Shop Talk: Bored of the Rings?

July 30, 2008

Spring retainers are only used to ensure that the tubes stay seated in the socket, and they have nothing to do with the ringing problem—which is likely caused by one or more preamp tubes that have become microphonic. High gain amps such as the Kitty Hawk are even more susceptible to this problem. Here’s why: A 12AX7 or ECC83 preamp tube has an amplification factor of 100, which means that for every 1-volt change on the input of the tube, there is a 100-volt change on the output. Preamp circuits have several gain stages, so you can see how the least bit of ringing on the first preamp tube can be amplified several thousand times. To troubleshoot this problem, turn the amp on and set the volume high enough for the ringing to occur. Sometimes tapping each preamp tube very lightly with your fingernail or dampening it with your fingers may encourage or discourage the problem. If you can’t figure out which tube is the offender, remove the first preamp tube, which is the one closest to the input jacks. If the noise stops, replace the tube with a new one that is not microphonic. If removing the first tube doesn’t stop the ringing, remove the second tube. Continue in a like manner until you find the offending tube. Sometimes a tube that is microphonic in the first tube position may be quiet in the second or third stage. Try swapping tubes around and putting the quietest one in the first socket, or just get yourself some new, non-microphonic tubes.
—Gerald Weber, Kendrick Amplifiers

I purchased a Fender Twin Reverb reissue last fall, and twice now, at the end of a 90-minute rock set, the amplifier lost about half its volume. What could be the problem?
—T. Burris, Lincoln, NE

You obviously have a power supply problem, which could be caused by the electrical power in the venue, a problem with the internal power supply on your amp, or both. All tube amps have to convert 120-volt AC wall voltage into high-voltage DC (to operate the tubes) and 6.3 volts AC (to feed the heater elements that warm the tubes up to operating temperature). If the 120-volt wall AC is low, then the heater voltage and the high-voltage in the amp will also be low, resulting in reduced volume. There are many possible causes for low wall voltage. For example, a beer cooler or air conditioner kicking on could cause a drop in wall voltage. Even a 10% drop could prevent the tubes from getting hot enough to function properly, and that would also make the high-voltage supply go down by 50 volts or more, thus reducing volume. The solution is to use a power conditioner, which will regulate the voltage to 120 volts, regardless of the wall AC voltage. I always use a 2400-watt power conditioner when performing.

The power supply inside your amp could be another problem. New production amplifiers often use Taiwanese-made, Illinois brand electrolytic capacitors, which have internal conductors that are etched to create more surface area. These caps can be made much smaller and for less money. When measuring one of these caps on a capacitance meter, it may read up to the rated microfarad value, but the meter tests using only a 9-volt battery. When this same cap is used in a guitar amp at 450 volts and heated up with a bunch of tubes for 90 minutes, it will not perform like an American-made capacitor of the same value. I’d recommend having a technician change your electrolytic capacitors to American-made versions for consistency and better tone.
—Gerald Weber, Kendrick Amplifiers

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