How to Set the Bias on a Tube Amplifier
Correctly biased tubes will draw an equal amount of current. However, as tubes deteriorate, their ability to manage the same workload diminishes over time. This means that other tubes in the guitar amplifier are forced to compensate by drawing more current from the power stage. Over worked tubes are at risk of burning out. The amplifier's sound will also suffer if the tubes are drawing unequal current. Biasing the tubes equalizes the amount of current received by each tube for optimal performance.
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Instructions
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1
Consult your amplifier manual. You need to establish exactly how you are going to adjust the bias. Measuring the bias is done with a multimeter, but the way you make the adjustment depends on your amplifier's make and model. Typically a tube amp will have at least one bias trim pot, normally two. Once you have located the bias trim pot, which is typically located beneath the tube housing on the amp chassis, you'll turn it to adjust the distribution of the idle current.
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2
Place the amp on your work surface. To avoid stooping to access the trim pot make sure it's at a comfortable height, because tube bias adjustments can take a while. Unscrew the back panel of the amplifier to expose the power section and pre-amp section. Both sections contain tubes.
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3
Unplug the first tube so that you can bias the socket. You'll need a bias tester, which is a special device that draws current from the amp, passes it through the tube and into the multimeter to be measured. The bias tester has three sockets: One receives the tube, the second one plugs into the empty tube socket on the amp and the third plugs into the multimeter. Switch on the amplifier.
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4
Make a note of the reading on the bias meter display. All amps are different, so there is no typical bias reading. Your amp manual may advise on recommended readings. You should take the average reading from all tubes and use that as a starting point, then tweak from there with a view to getting all tubes drawing the same current. It will measure the bias in volts. Repeat the previous step for all tubes, making notes of each tube's current. Consult with your amplifier's manual for information on bias limits so that you don't overwork any of the tubes.
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5
Adjust the bias trim pot with a small screwdriver. Now that you know what current each tube is drawing, you can adjust the trim pot to reduce or increase the current so that all tubes are equal. Some amps have a trim pot for each tube; others have one trim pot that acts as a master bias dial. These are less accurate as increasing one tube automatically decreases the others equally. Trim pot adjustments can take a while and may require a lot of delicate tweaking.
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Disconnect the bias tester and multimeter. Turn off the amp and replace the tubes. Gently slot them back into their sockets and close the back of the amp.
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Tips & Warnings
Wear latex gloves when handling the tubes. This avoids oil getting on the glass, which causes hot spots.
Exercise caution when attempting this. Amplifiers carry a potentially lethal current. Never unscrew an internal component when the amp is turned on.
References
Resources
- Photo Credit multimeter image by Aleksey Bakaleev from Fotolia.com
