Life expectancy of tubes.

LotusFool

LotusFool

Well-Known Member
I'm pretty sure this has been covered before, but what might the life of my power tubes (EH 7591) be?

When I got my used MC225 back in 2008, I took it into the shop where the guy replaced the caps in the power supply, and I asked him about testing the tubes. He looked at me as though I was crazy and said "Why, is there a problem in the way the amp sounds?" " No I said, it sounds wonderful". Then why do you want to check the tubes?

I didn't want to appear more foolish than I am, so I said nothing further. But I can remember going to the drug store back in the 60's and checking the tubes on their tube checker, so it seems logical to me to do.

I run at least 12 hours a week, so that's 624 hours a year. So I'm guessing that's nothing for the life of a tube, and the tech didn't seem to think it was worth the bother, but I still wonder?

Is there any guide lines as when you should begin checking for week tubes?
 
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Depends how hard the circuit run the tubes, complete electrical parameters, there is no precise answer.

Some of my power tubes (EL84 (6P14P-EV)) rated for minimum 5,000 hours runs for more than 25,000 hours, but they are 450V heavy duty military tubes on an amp that normally run standard EL84 at 300V, and in circuit, I added thermistors for soft-start, that keep currents low for the first minute when power is applied and tubes slowly warm up.

If some tubes are rated for 3,000 and run hard, maybe they can last 2,000, but if run soft, 20,000, or even 40,000 or 50,000, or more, depends how much soft they run.

Some small signal tubes can run even longer.

If you worry about, you can buy a simple emission tester on the bay, one of the most simple to use is my Sencore Mighty Mite VII TC162 , Not good at matching power tubes, but to know if one is weak, leaking, or present a short , useable or not.

PS: And in 25,000 hours, my tubes lost about 4% of their emission, when 70% = Good :D
 
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So.... customer relations aside, I agree with your tech.

I'd never test a tube unless there was some other indication of a problem. ...Well, OK, I think playing with my tube tester is fun, so I have on occasion just tested tubes for no reason, but that was more "testing the tester".

My 1950's Harman Kardon has either the original tubes, or tubes made early in it's life. It still works, balance is fine, and it sounds OK to me (I've replaced other parts). As mentioned, I have a tube tester sitting right next to me as I type this. But I can't see any reason to test the tubes in the old HK. The tube tester would surely tell me the tubes are old (weak). What would I gain?

I've found that most tube failures (in tube amps) are obvious -- heater not lighting -- short -- air intrusion.

I, too remember testing tubes at Radio Shack or Walgreens in my younger years. If you were trying to repair a tube TV, there is no question that they were nearly indispensable.

Also, if it is a stereo amp, it already comes with a tube tester -- the other channel.

If you suspect that the tubes are old, getting weak, etc., and you want to re-tube -- shop around and go for it, but again, a tube tester isn't really fundamental to that decision when talking about an amp that works/sounds OK.
 
Interesting thread I am wondering the same about my tung-sung 7591. I've only seen a 11bm8 tube die of mostly old age. Which was about 2 years of nearly daily use with long hours I was satisfied. I had it on for a few hours and I saw one tube red platting.
 
i don't have a lot of empirical data on it, but judging from my experience with CT and X-ray tubes as well as light bulbs, i've always wondered if it's not best to avoid turning tube gear on and off as much as possible, i.e. not more than once a day or so (or even less on something like a preamp). particularly on good vintage tubes, the thermal shock seems like it might be the most dangerous element to a tube's life expectancy. now that's assuming you're running those tubes well within the SOA -- if not, then tube-hours become much more important.
 
cdfac said:
i don't have a lot of empirical data on it, but judging from my experience with CT and X-ray tubes as well as light bulbs, i've always wondered if it's not best to avoid turning tube gear on and off as much as possible, i.e. not more than once a day or so (or even less on something like a preamp). particularly on good vintage tubes, the thermal shock seems like it might be the most dangerous element to a tube's life expectancy. now that's assuming you're running those tubes well within the SOA -- if not, then tube-hours become much more important.
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I agree regarding thermal shock. Most things i've had break do so during power on.
 
mroboto said:
.... I added thermistors for soft-start, that keep currents low for the first minute when power is applied and tubes slowly warm up.....
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I have one Amperite 6NO60 (6V, 60 seconds) delay relay bypassing the thermistors. Thermistors take longer to cool down than the tube so when power is disrupted for a minute or so I do not want the thermal shock. Primary reason was to keep the B+ for the driver tubes within maximum voltage - these heat up faster than the finals. (Using solid state rectification).
 
I had an Audio Research D76a amp for a lot of years. After several years I decided to have it checked out and have the tubes replaced. I didn’t hear any problems, just being proactive. They cleaned, biased, and replaced the tubes for me. When I got it back it was night and day better. I know I might be stirring up a hornets nest, but IMHO tubes age and as they age they change sound. It may sound OK to you because it has changed so slowly, but the sound has changed. I currently have a tube preamp that is very hard on tubes. Two-three years at best using very heavy duty tubes. IMHO
 
Output tubes are the ones most likely to wear out just because of what they do. Rectifiers are also in the high wear category. Depends on the amp which gets beat up more. Its not unreasonable to want to check those tubes for condition every 500 or so hours. They degrade gradually and its usually not at obvious until new ones get installed that anything was actually the matter. Running them through a decent quality tube tester just to get an idea of condition is not a bad thing.

A better test would be full power output on the amp. If its not meeting design specs, or at least matching what fresh tubes do by a significant amount, its time for tubes. I have a set of EL34 tubes here that are flat-out at the 6 watt mark. Good tubes in that same amp will make an honest 40. I've never actually popped those things in my tester to see what it has to say, but the power meter gives them a big fat FAIL.
 
I do not have any exact knowledge about the hour life of the various tubes but can only relate what I remember of the probably dozen amp clinics I hosted for Dave o'Brien and McIntosh when I managed one of the local McIntosh dealers.

If my memory serves me correctly only the first three, two in 1979 and the first in 1980, received the traditional full Mac tube amp clinic sevice.

DOB was adamant that the only way to test out the tubes was in the amp itself.

This involved a full bench test with the amp meeting his own expected performance limits which were 15 to 20 percent above what the published specs were.

If a unit tested just say 15% above related specs he would warn the owner to bring it to the next clinic or have our service shop "keep a eye on it" because service was coming due.

If the unit just met the published specs or did not meet specs the tubes went into the trash can and all new tubes were installed until it exceeded the specs he expected from it.

He was adamant about using the best modern new parts that protected the tubes so he would most likely be horrified in some of the questionable choices so many make in searching for a "sound"

By the first part of 1980 tube quality had receded to the point he could not reasonable invest the time to cull out the 50% of tubes that no longer would meet Mac's historical purchases made at the top of the bell curve of pricing. So 1980 was the last tube replacement clinic as best I can remember.

He did not give up his fondness for tube amps however. In the late 80s at the last clinic I hosted he set me up with the last of his personal 6l6cg output tubes. Culled from 100 GE/RCA tubes from the Kentucky plant he had 8 that met his expectations.....even though my MC240s are long gone as is DOB, I still have those tubes and the memories of working the test benches with him.

So from the man who tested more tube amps from more manufacturers than probably anyone else.......bench test the amp often at least twice a year.
 
cdfac said:
i don't have a lot of empirical data on it, but judging from my experience with CT and X-ray tubes as well as light bulbs, i've always wondered if it's not best to avoid turning tube gear on and off as much as possible, i.e. not more than once a day or so (or even less on something like a preamp). particularly on good vintage tubes, the thermal shock seems like it might be the most dangerous element to a tube's life expectancy. now that's assuming you're running those tubes well within the SOA -- if not, then tube-hours become much more important.
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I've talked with Kevin Hayes of VAC about this very subject- and from what I took away, he agrees with this sentiment, as far as we discussed it. Thermal shock is a primary cause of preamp tube failure (heaters, grid alignment, and other related causes due to expansion/contraction), but hours are usually what take out output tubes (since many output tubes are run near or at their limits).

Regards,
Gordon.
 
Tube life is essentially a function of stress imposed on the tubes and duration in a given amplifier. Higher quality equipment will generally be able to derive more useful life from output tubes than lower quality equipment.

Tube deterioration doesn't necessarily mean the equipment cannot continue to perform completely satisfactorily; it simply means that it won't be able to make "full output spec". For example, if the speakers are of extremely high sensitivity, tube deterioration is less critical than with the same amplifier (and tubes) used with much less sensitive (more power demanding) speakers.

There's a major difference between tube life in an amplifier averaging a watt or less with a pair of 105 dB K Horns and the same amplifier driving, say, a pair of 86 dB AR3s - with both pairs being driven to identical average SPLs.
 
If you began in 2008 with used tubes, and have since then put over 8000 hours on them, I think it would be prudent to at least check if the amp is still meeting spec. It is a lot of hours for power tubes any way you look at it, and new tubes are cheap relative to the value of an MC225.

When I repair an amplifier I do not test the tubes unless I suspect a bad one, but I do test the amp at full power, and worn out old tubes will fail this test even if it works well at low power.
 
^^^this is solid advice. The best tube tester is the device in which they are running. That being said, the cream of the crop tubes were sometimes given a 10000 hour guarantee. I'd at least check your power tubes once a year and if the performance drops, break out your wallet...
So much is circuit dependant that theres really no way to guess.
 
I'm sure this is a foolish question, but can the amp's performance be tested with an ohm meter at the speaker output terminals? Is that how it's done? And if that is how it is done, what do you check, voltage?
No laughing please.
 
Not a foolish question at all. Thats basically exactly how they are tested.

Hook it to a resistor of 8 ohms or whatever value your speaker is.

Drive a signal into it until the output clips

Measure the signal power out using the rms ac voltmeter on your dvom. (Or your HP 334 analyzer if you're me)

Rms voltage squared/ load (8 ohm resistor) should give you within a few watts of the published value in the data sheet.

Usually you can hear them sounding "thin" by that point I'd think.
 
LotusFool said:
I'm sure this is a foolish question, but can the amp's performance be tested with an ohm meter at the speaker output terminals? Is that how it's done? And if that is how it is done, what do you check, voltage?
No laughing please.
Click to expand...

At the very least you would need a signal generator, an oscilloscope, and a solid state or vacuum tube volt meter, and a dummy load. Optional would be a distortion meter. An ohm meter unfortunately is not really of use here.

The alternative is to simply buy new tubes every five years or so.
 
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