Valve deterioration

[Jimbo]

Do old valves approaching their end loose HF/treble ability and detail in general? What other audible signs do valves give to suggest they are about to die:eek:

[Radford Revival]

Any change in tonality associated with valve wear will be largely due to a rising output impedance (lowering damping factor) due to lowering Gm. If the valve amplifier in question has a feedback loop, then potentially wear of any of the valves inside the loop could contribute to this, or if a no feedback design then largely just the output valves (given the drive circuit is still functioning).

This tonality shift is the result of the interaction between the amplifier and speaker, and is speaker dependant.

Sometimes worn out signal stage valves can drift enough to dramatically shift the operating point of the circuit they're in, causing it to produce much higher distortion, which could be audible/noticeable depending on the design of the amplifier.

Valves themselves have no inherent "frequency response" aside from their ultimate high frequency limits which are determined by their geometry and construction but these are generally far above the audible range and are more generally defined by the circuit itself.

The best way to determine if an amplifier is working correctly with a specific set of valves is to simply measure its performance, which unfortunately isn't practical for most people.

[AlexM]

Will, that's a good explanation.

I asked the same question myself, and got some good info on this thread on TAS - see http://theaudiostandard.net/thread/196/valve-ageing

Regards,
Alex

[Jazid]

Mr RR has expained the technical side, may I humbly add a subjective component? Expect slow loss of dynamics.first, along with maybe noticeable shrinking of the frequency range, muted highs, muddled bass. Putting a known good replacement in will highlight the extent of this, otherwise we live at the mercy of our memories which are notoriously fickle!

[walpurgis]

I've never had a Hi-Fi valve fail, but on old radios and valve TVs the obvious signs were noise (or complete silence). 'motorboating' noise was common (caps?). I used to change dud valves in family electrical items as a kid (very many years ago). Dunno why I ended up doing it, 'the old man' was a major and renowned electronics wiz.

[Jazid]

Simple answer: Because he wasn't paid to. Classic case of the plumber's tap or the chippies door, if you do it for money 30 + hours a week then you won't want to do it for nothing in your time 'off'.