Lifespan of EL34/KT77 Tubes

Last year, I bought a quad of Gold Lion KT77s. While they sound great, one tube has died after only 14 months. I bought EH tubes five years ago and three of them went within a year. The Seimens EL34s, I replaced them with, lasted over four years.

What is a reasonable lifespan for EL34s or KT77s? It looks to me like the Seimens are the most reliable, but they are old stock tubes and get more expensive with time. Can anybody suggest some reliable new stock tubes?

 

Those are typical lifespans. 1 -5 years depending upon use. Some folks get a few years if the use the amp regularly. Others can get a decade if they don't use it very often. Turning an amp on and off is the most stressful thing a tube goes through, especially if it is done a few times in an evening. Then cumulative hours do add-up but do not in themselves have the same detrimental effects as on-off surges and excessive voltages and currents inside the amp.

I have found that the better brands of tubes are the premium ones and not only that but for output tubes, you also need to pay a premium for precision matching. Many of the biggest resellers claim to offer matching but from the results I have seen of those on amps that come through here for repair, they don't do a very thorough job. The best tubes from the new Mullard and Genelex brands are offered in a factory platinum matched version. That is all that I carry and the only way to achieve results similar to the old versions in my experience. That costs me a couple more dollars per tube but it's worth it.

Everyone on the net seems to be an expert and has some crazy story about "avoid this or that" brand due to some failure they have had. The truth is that all tubes will fail at some point. After years of working with all types and brands of tubes, I have simply come to the conclusion that getting a better brand and paying for the best QC testing and matching is what results in the best tubes. Even those fail but they will have stronger performance while they operate and can last a bit longer as well. Some amps are harder on tubes than others and you may or may not be able to adjust them for that.
-Bill

 

As Bill, has pointed out that's the usual life span for certain power tubes no matter the brand. But just in case you wanted to pick up another quad of those Siemens EL84's for less?.

Try looking for them labeled as RFT EL34's instead. It's the exact same tube and were in fact made by RFT for both Siemens and Telefunken NOS 1960/70's. But the ones of the newer production I've had the best luck with are the SED Winged =C= EL34, nicer tones as well.

Regards,Oscar

 

Different circuits and voltage and current levels stress tubes in different ways.

Certain high end manufacturers especially in the 80s designed circuits that ran the then-available, end-of-life production tubes (which were often built to take abuse well beyond specified design levels so they could be sold under various uprated special numbers) at the limit they could get away with. The imported tubes were often built to barely tolerate book limits and in some cases were blatantly mislabeled as to type.

So unless the equipment is modified, some of these units will NEVER work correctly with the tubes now available. And such modification will alter measured performance and sonics.

So who is to blame, the equipment manufacturer or the tube maker? Both? Neither?

It's a matter of who is looking at it when you get down to cases.

It's worth noting that one historic objection to the McIntosh circuit was that it stresses the heater-cathode voltage rating of the finals beyond design limits. But in practice it has never proved an issue. Tubes last forever in Mc amps. I must admit I'm astonished and disappointed no one is using the Mc circuit anymore now that the patents are long expired.

 

I've an amp that is supplied with new production tubes and I've used new production tubes in the amp, and its predecessor using the same tube type. I've had really good luck in terms of dependability and longevity using the Winged C EL34s. But lately I've found great sound from the new JJ 6CA7, great presentation, strong and rich. I've had flawlessly operation but I've only had them about eight months. Still, very strong still and I expect to get some good life out of them. I usually get a few years; I use my amps more than 12 hours a day, every day; they get a lot of use.

I've never really enjoyed the KT77 type. . . prefer the sonics of the EL34 and 6CA7, and my amp has a grid modification which maximizes the sound of these tubes, and not the KT77, so I don'tuse those.

 

You mean where the suppressor is connected only by a cap to ground?

 

I had always read about approximately 10,000 hours of use- is the lifespan of tubes.

 

http://www.decware.com/newsite/paper145.htm

 

Yes.

 

It varies entirely according to the tube and the application. WE made a couple of types with a design life of twenty times that. But for typical receiving tube apps that's not too far from the mark.

 

Worthwhile mod.

 

I'm using a matched quad of Russian 6P3S-E's in my Jolida 302b. If biased correctly, they should easily last as long as a new stock EL34. They are very rugged, and don't seem to mind the high B+ voltage present in the 302b. I have more than one amp, so the Jolida doesn't get much use, therefore the tubes will probably outlast me.

Your other option is to have the bias resistors changed to run KT88's. You may or may not get more life from them.

Are you leaving your amp on 24-7?

jeff

 

I've killed 3 electro-Harmonics EL34 tubes in my 30wpc amp in 4 years and I play it about 60 hours a week. No, I am not exaggerating the 60 hour bit, I play it all day.

 

I've had Siemens EL34 in my Music Reference RM9 last for years, never had one go bad, actually, worth the investment. The new Russian or Chinese sourced tubes just don't sound that good to my ears and they seem to age quickly.

 

Depends on the tube. Driver tubes like the 6SN7 GTA are thought to last 5,000 hours, while red based RCA 6SN7s are thought to go 10,000 hours.

I have a pair of 6L6 power tubes that now have 1,800 hours on them and are still going very strongly.

 

The design is a huge factor. Plate voltages, screen voltages, and bias voltages are key. Some amp designs run the tubes very hot, which tends to shorten life.

 

That is the design maximum for receiving tubes such as the 12AX7. As I tried to indicate in my post and as others have stated more clearly, the application is as large a factor as the tube design itself. The design of the amplifier and its operating points will determine the actual life of the tube. Output tubes don't fair as well.
-Bill

 

I've been turning my amp on two or three times a day. Would I be better off leaving it on until I'm finished for the night?

 

The RM9 design is easier on tubes than many amps. I had a C-J power amp that Siemens tubes made in W. Germany would only last a year in. In that case, they were not worth the investment.
-Bill

 

I would say that you are better off leaving the amp running for say, 4 o 6 hour stretches than turning it on and off for a few reasons. One is that the amp will sound its best after about an hour of warm-up, so you have only one period where the amp is warming-up vs three periods of warm-up. The other is the tube life is better when the tubes are stable in temperature. If the amp is operating at too high a voltage and the tubes are drawing too much current, then this is not true, but assuming that all is well in the amp, then letting them run doesn't do much harm. It is the on-off temperature and voltage swings that will wear the tubes out. Now, if the amp is set so that the tubes are drawing too much plate current, then they will be getting way too hot and also fail early due to heat damage.
-Bill

 

I agree with Bill.

Also, periodically check the bias on your EL34s. If it runs too hot (bias too high) it will prematurely age your tubes.

 

Yes. By leaving it on all day, the output tubes will receive much less thermal shock, and will probably last longer. Jolida's don't run in class "A", so they don't produce a lot of heat compared to other amps.

jeff

 

Thanks everyone for your responses. I will definitely leave the amp on until I'm finished for the day. I think I'll look into replacing getting the Siemens.

 

I would leave my amp on all day except that I would be apprehensive of doing so when I have to leave the house — as I'll be doing within the next 20 minutes. The fact that I have used the same amplifier in my system almost continuously since July, 1983 doesn't diminish the prospect that it could present a fire hazard.

My MS-190 uses 4 EL34s per side; I check their bias once a year and change them out every three years. Next time I'll try a set of fancy schmancy Mullard copies that set me back a good bit.