An amp's bias is a voltage measured at the tube's control grids. This voltage affects the amount of idle current that flows through the tube. There is variance from one tube to the next in how much idle current results from a particular voltage going to the grids.
If you're replacing power tubes, biasing may be necessary in some amps and not in others. There are three approaches I know of to the bias issue:
- Fixed bias
- Cathode bias
- Hard wired bias
Fixed bias amps are common and when you hear of someone having the bias adjusted, they're talking about a "fixed bias" amp. The amp has trim pots that allow you (or a tech) to adjust the amount of grid voltage for a particular tube. If you have an amp with 6L6 tubes, you may get another set with tolerances that are far enough off to require a bias adjustment. If the amp's bias can be adjusted, it should be, as the tube set you get for the amp may have enough variation that the new set you put in doesn't work properly with the bias settings for the old tubes. Incidentally, bias can wander due to vibration as well.
Cathode bias amps usually have a resistor of a fixed value between the tube cathode pin and ground. Current flowing through the resistor produces a voltage. As with fixed bias amps, the voltage regulates the idle current flowing through the tube. If a new tube has different characteristics than the old one, the current from the cathode will be different, resulting in a different voltage out of the resistor, which in turn establishes the correct idle current through the tube. Amps of this type are said to be self biasing.
Hard wired bias is an approach that Randall Smith, founder of Mesa Boogie uses in his amps. Instead of a trim pot, he uses a fixed resistor. The tubes used in Boogies must operate within a narrower specification than you might generally find among tube of a particular type. Mesa sells tubes that they have tested and matched not only to the others within the set, but to the narrower specification as well. Other people also sell tubes that are compatible with Mesa amps' operating specifications.
Of the three types above, Cathode biased and hard wired biased amps are plug and play when it comes to changing tubes. Get a matched set of tubes intended for that amp, clean the tube sockets, plug in the new tubes and start playing. Fixed bias amps need to be re-biased. If you have a volt meter, know what the bias voltage should be and at which points to measure it, you can set the bias yourself. If you don't, have an amp tech do it.
A tube supplier I have had good experiences with is eurotubes.com. They sell tube sets for many common amps and can make up sets for you, if needed. Prompt delivery and good return policy if you happen to receive a bad tube.
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. I'm kinda stuck on the "hair nation" channel on sirius, so any 80s metal song is well within my limit although Metallica, Megadeth, Accept is my cup of tea.
So I'll still be stuck with that crappy SS Marshall combo. 
