How to Bias a Blue Voodoo Amp
Blue Voodoo is a line of tube amplifiers from Crate. There are two models, the 60-watt BV60H and the 120-watt BV120H. Despite a difference in power handling, the amp controls are almost identical. The only difference concerns the impedance controls on the rear. Bias refers to the flow of current from the power section to the tubes. In a correctly biased amp, each tube draws an identical or as near-to-identical current as possible. The Blue Voodoo line of amps feature a bias-adjustment control inside the chassis; from there you can set the bias correctly.
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Instructions
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Turn off the amp. The “On/Off” switch is on the left of the front control panel. If you’ve been using the amp, let it cool down for 20 minutes. Moving an amp when the tubes are still hot is dangerous, as the tubes may shatter.
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2
Place the amp facing away from you on a stable, flat surface.
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3
Unplug all one-quarter-inch jack cables connected to the back of the chassis.
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Unscrew the back panel of the amp. Remove the back panel and place it to one side. Put the screws, in the order you removed them, on a piece of electrical tape. This prevents them from rolling away and also helps you put them back in order.
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Slide the chassis toward you. This is the black metal box inside the cabinet on which the tubes are mounted. The wires running from the chassis to the speaker are long enough to permit partial removal without the need for disconnection.
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6
Unscrew the lid of the chassis; this exposes the underside of the tube sockets, where the cathode pins are connected.
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Connect the multimeter cables to the positive and negative terminals, respectively, on the first tube socket cathode pin. For simplicity, start with the left-most tube. Turn on the amp and note the reading. Turn off the amp, disconnect the cables, connect them to the next tube along, turn on the amp and note the next reading.
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Turn off the amp. Calculate the average bias from all the readings by adding them together and dividing them by the number of tubes. For example, the BV120H has five.
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9
Replace each tube. Although you can bias old tubes, it’s a lot easier to replace the tubes for a new set. This way the tubes are a lot more likely to draw an equal amount of bias as none will be worn out.
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10
Fit a small, flat-head screwdriver to the bias trim pot on the circuit. This is the small plastic dial on the rear panel.
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Adjust the bias trim pot. Turn it clockwise to increase the flow of current to the tubes, clockwise to decrease it.
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Retest the bias of each tube. Biasing is a delicate tweaking process. Keep adjusting the bias trim pot until each tube’s bias is either identical or close to identical. A 5 percent tolerance range for each tube is typically sufficient to improve tone and maintain tube life.
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Tips & Warnings
Wear latex gloves when handling new tubes. This prevents oils from your skin creating hot spots on the glass.
Only perform bias adjustments if you're absolutely sure of what you're doing. If in doubt, consult a professional. Biasing at home may void the warranty.
References
- Photo Credit Jupiterimages/liquidlibrary/Getty Images
