Overdriving valves

[Primalsea]

This is not hifi related, but rather related to my guitar amp that has 12AX7 / ECC83 valves fitted. Currently these are just some generic Chinese valves.

What happens if you drive a valve with too high a signal, ie. overdrive. Does it do any damage?

The reason I ask is that I have been overdriving the amp with a boost pedal and two times now after a while the sound has become harsh and brittle, rather than overblown and full. Admittedly both times it has been cheap generic valves that have failed.

Obviously guitar amps are supposed to be designed to be overdriven but I wonder if you can overdo it. Possibly it just been the cheap valves, but I dont want to fit something decent and ruin them too?

Also could excessive vibration increase a valves microphonics after a time? Its a small amp with a 12" speaker and the sound is a bit like feed back via a overly microphonic valve that's being shaken to buggery.

[Stryder5]

Can't tell you directly, but Watford Valves have lots of info regarding overdriven valves and which tolerate this, just search ECC83, and read comments.

This is not hifi related, but rather related to my guitar amp that has 12AX7 / ECC83 valves fitted. Currently these are just some generic Chinese valves.

What happens if you drive a valve with too high a signal, ie. overdrive. Does it do any damage?

The reason I ask is that I have been overdriving the amp with a boost pedal and two times now after a while the sound has become harsh and brittle, rather than overblown and full. Admittedly both times it has been cheap generic valves that have failed.

Obviously guitar amps are supposed to be designed to be overdriven but I wonder if you can overdo it. Possibly it just been the cheap valves, but I dont want to fit something decent and ruin them too?

Also could excessive vibration increase a valves microphonics after a time? Its a small amp with a 12" speaker and the sound is a bit like feed back via a overly microphonic valve that's being shaken to buggery.

[Blackdog]

Over-driving a tube shouldn't be a problem. Guitar amps are designed for this. However you may be on to something with the microphonics. The vibration from the speaker will cause a tube to go microphonic, especially cheap ones. That's probably what you are hearing. There are a few modern ECC83's that are not recommended for combo guitar amps due to their internal structure.