The Bogner Barcelona tube guitar combo amp was designed for a nice high headroom clean sound. 6CA7 power tubes where chosen for their nice balanced tone. These have the same output impedance as an EL34 so you can use EL34's in your Barcelona with just a rebias.
The knob can be pulled to engage the pull fat mode. This will give you more overall bass as well as slightly more gain. The pull fat mode can also be used similar to a bright switch. If you set the gain to a desired level, pull the control and then turn the gain down so the overall volume is the same as before you will see that you have a more dynamic brighter sound with slightly more bass. The response has more spank with a softer feel.
Treble, Bass Control and Mid EQ Switch
The treble and bass controls let you set the high and low frequencies. The 3 position mid switch gives you the choice to further shape the sound, it also affects the feel. The low position has a more scooped sound reminiscent of the old blackface amps. The mid and high position is better for higher gain. With a 1x12 speaker, mid switch set to low and then toggled to the mid position is reminiscent of a 2x10 or 4x10 speaker configuration.
The Volume sets overall loudness. If you want maximum openness from the amp turn it all the way up and adjust gain and EQ as desired. For maximum clean headroom best to set it at notch or two below max, depending on where the gain is set.
Standby, Hi - Low Power Switch
After you turned the amp on and let the tubes heat up a few minutes you have the choice to switch the amp off Standby into the Hi or Low position. Hi has the full 40 watts available while Lo has only 20 watts. The amp will feel looser in the Lo position; readjust controls to your taste.