The pre-amp tubes in your amplifier play an important role in the performance and tone of your amplifier. Although characteristically different from power tubes, pre-amp tubes can effect the sound of your amplifier in a negative or a positive way depending on the quality of tubes you are using and the frequency at which you replace them. As far as tube quality goes, this old adage usually applies. You get what you pay for. Basically this means that bargain tubes are only a bargain if your goal is to produce bad tone, hum and microphonics. Here's a bit of information that may help you in determining if its time for new pre-amp tubes. But first let me give you a few pre-amp tube facts. Pre-amp tubes do not have to be matched because they do not work in pairs as power tubes do, nor do new pre-amp tubes need biasing as do power tubes. Because of these facts, you can mix your pre-amp tubes up however you wish. Older tubes with newer tubes, mixing tubes from different manufacturers or replacing just a single tube at a time, etc. The only consideration should be that they sound good. Here are some general guide lines for changing pre-amp tubes. If your pre-amp tubes are two years old or more, replace them. If you can't remember the last time your pre-amp tubes were replaced, replace them. For those of you who like to experiment, try pre-amp tubes from various manufacturers, they do sound different and through experimentation you will figure out which ones you like the most. As a general rule it is a good idea to purchase tubes from a reputable company that guarantees their tubes and will replace defective tubes promptly. Here are some tell tail signs of a defective pre-amp tube and how to be certain that you have diagnosed the problem correctly. The most common pre-amp tube problem and also the easiest to diagnose is the microphonic pre-amp tube, which makes a high pitched ringing sound, especially at higher volume levels. By tapping lightly on each pre-amp tube with a plastic pen or something simi lar you will be able to determine which pre-amp tube is microphonic. Other symptoms may include hum, various unusual static noises, or no sound at all. The microphonic pre-amp tube is the easiest to find due to the fact that you can tap test it. Although replacing a microphonic pre-amp tube with a new tube usually cures the problem, beware of new pre-amp tubes that are microphonic right out of the box. As for testing for other pre-amp tube problems, try replacing one tube at a time until you find the defective tube. Also, any tube that is cracked or looks to have a white coating inside should be replaced, as it has lost its vacuum and will no longer function. If you have tried replacing each of the pre-amp tubes and the problem still persists, your amp should be serviced by a qualified tube amp technician.