Do tubes degrade in SQ over time? How long does it last?

Yes, tubes degrade over time and eventually have to be replaced. The good news is that they continue to sound good up until the time they need to be replaced. When tubes die, they either stop working or you'll hear that they aren't doing well.

One side benefit is that the vast majority of tubed headphone are auto-biasing. The bias changes as a tube ages. Some amps have to be manually biased every so often - my speaker amp does this. It's nothing terrible; I have to turn a screw until a LED turns on, then back off the screw until the LED goes out. Other amps use different methods to bias, but it's nothing difficult. Again, don't worry about having to do this with tubed headphone amps.

The lifetime of a tube depends on the amp and how hard you run the tubes. Going back to bias, tubes can be run on a variety of voltages. For example, a tube could be run between 200V and 300V. Where the designer sets that voltage determines how long the tube lasts. Generally, the more power applied, the shorter the tube life. If the same tube is run at 200V in one amp and 300V in another, the amp running it at 200V will make the tube last longer. If you're wondering why all amps don't run at the minimum, it's because a tube's characteristics change depending on the voltage. A tube might sound better at 300V than 200V, or whatever goal the designer is shooting for.

How hard you run the tube also makes a difference. You'll see guitar players chew through tubes every few months if they run them hard. Power output tubes take a beating. On the other hand, I have gently driven signal tubes for old radios that are still working 60 or 70 years on.

Very generally speaking, you should get two to three years from a set of tubes with regular use. That all depends, but you shouldn't worry too much about regular changes.

I'll leave the amp recommendations to others.

Very nice and helpful write up Erik.

Can you also tell by looking if a tube needs replacement? Obviously, if it doesn't glow, I'd assume it's not working, but are there more subtler signs that it's seen better days? Maybe darkening or other tell tale features?

I really wouldn't worry.  I've now had my Little-Dot II+ for over 4 years.  It came with Mullard EF92 driver tubes.  They have seen a LOT of usage over the years, probably averaging 4+ hours a day.  The original tubes still sound just as they did when they were new and identical to newer mullard manufactured EF92 tubes i've purchased since.

i can't believe I'm agreeing with Uncle Erik --- a tube will sound great right up until the moment it pops ... and yes i've been in the middle of a listening session when a tube went south on me

I have a question: is tube life counted purely by the time it spends on or predominantly when it's actually being used, i.e. driving headphones, speakers?

 

I ask as my usual routine is get home from work, turn on my tube preamp and not turn it off until just before I go to sleep some 5 hours later. On days off this "on" period is longer as I turn on my preamp in the morning so it's on for about 12 hours or more. Is this a bad thing?

The tube life will shorten with hours. I have used my tube amp for 8-10 hrs most days and never leave them on all the time. Depending on the tubes used 3000-5000 hours is what most tubes last at least the time I use them. When I had the little Dot MK11 I used them same way and tubes were still good after a year.

Looks tends to be deceiving. From my experience, testing and actually using the tube is the only way to tell.

Tubes usually look fine until they die - you usually can't tell unless you put one into a tube tester. Occasionally, they develop a purple haze (seriously) which looks cool, but is just a step away from death. If you see a purple haze, you want to turn the amp off and replace the tube.

Gazza, tubes do wear down if you leave them on. They're always conducting when turned on and that reduces their life. Not as much as if you were running them at full power, but it's still there. Sort of like how a car continues to use gas while it is running but not moving.

There's a lot of debate over leaving tubes on and the "damage" that's caused by turning them on and off. I don't buy the argument for leaving them on all the time. First, tubes aren't the only items that wear out. So do the resistors and capacitors inside the amp. The longer they run and the more heat they're exposed to, the more likely it is that they fail. They are only rated for a certain amount of hours. If you know the part, you can find the MTBF ratings of the parts at Digikey or Mouser.

Fortunatley, caps and resistors don't fail spectacularly like they used to. However, they usually just go out of spec. A .1 uF cap might turn into a .25 uF cap after it has been on for a few thousand hours. You probably won't hear it because the change will be gradual. Same with the resistors and everything else. It might still "work," but the values of the parts inside will not be what they left the factory at. Considering what people spend on various tweaks, they seem oblivious to a resistor doubling its value. This is something that is easily measured with a $5 DMM. I wonder how people hear night/day differences with various things but cannot tell that their amp is out of spec.

I don't care if I have to replace tubes since they're wear items. For the same reason, I skip the NOS tube fetish stuff. Yes, some of the NOS tubes are quite good. But they're going to die sooner or later. So I buy either current production tubes or commonly found NOS tubes without the magic cachet. The Zana runs a RCA 6SL7 that I got for $5 or $6. Sounds good enough. The 6C33Cs and rectifiers are current production. If any of those die, I'm not going to be upset.

And to drag things completely off topic, this is the same reason I run a DL103 on the turntable. A phono cartridge is a wear item. It's going to die and it's also easy to screw one up. I'm sure those $2,000 cartridges sound good, but after so many hours, you have to send them in to be re-tipped. Re-tipping is usually about half the cost of a new one, so you're out $2,000 and get stuck for $1,000 every couple of years. No thanks. The DL103 sounds awfully good and turns up new for $150-$200.

After replacing the tube, is there such thing as tube "burn-in" or will it sound the same before and after 100s of hours. 

A tube will usually die in the first few days if it's bad. After that, it just begins a slow decline into not working. They wear out. Some might call it burn-in, but that implies that it is improving after time. But readings are never better than when a tube is new.

I am not familiar with new production/new tubes.

 

But from my experience, it's more like 'wake up' time. 

Once the tube is not used for a while - no idea how to define it, but seems like 6 month to a year does it - it needs some time to sound at it's optimum. For the 6sn7s I use, it seems like few to 10 hours does it.

Great advice here. 

 

I was recently getting bored with my MAD Ear+ HD tube amp because it was sounding about as good as my Clip+ --> CMoyBB 2.02 setup with my Grado RS-1s.  I thought I was losing my mind as I recall being blown away when I first got the MAD Ear+ HD several years ago.  Then, while listening in dim light, I happened to notice the "purple haze" in both 12B4As in my amp upon powering on.  Recalling Uncle Erik's advice I promptly went through my 12B4A collection until I found a pair with no power-up haze & tried listening again.  Audio bliss, just the way I remembered back when I first bought it.  Point is, the tubes don't always go south in obvious ways & it is good to swap out with known good tubes to insure that you are getting the best performance out of them.

 

Thanks Uncle Erik!  I was seriously considering selling my MAD Ear+ until this happened.