Treble Trouble

I have a 1969 silverface Fender Champ with volume, treble, and bass controls. Recently the treble control has started to act like a volume control in reverse. When I turn it counter-clockwise, the volume goes up, and if I turn it clockwise, the volume goes down. The actual volume control seems to top out at around 7 now, and the bass control doesn’t seem to be working at all. About two years ago, I replaced the speaker and the tubes, and the amp worked fine up until recently. Do you have any idea what is causing this problem? —Hutty Green

Your Champ is one of the few silverface Fender amps that I prefer over its blackface counterpart. Like the blackface, it has a tone stack circuit that is identical to the Deluxe, Princeton Reverb, and Twin Reverb. On this type of circuit, when the treble control malfunctions and acts like a reverse volume control, the problem is always related to the 250pf treble capacitor circuitry. This small ceramic cap is located on the component board, but connected to the right lead of the treble pot (looking from the back of the pot) with a white wire (see Diagram 1). The problem could be that the 250pf cap is bad, one of the solder joints is bad, or the white wire connecting the cap to the right lead of the pot is broken or not soldered properly. The most likely cause would be one of the solder joints not having good connectivity. Before working on an amp, the stored electricity should be discharged. The easy way to discharge all electrical power is to unplug the amp, remove the chassis, and then connect a jumper wire from pin 1 of the 12AX7 preamp tube socket to the chassis ground. Leave this connected for a couple of minutes and all of the electricity will discharge from the amp.

If you re-cook the three solder joints associated with the pot and treble capacitor as shown in red in the diagram, there is a 99 percent chance the problem will disappear. If that doesn’t solve your problem, then you’ll need to replace the 250pf/500-volt ceramic capacitor.—Gerald Weber, Kendrick Amplifiers

On a silverface fixed-biased Fender amp, is it possible to install a 15k½-20k½ cermet-element pot on the bias board before the bias balance pot to adjust the overall negative bias, and then use the bias balance pot to even out the negative bias voltage going to the tubes? This seems like it would be the best of both worlds. Am I missing something? —Randy Hobbs

That’s a brilliant idea, but I would not attempt to modify the bias board. Instead, I would change the load resistor (usually 15k½) that is mounted and grounded on the actual bias balance pot. I wouldn’t change it for just a cermet pot, either. Use a 20-turn cermetelement pot in series with a 6.9k½ 1/2-watt resistor instead of the stock 15k½ load resistor going to ground. Look at Diagram 2. These two parts could be mounted on the existing bias balance pot. You would balance the tubes first by adjusting the balance control for the least amount of audible hum. This assures that you are getting maximum phase cancellation, which means the tubes are closely balanced. Then you could adjust the cermetpot for correct bias. —Gerald Weber, Kendrick Amplifiers