are overdriving the tubes; around 120ma, coincides with
the maximum power.
It is not important if you overdrive the tube in short
periods, but if you do it continuously, it will shorten the life
expectancy of the tubes. To change the tubes just unplug
the A.C. power line cord, pull out the old ones, and insert
the new tubes; the active bias will balance them.
We recommend matched pair output tubes, but this is not
indispensable.
Do not substitute with other type the two 12AU7 tubes in
the back of the amplifier. You will unbalance the circuit
and will raise the distortion.
Figure 1. Interconnection diagram
Figure 2. Front and back of the U280
