OT: tube amp questions.....

[marrstians]

what are the problems with a tube being overbiased? i know that underbiasing can fry the tubes and your tranny but is there any true problems, besides bad sound, from overbias? ..... also if you get a blatty unmusical distortion from your amp and your speakers aren't blown could it be the tubes them selves? it only does it when it's getting loud... not a tube sounding break-up... more of a buzz like a fret rattle...

[Paul Marossy]

Are you talking about power tubes? Overbiasing a power tube makes it sound bad, in my experience. Are you sure your speakers aren't loose, or your speaker baffle? On old Fender amps the speaker grille can also rattle. Sometimes a failing filter cap(s) can cause weird out of tune sounds.

[R.G.]

There's another set of data at GEO - lots of stuff on tube amps. I highly recommend reading the tube amp FAQ and the Tube Amp Debugging page.

As to your question - are there problems with overbiasing a tube:
Yes. "Overbiasing" as the term is commonly used means applying too much negative biase to output tubes. This turns the tubes off more, and moves the amp from class AB biasing (both tubes slightly on) toward Class B where there is no "overlap" where both tubes are on as the signal crosses 0V. This leads to crossover distortion, a gritty, unpleasant and harsh sound that gets worse as you play more quietly. No, it won't damage your amp, just sounds bad.

The situation you describe, "blatty unmusical distortion from your amp and your speakers aren't blown " could be any one of a number of things, all related to the loudness. Something is being shaken around and causing the blatty distortion at loud volumes. Could be a tube. Could be a loose ground connection, could be a cracked but still held-together resistor or cap, could be a loose tube socket. Tap things inside the amp with a wooden dowel to see if you can locate what makes a noise when it's tapped.