In the recent issue of a well known guitar magazine, a well known personality in the tube world answered the question "What causes an amp to lose it's bias?" with "Nothing, What happens is the tubes wear out." And he goes on to say "buy a new set of tubes and be done with it. "
In our opinion that is a rather incomplete as well as inaccurate answer followed by a suggested cure that is risky at best and potentially an amp killer at worst.
The "bias" in a tube amp is a control voltage that sets the idle current for the output stage tubes. If the bias is not there or set too low, the output tubes draw too much current which will damage the tubes and may destroy the power and/or the output transformer. If the bias is too high, no damage will result but the amp won't sound very good.
While a shorted tube can bring down the bias supply, a worn or old tube will not. Just putting in a new tube without checking the output stage and the bias supply is bad idea. The tube, when it shorted may have damaged the bias supply or other components in the amp. This is especially important in amplifiers that rely on cathode bias. The cathode bias resistor may have been severely burned and changed value. In which case the amp will be incorrectly biased.
A variety of other problems can cause insufficient bias voltage. The bias supply is built with electronic components such as capacitors and resistors all of which age and can fail. Don't forget, guitar and bass amps get moved around a lot and suffer physical abuse. Broken connections and wires are common. When an amp blows a fuse or you change the tubes, the prudent thing to do is, CHECK THE BIAS!!