Shop Talk: Ticks & Glows

February 27, 2008

The ticking sound can be quieted with a simple modification that involves adding only one capacitor. Have your technician locate the 10MEG (10 million ohm) resistor (color code: brown, black, blue) that connects to the vibrato circuit optocoupler shown in Fig. 1. The optocoupler is a small electronic component mounted near the middle of the amp on the component board. It has black shrink tube covering, and there are four leads coming out of it. Solder a .022uf 400-volt capacitor across this resistor. In other words, the capacitor will be connected in parallel with the 10MEG resistor. It doesn’t really matter what kind of capacitor you use, as this component is not in the signal path. Any .022uf at 400 or higher volts will work. This modification will significantly reduce the ticking sound without altering anything else about the amp.
—Gerald Weber, Kendrick Amplifiers

I have a Gibson GA-30RVS 2x12 combo amp with four EL84 power tubes. Lately, I’ve noticed that one of the power tubes starts glowing bright red after playing the amp for about ten minutes. Although it’s not affecting the sound, I know it’s not a good thing. What should I do?
—Roy Sharp, Biloxi, Mississippi

When a tube starts glowing cherry red, it’s because there is too much current going through it. A simple troubleshooting technique known as “tube substitution” is something that can be used on any tube amp with a similar problem. Here’s how to do it: Turn on the amp, and play it until you see the tube start to glow red. Then, turn off the amp, and swap the EL84 tube that was glowing with any of the other EL84s. You’ll want to use an oven mitt or glove, because the tubes will be too hot for bare hands. The basic idea is to put a tube that you know was not glowing cherry red into the socket where the problematic tube had been. Now, turn on the amp, and start playing. Did the problem move with the tube, or stay with the socket? If the same EL84 is glowing bright red in a different socket, then you have a bad tube. If the problem stayed with the socket, then there is a problem with the amp—possibly a bad grid resistor ground connection on that socket. You’ll need the help of an amp tech to fix either of these problems, but, if it’s just a bad tube, that’s something you can easily replace yourself.
—Gerald Weber, Kendrick Amplifiers


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