How to Adjust the Bias on a Fender Super Champ
The Fender Super Champ is a 15-watt tube amplifier. As with all tube amps, it necessary to adjust the bias of the tubes. The tube bias is the ratio of current that is drawn to each tube. In a biased amplifier, these currents are equal. If a tube becomes worn out, the other tubes draw more current to compensate. This puts extra demands on the other tubes and often leads to problems. It's necessary to bias the tubes if you have installed an unmatched set. Avoid the need to bias by replacing your tubes in matched pairs or sets of four.
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Instructions
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The trim pots on Fender amps are hard to access. Turn off your amplifier and remove the power source. This amplifier carries potentially fatal current. If you have been using the amplifier, leave it for 10 minutes so that the tubes can cool down. Moving an amplifier while the tubes are still hot can cause the tubes to shatter.
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Unscrew the back of the amplifier. Unscrew the four screws on top of the Fender Super Champ that connect the cabinet to the chassis. Unplug the speaker wires; this is for added safety while you're handling the chassis. Next, remove the amplifier chassis by sliding it toward you. Some amplifiers have an external bias trim pot, which makes for easy bias adjustments. The Fender Super Champ's bias trim pot is located on the chassis.
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Reconnect the speaker once you have access to the chassis. To check the bias, there needs to be a complete circuit in the amp.
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Power up the amplifier. You need power running through the amp to measure the bias. It's imperative to exercise extreme caution from this stage on.
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Connect the black wire from your voltmeter to the chassis. The chassis is grounded, so this is the safest place to do it. Be careful not to touch any other parts of the chassis.
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Voltmeters are also called multimeters. Check the meter reading. A correctly biased Fender Super Champ should give a reading of 40 millivolts. Manually adjust the blue trim pot located on the right of the chassis to bias the amplifier tubes and check the reading again. It's a tricky process and usually takes a few attempts. The trim pot adjustments balance the current drawn by each tube so that they share equal load. If you find it impossible to get the bias to 40 millivolts, you may have a faulty or under-performing tube. If this is the case, disconnect the power, replace all the tubes and retry.
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Disconnect the voltmeter from the chassis and disconnect the speaker wire, then turn off the power. Slide the chassis back in place and reconnect the speaker wire. Screw in the four screws on the top of the cabinet.
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Tips & Warnings
Wear latex gloves when handling amp tubes. Oil from your skin can create hot spots on the glass.
Unless you are entirely confident dealing with fatal voltages, do not attempt this project.
References
Resources
- Photo Credit ICHIRO/Stockbyte/Getty Images amplifier image by Darko Draskovic from Fotolia.com multimeter image by Aleksey Bakaleev from Fotolia.com
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Comments
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Ok. So you've told us to attach the black lead from the voltmeter to the chassis - but where are we placing the red lead to get the reading? Thanks!
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you gorgot to mention where to put the red leed. I believe is is R20.
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I asked my amp tech to make this adjustment and he was a little surprised. He said they currently read around 17 which is what his chart says it should be biased at, and the 40 was more than double this. I have read that they typically come in cold at around 36 mv, and to bump up to 40. Mine is reading 17 currently.
