A question about valve-state

[jetset]

Hello, I have a question about valve state.
Ok I have a guitar amplifier that has a Sovtek 12AX7 in the pre-amp and the rest is solid state (power amp, power source, effects, etc). The amp now sounds really crappy, sounds like farts sounds, so I thought it got broken. So I opened it to figure out what the problem is, I meassured all voltages and everything seems to be ok, but when I looked at the tube
(12AX7) I could see it has like a dark layer inside of it.
So I have read that as the time passes, the tubes lose their quality and stuff, and my question is, Do this black layer on the tube is sign that I have to change it?

 

[flatulent]

This could be the "getter" which is a silver coloured area on the envelope in about a cm squared area. This is a process where the residual gas is removed and trapped in the metal that deposits on the surface.

Do you have an oscilloscope? If so, look at the waveform (using AC coupling) at the input grid and output plate of the valve/tube.

Usually, when they age the cathode emission declines which reduces the plate current which raises the plate bias voltage.

 

[eltonjohn]

Now ..everybody will know that i'm an oldtimer .
Listen friend with tubes ( we also say valves in french) You can't know
visually if the tube is bad or not ..Unless the filament in not on .Otherwise a black and silver spot is just a reaction of some oxigen reactive compound that was introduce when the tube was manufactured .. But yes TUBES have a LIFE .. and is commun to reaplace it after 5 to 10 years of continous use .. So it sounds like crap .. well change the tube and go back to rock n' roll

cheers

 

[papyaki]

Hi,

Tubes have a life, but also electrolitic capacitors. after many years of fonctionement, electrolitic capacitors lost some of their properties, especially when used in heat environment like power amplifiers.

If your guitar amplifier is quite old, I think you can also replace some high voltage electrolitic capacitors arround the valve, specialy those used to filter the power supplies.

Regards

 

[jetset]

Hi, thanks for your replies. I could fix the amplifier, it was a solid-state problem. :roll:
It's a vale-solid-state mixed amp not so old.