What are the symptoms when tubes are starting to fail

tubes get grimy and nasty just from being in place. Ideally yes, they should be clean but touching one isn't going to make them explode either. I actually don't really like to clean tubes too much just because it wipes the markings off. Ever look through a box of shiny tubes that have no numbers? I have lots of them, and I've been slowly throwing the things away that I can't ID.
 
OK, OK, you've had your experience, :boring: but what about my experience? I am also a tech from the late 70's, and still I have never seen the things you have described here. So.......am I un-experienced? Not really :no: but........I still have not seen catastrophic failure that you have described here. Maybe all of the amplifiers and receivers that have been through my fingers have never been abused or have had the tubes touched by human fingers. This is just my experience over a 30 year period. I don't dis-agree with you, but I have never seen it.:no:

Fortunately you are right in the vast majority of cases. I, too, have seen a lot of filthy tubes. God knows that when you work on musical instrument amplifiers you never know what you'll find. I've found things in guitar amps that don't even have a name yet. Usually the ouchputs and rectumfriers run well below critical heat. Bulbs that really get hot are usually projector bulbs and halogen lamps, where one is warned not to get fingerprints on the glass.

I agree that it is possible over a 30 year period not to have had such a misfortune with output tubes. I--unfortunately--haven't had that kind of luck.

One of the more embarassing moments was when as a kid I was building an amplifier and running tests I lost a pair of 6CU6s in one sneeze. :eek: :D
 
Halogen lamps and progector lamps need the "kid glove" treatment.

I think halogen lamps and projector lamps need special handling (no finger oils or other contamination) because of the extrema high temperature these items run at.

Tubes run cooler and may survive, but keeping them clean is a good idea too.

Going back to the original topic, my Sansui 1000A was running used tubes (7591A) and I ran them as long as I could until one had lost its silver getter-ing and I was afraid it would go gassy and short out.

I got some used old stock from another AK member, and combining the 3 tubes he got me, and a spare I had, I made up a reasonably matched set and noticed a mediate increase in the quality of the overall sound, Bass was fuller and more "tight", highs where stronger and more detailed then before, a general improvement and satisfaction that the getters in the tubes would last a good while longer (based on how long the previous tubes lasted).

Didn't mean to hijack the thread, but we got slightly off topic:D

Mark T.:music:
 
And the amorphous nature of glass leads to many an argument as to its state as solid, or perhaps a very high viscosity liquid.

I studied and worked with bio-active glass in graduate school, including mixing and firing my own Ca-Na-P-Si mixtures in platinum crucibles. I was taught that glass is a supercooled liquid, but that is subject to argument. For a nice discussion on glass, look here

I would also agree with the differential heating theory, but that would also indicate that the printed lettering should be removed precisely for the same reasons.....:dunno:
 
Leaving fingerprints on tubes is a bad practice if one wants to maintain strict adherance to protocol in the handling and use of vacuum tubes. While small signal tubes may bear finger oils without issue the tubes that run hotter in application will make cleanliness of the glass a more serious concern. An old 6L6G may do OK with grime on its glass, but a more modern 7591 or EL-34 may take exception, and suddenly have the envelope fail.

Owing to the high vacuum contained in a typical tube I have my doubts as to the porosity of the glass used in the containment of same. And the amorphous nature of glass leads to many an argument as to its state as solid, or perhaps a very high viscosity liquid.

The attention here needs to be aimed at the temperature signature over the area of the enclosing glass. If there is a foreign substance of a different specific heat than the surrounding air, and the glass beneath such contamination heats at a slower rate than the surrounding glass, there is an anomaly in that area as a result. If the area of clean glass temperature to the temperature of the area of anomaly has a sharp enough transition there will be an immediate and concentrated area of stress that may cause the glass to crack, thus causing immediate failure.

In the real world tubes get dirty, and--while usually they will be forgiving, and function for many hours--sometimes they fail for the contamination on them. If you can take steps to clean them, or can take steps to avoid getting tubes dirty in the first place, why take a chance....?

How many light bulbs have changed with gloves on? and the glass is much thinner.
 
Handling the tubes with your bare hands is quite fine. I might dust off a accumulation of dust, but I don't recommend wiping them, it can remove the fragile lettering on them. Some of the really old valuable tubes will lose their lettering if you just look at them too long... :)
 
Some SED Groove Tubes EL34s made a popping sound after about seven years use in my Marshall amp.
 
wear gloves when you handle tubers the oil in yer skin can permeate the glass which is porous.

Not true at all.
Finger oils are much larger molecules than the diatomic atmospheric gases the tubes hold vacuum against. Permeate by definition means to pass through.

I don't see that happening....

As too tube life, hard to say, but some tubes that test low sound just fine. I've run 12AX7s that test bad/low and sound fine w/o issue. I think I've discarded 3 tubes in the last 8 years due to going bad and run 40-50 year old tubes regularly.
 
Not true at all.
Finger oils are much larger molecules than the diatomic atmospheric gases the tubes hold vacuum against. Permeate by definition means to pass through.

I don't see that happening....

As too tube life, hard to say, but some tubes that test low sound just fine. I've run 12AX7s that test bad/low and sound fine w/o issue. I think I've discarded 3 tubes in the last 8 years due to going bad and run 40-50 year old tubes regularly.
Many tubes are cooled by radiating heat as IR light. When you leave oily spot on glass, it eventually gets darker. As a result it results in hot spot on glass envelope due to higher absorption of IR. This can end up with cracked glass over some time. Most problems happen with power tubes, more so - those with graphite plate. The same happens with HID bulbs in car's headlight, light source in video projectors and other places.
 
I have no problem with the oils cooking and damaging glass; that makes sense.

Oils passing through glass by diffusion; that's a different story...
 
I don't think the "normal" power tubes we use get nearly hot enough to carbonize oil. Thoriated tungsten? I might wear gloves for those, maybe.
Anyhow, the only tubes I ever had wear out were 6L6s and they started to sound flat and distorted.
 
only problem i was told handling valves or is it tubes these days ? anyway it rubs the writing off .. enuff said ..
failing ones will have many symptoms as already stated . its finding out why they failed is the more important bit
 
Thoriated tungsten will give off alpha particles when heated- not an issue with power tubes.

The amorphous glass does not require wearing gloves because of its structure and thickness. Think about it this way- if you think 50+ year-old tubes were handled with gloves- you have a bad memory. The temperatures and amorphous glass thickness don't require wearing gloves. Epitaxial glass and higher operating temperature would require gloves, but it's a non-issue for audio tubes.
 
If a finger leaves a oily spot on a power tube one could wait until the amp is fired up and ran hard for an hour or so then lick the oil off.
 
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