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How long do tubes last?

ron-c

ron-c

Addicted Member
A local dealer just sold a customer a new set of tubes for his almost three year old MC2000 amp. This customer uses the MC2000 to drive floor standing JM Labs Utopia speakers. I asked the dealer to hold the old tubes for evaluation. These tubes are the original set all branded McIntosh. The KT88 outputs are checked on a Hickok 539B and the small nine pins on a Hickok 752A. The amp has been in use for an estimated 4 to 5 thousand hours.
All eight of the used SED-Svetlana KT88s measured between 7500 and 8000 micromhos transconductance and a new pair I have measure 7500 mm. 6250 mm is the new tube spec.
The four 12at7a tubes, two US made and two Chinese all measured as new. The four 12ax7a tubes, all Russian, two were in spec and two were 1/2 normal transconductance. (McIntosh no longer uses this particular 12ax7a).
People always ask how long the output tubes last in a McIntosh amp and I usually say 5 to 10 thousand hours. I have some SED 6550C with over 8000 hours on them that measure as a new tube. Many tube amps will go through a set of outputs in 1000 hours but the Unity coupled output design is very kind to the tubes. I have some old Gold Lion KT88s out of an original MC275 and they also measure as new. Old tubes are not necessarily worn out.
Ron-C
 
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"All eight of the used SED-Svetlana KT88s measured between 7500 and 8000 "
The test data on my 539B shows they should read 5000 or above. So if
these tested up to 8000 then their doing fine. So I dont know why they
sold this person new tubes if they tested that high. Other than this
reason $$$$$$$$$$$$$$$$$$$$$$.
 
damn greedy dealers. :D
Yeah, I tell my customers around 10000 hours, or when they start to sound different. Then if they look at me weird, I ask them if they change their household lightbulbs even if they still work?
 
Glenn,

The tubes will be returned to the cutomer and he knows I am testing them. The customer wanted a new set so the dealer sold them. Very few dealers test tubes and very few MC2000 customers want to send their amp in to be tested although I have seen them at the factory service department for this.
The setting for a new KT88 is 6250. Some 6550 Tungsols, NOS GEC KT88s and the small bottle RCA / Sylvania 6550 all test at 9000 to 10,000 mm.
Ron-C
 

"The tubes will be returned to the cutomer and he knows I am testing them. The customer wanted a new set so the dealer sold them."
Good deal, for minuet I thought the dealer was selling something
the customer didnt really need.

"Very few dealers test tubes "
Your right its rare to find anyone that can anymore. However I was a bit
troubled when I seen somone that was doing service to amps and they didn't
own a tester.



Glen
 
In my own used I have found small tubes like 12AX7s, 12AU7s and 12AT7s generally last a good while. I popped a set of RCA 12AX7s in an old Electrohome preamp I use in one of my rigs, tested high but not quite new. After countless hours of use they still tested almost the same. Got a set of 6L6GCs in an amp that tested a tad lower that still have held out good to this day. My worst experiences have been with chinese 12AT7s, I have had supposedly new ones test half what a good NOS one did on my Dynamic 85 tester, not much of a concern now as I obtained a bag of new ones and the amp that uses this tube only uses one but worthy of note. Chinese 12AU7s tend to hold out fine but sound terrible IMHO. I put a set of NOS Sylvanias in my one amp and it has been happy ever since. For the record the only tubes in anything that I have had that ever needed replacing were an output tube (6V6 if I recall) in a circa 1936 Midwest radio, and several smaller tubes in a TV set. Had a Sovtek 5Y3GT that dropped off considerably in testing and sound quality in a relatively short period of time but there again I have enough 5Y3s that it makes little difference other than being an observation. Most tubes I have found hold out for a long time. Pulled a bunch out of a circa 1971 Zenith TV I have tha has received more use than most TV's will ever last, only had some tube replacements in 1979 and the set did not even receive most of its use until after that. While I was in the process of cleaning it and giving it a good once over I tested some of these and they still tested more than fine. IMHO as long as you stick to decent stuff, preferably not Chinesee, the tubs probably will hold out quite well.
 
I have a Grebe MU-1 from '27 that has the original ballooon tubes- 01-As- in it & they all test fine. Most of the failures I've found in older radios are due more to bad caps than bad tubes. -Sandy G.
 
With as many tube units I see each week I use about a tube a week, most of the bad ones are the indicators (EMM801) that have gone dim or 6U8's in the demultiplex with weak triode sections. It is frustrating when a customer asks for a retube for a tuner when all of the 6AU6's test like new, why waste the money and time when the 35 year old tubes are as good as new and most likley are original.
 
I think that the 6U8 is being pushed too hard, I usually find the pentode section OK (19K pilot amp), but the triode weak (stereo switch). What I find is it starts out strong then looses gain as if it is gassy, 5 minutes later it behaves dead. Haven't kept track of brands that tend to fail the most.
 
Tubes tested good

I had an experience with a H H Scott 330C that changed my mind about tube testing. The 330C behaived poorly. Signal strength was weak. The strongest station only caused the signal strength meter to deflect half way. I check all the tubes on my Hickok 6000. All tested very strong, as good as new tubes. I adjusted the alignment with no improvement. Finally, I substuted a fresh tube in the first i.f. stage. The signal strengh dramatically improved! Replaced the tube in the second i.f. stage and again a significant improvement and so on. Lesson learned, if an i.f. or r.f. tube tests good it may be good or it may be weak.

Not all tube testers can spot a weak tube. Some tube testers will be more accurate with some tube types than others. One needs to understand the limitations of the tube testers and try substatutions.

tcdriver
 
For a modern McIntosh that takes 12AX7, 12AT7 and KT-88/6550 tubes, are there any brands that really shine sonically and are known for their superior reliability? More importantly, are there any that should be avoided? I'm new to tubes but from what I understand most if not all tubes today are from overseas sources and the quality can vary greatly.
 
The best way to test a tube is in the actual circuit. If that fails tube substitution is the next step. Tube testers are usefull if you have alot of tubes to go through and weed out the shorted or burnt out filament tubes. The NOS american tubes are getting scarce and prices are shooting up - the new manufacture tubes are looking better every day.
 
TWinbrook46636 said:
For a modern McIntosh that takes 12AX7, 12AT7 and KT-88/6550 tubes, are there any brands that really shine sonically and are known for their superior reliability? More importantly, are there any that should be avoided? I'm new to tubes but from what I understand most if not all tubes today are from overseas sources and the quality can vary greatly.
Click to expand...
TWinbrook
I've tested all these tubes on my Mc275CE :
. 6550 : GE, McIntosh/Svetlana, Tung Sol
. KT88 : GEC(TT21), Tesla, Valve Arts
. 12AX7 : McIntosh (chinese ones), Tele, Sovtek Cryo, Mullard CV4004, Philips, Mazda, Siemens 5751
. 12AZ7 : McIntosh (chinese ones)
. 12AT7 : McIntosh (chinese ones), Philips, Siemens, Mullard etc ...

Great combinations are :
. the original one !
. McIntosh small tubes + GE
. whatever with TungSol, Tesla and GEC power tubes

A "bad" one is the McIntosh small tubes + Valve Arts KT88,
but with european 12xx7, the sound is better : dynamic is back

Happy listening
Emile
 
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Please excuse my ignorance, but what exactly does a "rectifier" tube, such as a 5U4 GB tube do?

Someone at work knew how much I love McIntosh gear & gave me a used "Made for McIntosh by RCA" tube as a conversation piece for my office.
-LOL- Inquiring minds want to know...
 
5U4 takes AC voltage from power transformer and converts it to a DC voltage, the tube version of two diodes.
 
R&R and Terry: Thanks for your replies & simplifed answers. I spent 10-15 minutes on "google" & came up empty. I know we don't need to understand everything there is to know about tubes to enjoy their sound, but can someone kindly recommend an "easy" book or a couple of "better" websites that I can start learning more about tubes & older audio gear? :nerd:

Thanks in advance for your time & expertise!
 
Out of hundreds of tube type products I have worked on, I can count very few tubes in audio equipment that have actually been bad enough to impact performance. Had a 6J7 that heated up in an RCA PA amp but was non-functional. Replaced a 6AU6 in a Curtis Mathes console in the FM section.
More often there are bad tubes in TV's although there they are still not all that common...have had to replace the HV tubes most often, 3A3, 6BK4, 6JE6.

Have a 1929 Sparton radio, still uses type 24 tubes and some directly heated triode output tubes...tubes still seem to work fine. Only problem with some of the really old tubes is that they can be more microphonic.
 
MWP said:
...can someone kindly recommend an "easy" book or a couple of "better" websites that I can start learning more about tubes & older audio gear? :nerd:
Click to expand...

I would suggest Vacuum Tube Valley. Order some back issues or some of the books .... its a very good source for vintage tube gear in print.
www.vacuumtube.com
:thmbsp:

Audio Asylum also hosts a vintage forum.
www.audioasylum.com
 
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