One of the most popular and revered guitar amplifiers in the world, the Blackface Super Reverb can be heard on countless recordings from the 60s. Like any tube amplifier, the Super Reverb requires periodic adjustment. Biasing a tube amplifier is an important aspect of maintenance and is essential to ensuring its proper operation. Like performing a tune-up on a vehicle, biasing an amp will make it sound better and help the power tubes last longer, providing years of musical enjoyment. Setting the bias on a Blackface Super Reverb allows you to adjust the sound based on your musical and playing style.
Turn off your amplifier. Since you will be dealing with high voltages, make sure you turn off the amp. You can even unplug it for added security. You will plug it back in and turn it on once you are ready to set the bias. If the amp has just been used, allow the tubes to cool down to avoid burning your fingers. Remove the screws securing the back cover to gain access to the power tubes.
Locate the power tubes. The power tubes are the two largest tubes. They should have 6L6 written on them. Remove them from the sockets by prying open the metal tabs, grasping them at the base and gently rocking them back and forth. Lay them on a soft cloth and avoid touching the glass, as the oils from your hands may cause them to malfunction. The smaller tubes nearby are the preamp tubes and do not need to be biased.
Connect the tubes to the bias tool. Some bias tools come with two sockets to make bias adjustments easier and quicker. Gently insert the tube(s) into the socket(s) that came with your bias tool, paying special attention to the key on the bottom of the tube to make sure it is properly connected to the socket. Connect the bias tool socket(s) to the sockets in the amplifier, again making sure the key is inserted correctly. If you are putting in new power tubes, make sure you purchase a matched set. This will make it easier to bias the amplifier.
Turn the amplifier on. Once the bias tool and tube(s) are connected to the amplifier, plug in the amp and flip the power switch but leave it on standby to allow the tubes to warm up. Find the trim pot on the chassis, identified by a small input jack with a screw head that allows you to adjust the bias. You may have to remove the chassis from the cabinet for easier access, but do not unplug the speakers. Flip the standby switch and turn on your bias tool. Current draw for the first tube will appear on the bias probe screen expressed in milliamps (mA).
Set the bias on the first tube. Insert a small screwdriver into the trim pot and slowly rotate it left to right to adjust the bias. On a Blackface Super Reverb, current draw should be between 38 and 44 mA. Setting the bias colder (closer to 38 mA) will thin out the sound but prolong the life of the tubes. Setting it hotter (closer to 44 mA) will decrease the life of the tubes but give you a warmer, more distorted sound. This gives you the opportunity to adjust bias according to your musical tastes. Switch to the other tube and try to match the current draw as close as possible between the two tubes.
Remove the bias tool. Once you have adjusted the bias for optimum performance, turn off the amplifier, unplug it, and remove the bias tool and tubes from the chassis. Put the power tubes back in their original sockets and replace the back cover.
Before you bias your Blackface Super Reverb, remember that working around high voltages can prove fatal. Always use a chopstick or another non-conductive tool when digging around the internals of an amplifier, and make sure you turn off the amp and unplug it before removing any components. Take it to a professional amp technician if you do not feel comfortable with this procedure.