Tubes on it's last legs
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N@Z

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Without the aid of any testing equipment, how would I know that the tubes (on my MG Head OTL be it the EL84's or the 12AX7's) are reaching the end of it's life?

I understand that the sound will degrade more & more as it approaches the end but what specifically would I be hearing?

Does the strength of the tube's glow deminish gradually or is there any other physical indications?

Finally, how will it end? Will it stop working altogether? Under what conditions would the tubes blow out?

Any other relevant information would be greatly appreciated too.
 
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N@Z

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Time for a little bump...

... & hopefully a little response... anyone?
 
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Hirsch

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This should be answered by someone who has been using tube equipment longer than I have, but:

A tube wears out, rather than blows out, AFAIK. However, I've seen one defective tube pop, like a bad light bulb, in my tester, before it had ever gone into an amp.

One clue that a tube will test poorly is that it will get very hot very fast. If tube is generating a lot more heat than is usual, it's probably reaching the end.

Odds are that poor sound will force you to pull the tube before it dies completely. However, excessive heat will increase tube wear, so once you reach the hot-running stage, the end will be near.
 
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Kevin P

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Tubes can fail in a number of ways, but the typical "wearing out" scenario is when the cathode gradually loses the ability to emit electrons and the tube's current carrying ability declines. This probably leads to the tube getting hot scenario mentioned in a prior post, but as the tube wears out it'll clip or distort more easily and won't sound as crisp.

In any case, the tube will probably be replaced for sounding bad long before it dies completely.

As for "total tube failure", the filament can burn out, in which case the tube would go dead (and not glow anymore), it can lose vacuum (particularly if it's mechanically damaged), or it can short out. These modes of failure are more often due to defects or mishandling than overuse.
 
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N@Z

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Thanks Hirsch & Kevin P.

Hirsch:
Any idea whether a tube that powers up (glows) faster than normally means it is running hotter than normal? In your experience what specifically is heard in terms of poor sound quality you mentioned?

Kevin P:
When you refer to worn out tubes losing its ability to emit electrons more & current ability declines, would that translate to a lower glow on the tube itself? Ditto my question on poor sound as above.

Sorry if my question sounds kinda silly but I never had any tube experience prior to this. I notice that different tubes have different glow strengths, powers up at different rates & was wondering whether it would be just type/brand related or due to its age or hours of use.
 
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Wodgy

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Quote:

Originally posted by N@Z
When you refer to worn out tubes losing its ability to emit electrons more & current ability declines, would that translate to a lower glow on the tube itself?


The part of the tube that glows orange (the filament) isn't the same part that emits electrons (the cathode). For this reason, you won't normally see a lower glow as the tube loses its ability to emit electrons. However, there is one exception. Sometimes if you watch power tubes (like the EL84s) in the dark, and especially if there are stray electromagnetic fields, you will see a light blue glow from the tubes. These are the electrons being emitted. The blue glow does decrease as the cathode ages. However, just because the blue glow disappears entirely from sight doesn't mean a tube is dead yet -- it may still have quite a long life. Also, not all power tubes exhibit this kind of glow.
 
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Budgie

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Easyiest way to test without a tube tester is to substitute a new tube and see if it makes a dramatic difference in the volume, or sound quality. You do have a spare set of tubes, don't you???
 
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Andrew LB

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About how long to tubes on the MG-Head OTL generally last?
 
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N@Z

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Quote:

Originally posted by Budgie
Easyiest way to test without a tube tester is to substitute a new tube and see if it makes a dramatic difference in the volume, or sound quality. You do have a spare set of tubes, don't you???


LOTS! But they're not the same brand & mostly used.
 
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Kevin P

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Quote:

Any idea whether a tube that powers up (glows) faster than normally means it is running hotter than normal? In your experience what specifically is heard in terms of poor sound quality you mentioned?


The warm-up time probably increases as the tube ages (remember when you could gauge the age of a TV by how long it took the picture to appear?), but it will start glowing just as quickly/slowly as before. Quote:

About how long to tubes on the MG-Head OTL generally last?


The MG Head doesn't drive the tubes hard so they should last a long time. In any case, the driver tube (the center 12AX7/5751) will likely outlast the power tubes (the EL84s).
 
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Budgie

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Sometimes you can get by without having the output tubes matched by brand. Your ears will let you know if they don't match well.
 
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briantf27

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The power tubes wearing out will typical manifest as a static-like sound in one or both ears, while the driver tube will typically start to clip. I recently ran into just this: sudden severe clipping. So I swapped out my driver tube (I was disappointed at its 6 month lifespan) only to have the problem remain. After switching out tubes numerous times and making several high voltage measurements on the board to verify internally that the amp was probably alright, it turned out to be my damned 1 year old Cambridge CD player. :-/

But back to your question... I typically see 12 to 14 months out of EL84 power tubes if the amp is left on 24/7 (which mine is). If you like to turn your amp on and off each day, perhaps the tube life would be extended an extra 6 months or so. Just a guess, as I can't say from experience...

FWIW

Brian.
 
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braillediver

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The life span numbers you state make sense:

12 months on 24/7 = 8760 hours

14 months on 24/7 = 10,200

They seem to agree with the estimated averages I see.
 
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N@Z

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Thanks for all the replies.

Based on what has been mentioned my tubes should still have lots of life yet in them.

I still have to pinch myself sometimes... can't believe I own a tube headphone amp!
SWEET!
 
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Andrew LB

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My MG-Head OTL will be arriving on wednesday (oct 30). Time to go music shopping before it arrives!
 
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