how do you rebias a tube amp

[Space_Ace113]

I have a 1986 laney aor pro tube 100 watt head and when i got new tubes i was told they had to be rebiased, fist question is why and seond is how do you do that

[Teahead]

Bump! I know somebody knows the answer to this, so I couldn't see it slip off the first page. Maybe you could post it on the "Ask Myles" thread? Good luck!

 

Tea.

[Gabriel E.]

Unless you are experienced with electronics and high voltages, this is something you should not do yourself or you might die. Take the amp to a tech to have it done.

 

Biasing is making sure the correct voltage is applied to the power tubes (preamp tubes don't require biasing). Different tubes have different bias points. Unless you are replacing tubes with the same tubes from a supplier who measures their tubes for consistency (like Groove Tubes), you need to rebias every time you put in new power tubes.

 

An underbiased amp will sound thin and weak. An overbiased amp will sound muddy. If too much voltage is applied to the plates, the tubes will burn out (and quite possibly take parts of the amp with them).

 

On most modern amps, theres a potentiometer somewhere in the circuit that regulates the bias voltage. Depending on the method used (in the amp geek world there's a lot of controversy surrounding the best way to bias an amp), the amp is hooked up to an oscilliscope and signal generator or a multimeter and the pot is turned until the desired bias point is reached.

 

Some amps (Mesa/Boogies, a lot of Fenders) don't have a bias adjustment pot. They have a resistor that must be replaced with one of a different value.

 

If you are good with electronics and have worked with high voltage, this is something you can do yourself as long as you are VERY, VERY CAREFUL. There are numerous resources on the net that describe the various methods. I prefer the transformer shunt method.

[Tele Kinesis]

I second Gabriel's answer about taking it to someone. For a modest fee they'll do it correctly.

Yeah, it's good to learn how to do it, but it takes the right knowledge about working on live high voltage and using the instruments. If you DO know the rules, know about electronics, and want to attempt it, check and double-check your procedures from several different sites. Although the first site with info that pops up may have info, dig in a little deeper to confirm that info.

You also may want to attempt to find a schematic to download. Then, before you do anything, you can give more detailed info if you need to ask more questions.

BE CAREFUL poking around in a live tube amp!

Here's a good place to get the skinny on stuff. These guys are pretty helpful.

http://www.firebottle.com/ampage/

 

Tele

[Tele Kinesis]

Another safety warning, while I think about it:

Even with the amp unlugged from AC, it can hold a large enough charge to knock you on your a$$. Always, before doing any work, (changing components, etc.), double-check that it is unplugged. DON'T ASSUME you are pulling out the correct plug, especially if you have multiple things plugged into a strip. Follow the cord from the amp to the plug. And then use a bleeder resistor to discharge the power supply caps.

(You are likely to hear this kind of safety advice over and over again in any forum with advice worth following. It's not a slam on your abilities or knowledge. We don't know your abilities or knowledge, so it's just responsible advice)

 

Tele

[BlueZet]

take it to an amp tech (that's what I do )