had a set that measured 50ma's and the next tube change you are getting 30ma's.The colder you set the bias on your power tubes,the less break-up you get out of your power tubes,can make for a very cold,sterile tone particularly in the clean channel.Mesa's get their distortion from the pre-amp being overdriven,usually refered to as "pre-amp distortion",so if you like that kind of distortion you can bias on the cold side and get away with it,but the clean channel may sound kind of sterile.If you bias them a little hotter you can get a little more grind out of your power tubes,but go too hot and the lead or overdrive channel may get kind of ratty.I prefer to set my own bias on the hot side,in my Boogie amps I use the clean channel mostly,but have switched my pre-amp tubes with a lower gain tube to keep the lead channel in check.
