Question How to tell when a valve is ready for replacement?

[RICHARDIIII]

7 months in with my Art Audio Jota I have noticed that I have to turn the volume knob higher than I used to (very subjective!). I'm not certain, but it has made me wonder when valves need replacing?

The small driver valves have been replaced with NOS recently, but the main 300B XLS had quite a few hours on them and I must have had it on 4-500 hours (I'm a sod for falling asleep listening and waking up at 4am!)

Will they just "pop" or gradually loose power? Any other tell tale signs?

Cheers,

Rich.

 

[Fullrange]

Hi Rich.
Generally speaking power valves should give you 2000 to 5000 hours. The small valves could last at least double that or more.
500 hours they are just running in.

 

[Jazid]

Along with the above, I might add that there's several failure modes for a DHT like the 300b, but *typically* they just run out of steam, and slowly usually. Very slowly occurring loss of volume is indeed a sign of tired valves, but often I have found that more vigour with the volume is because of a more refined or less abrasive sound, all things being equal which of course they never are!

Overall I'd concur that most likely things are just settling in. That is what I'd expect as the valves reach early mid-life. Sit back and enjoy 😊

 

[pmcuk]

As above - probably the 300Bs are in mid life. No big deal for now. Just look for a good deal for a future purchase.

Input tubes last a long time unless they are over-driven. This can happen, but there's usually no need to design for over-driving an input tube.

 

[audio_PHIL_e]

there's usually no need to design for over-driving an input tube.

unless you're a hard rock guitarist

 

[rabski]

Directly-heated triodes (e.g. 300B in this case) generally last longer than indirectly heated valves. With absolutely any valve it very much depends on the circuit and on how 'hard' it is run. With DHTs in particular, it also depends on the filament voltage being accurate.

Assuming a decent circuit with proper operating points (with the Art is highly likely to have) then depending on the make, you should expect 5,000 to 10,000 hours for 300Bs. The 'real' Western Electric were rated at over 30,000 but some of the modern production 'hyper' versions are apparently closer to 5,000. Valves sometimes fail quite spectacularly, but unless the bias is way off, DHTs tend to just tail off. Usual signs to my ears are a loss of definition and top-end clarity, but again it depends on the circuit.

Either way, bearing in mind that DHTs generally take a fair while to stabilise properly, at 500-1,000 hours, they're just about ready to rock

 

[enrae8]

I only noticed this had happened when I got up to change a record 😎

 

[toprepairman]

Oh dear, most unfortunate.
Is it push/pull or parallel SE?
Did it still work?

 

[audio_PHIL_e]

Is it push/pull or parallel SE?

If it were push-pull I think he'd have noticed before the end of the record.

The heaters can't be in series otherwise they'd all be out.

 

[toprepairman]

Not necessarily if he's been listening quietly.

 

[JANDL100]

Thank you to whoever put the 2nd v into valve.

That was driving me crazy. 🤪

 

[audio_PHIL_e]

Not necessarily if he's been listening quietly.

Are you sure, Sir? I thought that if you took out half of the waveform you'd end up with serious distortion. I'd have agreed with you if a parallel valve goes dead at low volume you may not notice it has gone but Push-Pull? Unless you were listening at such low volume that the o/p stage is always in class A, so that the valve which is still operating continues to work over the entire waveform ... maybe my understanding is at fault: if so Pleez enlighten me!

 

[toprepairman]

Quite sure. The amp will be mostly if not entirely in class A.