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Tube Amp #1 - Use Your Standby Switch!

The standby switch on a tube amp is not just for muting the amp during breaks. Correctly used, it can substantially prolong the life of tubes and power supply capacitors.

Before we get fully into the explanation of the standby switch, let’s first say that if your amp has a tube rectifier, the rules for the standby switch are not nearly so critical as with solid state rectifiers. A tube rectifier warms up more slowly than solid state rectifiers, and is very forgiving of any misuse of the standby. However, most tube amps have solid state rectifiers so if you’re not sure which you have, it’s best to follow the guidelines given below. Any amp repair technician can look at your amp and tell you which circuit you have.

Here’s how to use the standby switch to preserve the life of the amp. Turn on the power while in standby mode. Let the tubes warm up for at least thirty seconds. Turn the standby switch to the play mode. It’s as simple as that! What we’re doing here is turning on only the low voltage section of the power supply until the tubes are warm. Then the high voltage comes on when we flip the standby on. No harm is done by letting the amp run in standby mode for an hour or so, but thirty seconds is about the minimum and several minutes is probably better. When the tubes are warm, you can turn the standby on or off any time you choose. An amp is considered cold if it’s been turned off for fifteen minutes or more.

Don’t turn on the power and standby simultaneously. Don’t have the standby switch on while powering up. Either causes a high voltage surge in the power supply and tubes. Let’s look at what happens during an incorrect power up.

Two elements in the tube, the cathode and the plate, work harmoniously when the tube is warm. A warm cathode emits electrons, and a highly charged plate, at about 400 volts, pulls electrons from the cathode, setting up current flow in the tube. All is well when this happens in a warm tube. A cold cathode, however, cannot emit electrons, and the force of a highly charged plate near a cold cathode literally pulls physical material from the chemical coating on the cathode. This chemical coating then leaves the cathode, where it’s supposed to be, and becomes deposited on the plate, where it’s not supposed to be. This puts wear and age on the tube with no return benefit.

There’s also another kind of damage during an incorrect power up. The amplifier’s power supply was designed to deliver the correct voltages to the tubes at operating temperature, where the load of the tubes draws supply voltages down to their correct rating. Cold tubes don’t draw any current. If the tubes aren’t drawing any current, the power supply delivers more voltage than either it or the tubes were designed to withstand.

Let’s make a brief comparison. Driving your car up a long grade, full throttle, at highway speed is unlikely to damage the engine. The load imposed by friction, wind, and weight hold the engine RPM to within the design limits. On the other hand, holding the accelerator pedal to the floor with a cold engine in neutral (no load) would race the engine to the point where mechanical parts are operating beyond their design limit.

Power supply capacitors in a tube amp have a voltage design limit of 450 to 500 volts. If we fire up the power supply into a bunch of cold tubes, there’s no load, and the voltage can temporarily exceed this maximum by as much as 150 volts. This breaks down the insulating materials inside the capacitor, causing poor performance and premature wear. An amp with poorly performing power supply capacitors is unresponsive and lacks punch and clarity. In extreme cases, it can even make the guitar seem out of tune on certain notes that are actually well tuned. It’s a fairly expensive proposition to have them replaced, and a thorough bench evaluation of their condition would likely cost more than simply replacing them.

Take care of those tubes and caps by using the standby switch. As a final note, the amp can be turned off in any sequence with no harm. Flipping both the standby and power off simultaneously is fine and leaves the standby in the right position for the next power up.