Dear af;
The external adjustment on your amp make this much easier, but be forwarned.....tube amps contain lethal (I'm not kidding here!) voltages so if you try biasing the amp yourself it's at your own risk!
First, be sure to buy a matched set of output tubes. Un-matched tubes will draw different amounts of current, causing distortion.
You will need a VOM (volt-ohm-meter) and a flat-blade scewdriver. Before you put the tubes in, roughly set the voltage......set the VOM to the highest DC scale it has (usually 500v or 1000v....the higher the better).Turn the amp on and touch the positive lead to the metal chassis and the negative lead to the bias test jack. Insert the screwdriver into the bias adjust pot and set it to -55v. Then turn the amp off, install the tubes, (be sure to plug in the speaker) and turn the amp back on. Repeat the voltage test and tweak the bias adjust for -55v. You can raise or lower the voltage by about 5v for tone preference but watch the tubes to be sure the plates don't go cherry red! This will shorten the tube life to minutes!
I'd advise that you read Gerald Weber's first two books for more detail on amp biasing. If you are not confident with any of this, take your amp to am amp tech! Call your local guitar store for a referral...there is probably a local tech that can help you out. It's not expensive and you'll avoid a possible electrical jolt!
Good Luck!
Ryan
