Today’s TTOTD question we made up. It is actually a compilation of similar questions we’ve received over the past few weeks that we think we can take care of in one shot. “I have several tube guitar amps and I’m curious to know how often to replace tubes or how do I tell when they need to be changed?”
Most tubes tend to have an indeterminate life span, rather like a light bulb. There is no real way of categorically saying what sort of life they will have. Generally speaking, if used two or three times per week, a set of EL34 or 5881 output tubes will last 18 to 24 months. Preamp tubes (ECC83/12AX7) as a rule have about double the life of the power tubes, so about 3 to 4 years under normal playing conditions. Making sure your amp is biased properly for the tubes you are using is a key to making them last longer.
When the time comes to change the power tubes you will definitely know as they generally only have two states, working or not working. The most common symptom of power tube failure is the blowing of fuses in the amp. On some occasions after replacing the fuse the amp will work for a while but will blow the fuse again when it reaches full working temperature. This is the first warning sign that a tube is failing. Although not absolutely essential, it is always recommended that the entire set of power tubes should be changed together.
Where preamp tubes are concerned, the most common fault is that they get microphonic. This is often caused by the first preamp tube in the chain and can sometimes be cured by simply swapping preamp tubes around. Other preamp tube related problems include; lack of volume, loss of bottom end, thinning of the sound and loss of all top-end frequencies. Again it is best to replace all the preamp tubes at the same time.