Bias adjustment

Does anyone know how to and when to adjust the bias on a tube amp. I used to have a Crate vintage club 30 & 50 and with those amps you didn't have to adjust the bias when changing tubes. I'm not sure why but you didn't have to worry about it. I now have a Epiphone Blues Custom 30. Its a 30 watt class A/B and 15 watt class A, selected by a switch. I called Epiphone (Gibson) tech support and they put me threw to their "Amp expert" and he said when replacing tubes make sure you adjust the bias and make sure the tubes are matched. I said OK, but then wished I'd asked a few more questions. It can't be to hard if you have the right tools and instructions, can it? And how often do you need to do this?

Hmmm, as far as the length of time between changing your tubes goes, this will fluctuate heavily depending on your usage of the amp. If you're just noodling around in your room the tubes may well last you a couple of years––but if you're practicing at good volume every day, or playing lots of gigs, you might only get 6-months out of them.

You don't need to bias your tubes until you replace them, but unfortunately I'm not the one to answer biasing question, as I've always found the subject fairly confusing. If all esle fails you can leave it to your local guitar tech to switch out and bias the tubes for you, though it would be a lot nicer (and cheaper!) to be able to do it yourself.

I'll see if I can get one of my tube-amp-building acquaintances to give us the run-down...

Hold tight.

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egr :: guitar spoken here

Just got an email from Gibson and I stand corrected. The Blues Custom 30 amp uses a cathode bias method. There is no bias adjustment. But I still would like to know more about it. And what it cathode bias?
Paul

If you want to know about the technical side of tube amps, it's hard to beat Mesa Boogie's site. For the rundown on biasing, check out this bias adjusting article on their website.

The only occasion I've ever had bias adjusted on an amp was at a re-tubing. It doesn't "wear out", you actually adjust the current (bias) to the new tubes, because no set of tubes has the exact same operating levels.

I wonder if the email from Gibson was meaning to imply "no user serviceable bias adjustment"?

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