This is a question often asked, and with good reason because there is no single, obvious symptom which screams out at you to change your valves.
As valves age they just become less and less efficient at what they’re designed to do. Trouble is, the degradation is imperceptibly slow and so gig to gig you won’t notice anything dramatically wrong with your valve amplifier.
I have met several guitarists who didn’t even realise that valves needed changing in their valve guitar amplifier! An analogy might be windscreen wiper blades. You don’t notice them getting worse and worse from journey to journey.
Please be aware that output valves (the big ones!) are the main valves which need changing in guitar amplifiers as they take all the punishment.
So, given that there is no obvious single symptom what should you look out for?
Here are some pointers:
1. Age. It’s not how old the valve is that counts, it’s the amount it has been used. An analogy might be car tyres. It’s the miles driven which contributes to wear, not the time in storage. So a 50 year old, unused New Old Stock (NOS) valve should sound as good as when it was first manufactured. As a very rough rule of thumb, consider a valve change after 1,000-2,000 hours of playing.
2. Flabbiness. The amp just lacks the punch it used to have. This usually indicates the output valves need changing in your guitar amplifier.
3. Noises, pops crackles etc MAY indicate valve trouble. This is the first thing a valve amplifier technician will look at. There are many other causes of this fault though.
4. Microphony. This is usually associated with preamp valves (the small ones). The elements in the valve become slightly detached and loose. They acoustically pick up sound from the speaker and vibrate, thus causing unwanted harmonics and squeals in your valve amplifier. Try tapping the preamp valves gently. This will usually show up any ‘microphony’. As the name suggests this is the valve acting as a microphone – which is shouldn’t!
5. Redplating. If you notice that an output tube is glowing cherry red (not just the heater, the whole element) this means that valve is drawing way too much current. If it has done this for any length of time the valve needs replacing and that usually means replacing the whole set of output tubes in your guitar amplifier. Redplating often means there is a problem with the bias which sets the ‘idle’ current of the valve. Get it checked out by a valve amplifier technician.