How to bias an amp?

cerfew

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I'm getting a Bugera 333XL here soon, and I want to make sure I get it biased properly. I'm a DIY kinda guy, and I have some decent experience with electronics, so I feel pretty confident I can do it myself- I just need to know HOW to do it. I've been doing a lot of searching and haven't found anything clear and cohesive. All I know is not to use the RCA jack on the back for testing xD

So basically, I need someone to give me a really clear description of how to do it, or find me a website with a really easy tutorial.

I also need to know some of the lingo-such as, what is the plate? And the plate voltage?
 

Dead Undead

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I'm getting a Bugera 333XL here soon, and I want to make sure I get it biased properly. I'm a DIY kinda guy, and I have some decent experience with electronics, so I feel pretty confident I can do it myself- I just need to know HOW to do it. I've been doing a lot of searching and haven't found anything clear and cohesive. All I know is not to use the RCA jack on the back for testing xD

So basically, I need someone to give me a really clear description of how to do it, or find me a website with a really easy tutorial.

I also need to know some of the lingo-such as, what is the plate? And the plate voltage?

Look up vacuum tubes on google or something. Wikipedia will give you more info than you would ever want on them. IIRC The plate is the anode, or positive, side of the tube. "Starved plate" means it's not running at full voltage. I can't remember what effect that has on the tone though.
My friend on another forum said he biased his tubes in his Hughes & Kettner so hot that they ignite anything that comes in contact with them (exaggeration of course).
As for biasing them, I have no idea how to do it either. Hopefully this thread will benefit more than just you and me. :lol: It's useful information!
 

cerfew

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MrMcSick

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A super simple way on my amp but maybe not for all is to take out the power tube fuse and connect a multi meter to the positive and neg of the fuse holder, completing the circuit. This will give you a general reading for all the tubes combined. Mine are supposed to read 120, so divided by 4 that is 30 for each tube. This is assuming all 4 of your tubes are matched and functioning equally. I would use a better method if you need serious work or whatnot as this is just a ballpark figure.
 

shogunate

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The bitch of it is most amps you have to take out of the chassis and dig around for the bias points, they're a bitch to get to on some amps though. IIRC on most amps they'll look pretty much the same, like little flat head screws, usually white and plastic. Do NOT use a metal screwdriver to adjust them, they will turn the current running through the tubes either more or less, depending on the total voltage recommended for particular tubes.

Also, if you're ever going to rummage around inside your amp, unplug it from power and let it sit for half a day or more to let the capacitors (fancy word for doohickeys that store potentially deadly current even without active electricity flowing through them) drain out.
 

Rook

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They don't store current they store energy more specifically charge, very different. Still painful though. It's very easy to drain the capacitors. They're the big scary looking ones, just connect the poles of the capacitor together with a 1meg resistor. Most modern amps have filter cap drain resistors, that will discharge them within minutes but better safe than sorry. When connecting the resistor, use a long cable with croc clips and connet one side at a time using only one hand, never use both hands at the same time.

If you didn't already know this I wouldn't do it...

As for biasing, you can normally do it using the bias pot (if the amp has one, many don't) and just watch the power tubes, they'll glow a deep blue, if it's too hot you'll get a lot of noise. If you want to do it by numbers, you can connect two pins of the power tubes together (not any two, 5 and 8 I believe) multimeter and then use the pot. I prefer to do it qualititively, but numbers are easier I guess.

Sorry to seem vague, I'm trying to be informative without encouraging people to do it. I really don't suggest screwing with this stuff unless you know a decent amount about amps.
 

cerfew

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The amp I'm getting has a knob on the back for adjusting- real easy. It's just setting it in the right point for the best combination of tone and longevity that i'm worried about.
 

Jzbass25

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Get a bias probe that has a plate voltage setting and get either a multimeter or a biasprobe with a built in meter. It will cost more than just a multimeter but it makes the job easier and more safe and it pays for itself instead of having a tech do it.

Also if you know the average plate voltage/mA your amp model/tubes like to run at, then you can technically safely bias without plate voltage and all that jazz since your plate dissipation will only be off by a few percent from "perfect". But chaos theory is always at work so might want to be more safe then sorry.

Also if you dont even need to take out the chassis to adjust the bias trimpot then awesome!! I want to mod my legacy so I can just test and adjust the bias from the back panel. I wish all amps came that way.
 

cerfew

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I really want to use the method of rewiring the ground wire with a resistor and measuring the current across it, because that'd be cheapest-- but a probe would be easiest. But they're like 25 bucks, and then I'd have to wait for shipping without being able to play my new amp, not feeling safe not having it adjusted.
 

Digideus

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Bugera include Biasing info in their manuals.

As I understand it, if the tubes and the treadplate are glowing bright red, its got too much juice running through it!. Im pretty sure that the 333 has a point on the back of the amp where you can plug in a volt meter, and the manual has suggested voltages for different Power Amp tubes. Im sure you can find a PDF of it online somewhere (I did, but I cant remember where - try the Bugera site?) or check Youtube
 

cerfew

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I was looking at that. But I was reading about different ways of biasing, and that is the most inaccurate way to bias an amp. I opened her up and found out that I can't actually rewire a resistor in place of the ground wire-- the whole circuit is on a printed PCB. Oh well. I just set the bias back to it's factory setting. Sounds fine and the plates aren't melting- of course I haven't cranked it up yet though. They're just Bugera tubes-- I figure if anything DOES happen, I'll get a proper bias probe along with a decent set of tubes. Thanks for all the help guys.
 


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