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Question about tube amps

#2
Depends on the design of the amp, but for average use I very often see tubes that are between 10 and 20 years old that are completely fine. I also see a LOT of perfectly good tubes discarded in the quest for the "perfect" tone.
 
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#3
Pre-amp tubes can last close to forever. Power amp tubes it depends on how "hot" the circuit runs them and how they are used. I have 1985 guitar amp with all original tubes which gets played fairly frequently. New caps but old tubes. If you are playing out five nights a week and being reasonable with your volume you should get at least two years from most amps.
 
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#7
agedhorse said:
Depends on the design of the amp, but for average use I very often see tubes that are between 10 and 20 years old that are completely fine. I also see a LOT of perfectly good tubes discarded in the quest for the "perfect" tone.
Click to expand...

:thumbsup:

I would also say it depends upon how your amp uses the tubes to some extent. For example in many amps the signal passes through a series of preamp tube stages and then hits the phase inverter which splits the signal into two paths for the output section. With a signal path like this, it might be a good idea to have one spare of each type of preamp tube used in the amp. I.E. if you amp uses three 12AX7s, you probably only need one good spare 12AX7. Then if you have a problem that you suspect is a tube, you can replace the tubes one at a time with the proper type to see if the problem is corrected.

Some amps also have a driver section between the phase inverter and output section. In some amps, driver duty is handled by two sections in one tube. So if the driver is the same tube type as another tube in the amp, you may still be fine with just one spare tube.

Other amps like the Ampeg SVT use a pair of drivers tubes. Although it's not essential, IMHO it's probably best if these tubes are replaced in pairs. If you have an amp that uses two driver tubes you may even consider getting a pair of tubes that have been tested and gain matched.

With the output tubes, ideally you want a matched set. So you when you buy new tubes get a matched set and keep any of the old tubes that still test within spec for spares. The role of the old tubes is sort of like the little donut spare that comes with most cars these days, I.E. they help you limp through if you have an unexpected failure.

A strategy you can use to increase compatibility between old and new tubes is to always buy tubes with the same plate current and transconductance ratings. Keep in mind that tube characteristics change as tubes age, so old tubes will not perform exactly as they did when they were new. Also, in order for this to work, you need to always buy from the same trusted vendor. The reason is because each vendor uses different equipment and processes to test and match their tubes. I.E. the plate current and transconductance ratings are not standardized across vendors.

Something to keep in mind with output tubes. Often when an output tube fails it damages a component inside of the amp that must be replaced before the amp will function properly again. Whenever I have an output tube go bad, I open up the amp, do a visual inspection, and check working voltages in the output section to make sure everything is okay.

Also, I usually have a few old tubes I have pulled from various amps. When a tubes fails I go through my collection of old tubes to see if I can find one that biases at the same approximately current level as the remaining good tubes in the amp. In order to do this I need test equipment. If I can find a tube that biases with 15% of the other tubes and the amp sounds good, I don't bother buying a new matched set.
 
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#11
One problem with buying a bunch of tubes for spares is they typically have a pretty short warranty period. So you buy a spare matched sextet of 6550s for your SVT and don't use them for three years. Then when you do use the tubes, you experience a failure. By this time the warranty is long gone, and you're probably going to be upset because you feel like the vendor sold you a bad tube. Unfortunately you need to buy from them again if you want a matched set.

I have a fairly large collection of small signal tubes. I bought most of the popular current production tubes to explore tube rolling. I had good results and then wound up buying some of the more coveted vintage tubes. I am sure I will get burned on some of them...but it's single tubes, and most of them were not very expensive.

When you buy output tubes you typically buy a set. For example you need six 6550s for an SVT, so you are probably going to be spending around $300 for a matched set. I buy output tubes as they are needed, so I can take advantage of the warranty. In my experience output tubes are most likely to fail when they are new.
 
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#13
AGCurry said:
It's never a bad idea to have spare ones. But the answer to "how often" is "when it goes bad." I miss the days when you could find a tube-testing machine in most dime stores (I guess those are gone too), drugstores, and hardware stores.
Click to expand...

Yeah, they had one at the Thrifty drug store up the street from my house.
You had to wait in line behind the guys trying to fix their TVs! :laugh:
 
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#15
When I played guitar and played out a lot with tube amps, I always carried a spare pair of output tubes, and one preamp tube. Never had to replace a tube at a gig, though I did have a few go bad at other times. I bought a Vox amp once that had a bunch of old mis-matched tubes - no 2 preamp tubes were the same make (even the 2 output tubes were from different manufacturers). At the NAMM show, the guys from Groove tubes gave me a deal on a set of tubes, so I had all matched input tubes and a matched pair of output tubes. A while later, a preamp tube became microphonic, then another, then another. Within a year, all of the new tubes had developed issues, and I played the thing for several years with...the mismatched "set" of tubes it came with - although they looked like a motley crew, they worked just fine. Tubes can be...fickle things. But some of them can last a very long time.
 
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#16
I have a 1970 Ampeg V2 with the original 7027A tubes. It’s had a lot of use through the years, but not much recently. It still sounds great. I figure this is a very big exception, but it does show what tubes can do in rare circumstances.
 
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#18
I have some tubes on my shelf that I use for comparison testing that are from the 1940's, that's 70+ years and I still use them as benchmarks for some applications. I also have some 12AX7 tubes that have about 20,000 hours on them and they are still just fine, meet the original specs within 20% (which is better than random new tubes from the same batch). I also tend to design fairly conservatively.

The point is that tubes tend to last a long time unless they become damaged rather than worn.
 
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