"Super-awesome tube-pulling thingamajig for the VC30 By MrCarrotSay the tubes are aligned like this from left to right, looking in straight from the back:X X X XYou can pull any of these combinations, O will show no tube.O X O XX O X OX O O XO X X OAlso, make sure to set the impedance switch to 4 ohms when using an 8 ohm load with it.Mine is currently set to O X O X. I've also done X O X O safely. I've not tried the others although I hear they are safe, but may as well stick with what you know is safe."Taken from http://ultimate-guitar.com/forum/showthread.php?t=821044