Determining whether you have a blown preamp tube can be tricky. Sometimes it is fairly obvious that a blown tube has caused a problem; other times it is harder to diagnose. Preamp tubes are inexpensive and easy to replace. If you cannot figure out which tube is giving you trouble, try replacing each tube one by one until your amp works properly again.
A blown preamp tube will often appear normal. But if the tube is cracked or has a whitish film on the inside, this is a sure sign the tube needs to be replaced.
The easiest way to tell whether a tube is blown is when it goes microphonic. When a tube is microphonic, your amp will release a high-pitched squeal as you turn up the volume -- with or without a plugged-in instrument. You can test which tube has gone microphonic by gently tapping each tube with a chopstick or a pencil. Listen as you tap each one; it will be obvious which tube has become microphonic because it will sound different from the others.
If you hear unwanted distortion or crackling noises coming from your amp, you most likely have a blown preamp tube. Put your amp on the cleanest setting (in other words, turn off all distortion). If you can still hear distortion or crackling noises, you probably have a blown preamp tube.
Weak Signal or Loss of Sound
A blown preamp tube can cause your amp to produce a weak signal or lose all sound. A weak signal will diminish the amp's volume and cause it to emit a low buzzing sound. A weak signal will eventually turn into a complete loss of sound.
Lack of EQ Control
Preamp tubes control the EQ of your amp. If adjusting the EQ knobs on your guitar does not change the tone of your amp, you may have a blown preamp tube. This is especially noticeable at the high frequency range of your EQ.
In older tube amp models, the reverb is controlled by the pre-amp tubes. If your reverb begins to sound strange, or you lose all reverb, it can be a sign of a blown pre-amp tube.