The Fender Concert is a now discontinued tube amplifier, which originally was introduced to market in 1960 and was discontinued 1965. The Concert was reissued in 1982. Correctly biased vacuum tubes draw an equal amount of current. When the tubes are not-biased, the distribution is unequal and puts extra stress on the tubes that are drawing the higher current, causing them to burn out quicker. Incorrectly biased tubes sound poorly and cost money when they burn out.
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Things you need
- Soldering iron
- Replacement resistor
- Mini trim pot
- Bias meter
- Latex gloves
Read and study the wiring schematic for the Fender Concert. It is a fixed-bias amp and doesn't have a bias trim pot with which to adjust the current distribution to the tubes. In order to adjust the bias on any fixed-bias amp, it's necessary to manually modify the signal chain. Because this amp has been reissued, there are numerous wiring schemes. Find the schematic that matches the model. Use the schematic to locate the resistor responsible for governing the flow of plate current, which typically is a 56k bias resistor.
Unplug the amplifier, and let it to cool for 10 minutes if it's been in use. Remove the chassis by unscrewing the rear panel and sliding it forward. The wires connected to the chassis are long enough to partially remove the chassis without disconnecting them.
Locate the bias resistor. To adjust the bias, remove this resistor and replace it with a resistor/potentiometer combination pot. A potentiometer, or pot, is a manually operated resistor and governs the flow of current, depending on the position of the potentiometer.
Disconnect the bias resistor. Solder in a new resistor of lower value, which enables more current to pass. The potentiometer has three terminals: Two outside and one centre. Solder a wire between the new resistor to the top, outside terminal of the pot. Solder another wire between the centre terminal and the remaining outside terminal on the pot. Solder the potentiometer onto the circuit board next to the resistor.
Attach a voltmeter probe to the resistor. Turn on the amplifier and observe the reading, which should be within a five per cent variance of the resistor value. Adjust the pot counter-clockwise and watch the reading decrease. Set the pot to the lowest resistance, which establishes a low starting point from which you can increase the current flowing to the tubes. This enables you to manually adjust the bias. Turn off the amp.
Remove the tubes, and put one bias meter probe in the tube base, one on the tube and the third into the volt meter. Turn on the amp and monitor the reading on the volt meter. Adjust the new trim pot to bias the tubes. Write down the starting bias of each tube. Adjust each tube so that all draw as equal an amount of current as possible.
Tips and warnings
- Always change tubes in matched sets, which reduces the need for biasing.
- Consult an electrician if unsure of how to measure the resistor. The Fender Concert is a 60-watt amp, which is enough electricity to be fatal.
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