valve life

mattacjones

I had a minor issue recently, swapped the existing valves with some 'spares' - all of which were working once, all of which looked almost brand new. It made no difference, so took it to an amp tech who has told me, he put ALL my premp valves in a valve tester and they were ALL duff i.e. not producing anything like what they should. I'm not a pro, but the amps get used regularly, but I always thought valves worked or didn't work. So this was a shocking discovery - they wear out and degrade. How could I not know this, is it true? Admittedly some were 5-6 years old, but how do you tell if they are on the way out, or should you simply replace after xx years? Cheers 

Voxman

Yes, valves can degrade with use over time.  Even an amp that is working fine can sound better with a new set of valves, if the amp has been well used and the valves are older.  There's no absolute here, but with reasonably regular use, a tube change every 2-3 years will help keep your tone optimal. 

ecc83

This an area, like so much to do with guitar amps, that could benefit from some hard research. Yes, valves lose "emission" over time but how that affects tone is largely unknown and untested AFAIK.

One parameter, "mu" or amplification factor does not change much with age so it seems unlikely that even quite degraded pre amp valves with have much effect? An output stage (inc' PI) that employs negative feedback will change much less as valves age.

Then there is "conformational bias". IF you have just shelled out 100 quid on a full set of bottles you WANT them to make a diff! Does not hurt certain valve tech's wallets to promulgate the idea either!

Dave.

ICBM

I would be careful about reading too much into that. Yes, valves do degrade slowly in use over many years, but the tester measurements don't necessarily mean a lot in terms of real-world performance. It's not unusual to find preamp valves from the 1960s which have been in use for decades which still work perfectly well. Also, a lot of new-production valves don't actually meet the original design spec for the types even when new, so it's not necessarily an indication that they've degraded.

If every single valve tested as weak it's also possible that the tester is out of calibration.

jpfamps

As ICBM says tester calibration is an issue, and even if it calibrated correctly to the manufacturers spec, these can vary too as there is little hard data on say emissions as to what is in spec.

It's also seems that some modern 12AX7s are purposely lower mu than spec so that they are less microphonic, which is a major issue for manufacturers in combos.

Regardless, if my tester tester all the pre-amp valves duff, then I would check it with some new valves. If these tested duff then I would want to check that my tester is working properly.

The ultimate test though is in the piece of equipment they are being used in.

For example I had a 20W Marshall head in the other day that was putting out only 10 W. The power valves tested OK for emissions, however putting a new pair of EL84s in the amp restored the power 20 W, so cleary the valves were very worn.

Valves in combos have quite a hard life as they are vibrated hard in use and also ventilation may not be optimum. 

Many valves in this environment fail mechanically before they fail electrically, eg become microphonic due to the internal structure become loose.

This is particularly true of pre-amp valves.

My attitude to valve changing on amps I'm working on is that if the valves are obviously defective I will change them.

If the amp is not sounding right, I may try new valves, but unless this rectifies the situation I will put the old valves back.

Else unless the customer particularly want new valves I'll leave the old ones in.

mattacjones

Interesting. Locally, well reputed chap. Could well be that my pristine looking collection of spares are indeed clapped, I really never kept track of them. I shall report back!