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I think this would be a wonderfully instructive question to people who are new to tube amplifiers. In short, how do I know that it's time to change a tube, and what's a good strategy for figuring out which one to change (preamp vs power vs rectifier)?

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thanks for all the great advice this helps as I am at the point of wanting to blow my Blues deluxe to bits... this being said after playing a huge gig and my amp just not sounding the way I want....this makes for a night of hard work and little to no fun within. –  user6561 Jun 30 '13 at 19:52
I think it's time to change the tubes when the amp looses its lively sound. its dementionality. bad tubes will result in a drab sounding guitar tone in other words a tele will lose its twang. –  user6798 Aug 2 '13 at 2:30
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2 Answers

up vote 11 down vote accepted

An important thing to note is the different types of valves can be changed independently of each other; eg: if you change the preamp valves you need not change the power amp valves etc.

With power amp valves you will notice very quickly when one or more have gone. The amp will give out noticeable and unpleasant tones/white noises and will be reduced in power; the glow that a healthy valve gives off may be different in colour or gone altogether, there may even be flames in the blown valves, which is pretty hard to miss. When changing power amp valves ensure that the amp has been cut off from the power for some time before you start, valves store a lot of power and can be dangerous if removed straight away.

Preamp valves tend to last many times longer than power valves; so check the power valves first if they look ok then inspect the preamp valves closely; a blown preamp valve might also give off different noises than a power amp valve; such as excess feedback and other squeaks/screeches. Also if a preamp valves goes you may not notice a reduction in power; which is another way to identify the type of valve which has gone.

A good rule of thumb is; if you get a drastic change in tone or hum from your amp; or it wont power on at all; its time to check the valves. If the valves look ok check all the cables carrying signals to the amp including the effects loop; if the amp wont power up check the power cable/fuses first. then check the valves. With no power its not possible to check the glow colour of a valve so look for valves which are opaque or have burn residue on them.

If you unsure take it to a dealer for a service, those this shouldn't be needed.

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"noticeable and unpleasant tones", especially on low notes which cause a "blat" sound instead of kicking you in the gut the way you expect. –  Anonymous Jan 16 '11 at 21:56
Pretty sure valves don't store any power, that's the function of the capacitors in the circuit. That said, it's still a good idea to unplug the amp entirely and let it sit for a few minutes before sticking your hand in there. Any good-sized caps should have "bleeder" resistors to dissipate their charge, but these will take awhile to bring the voltage down to safe levels. It'll also give the tubes a chance to cool down so you don't burn yourself when unplugging them. –  Anonymous Mar 28 '11 at 17:42
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Two additional problems I've seen with some people's tube amps, related to tubes going bad, is their sloppy manner of treating the tubes.

Some people don't hesitate to use their fingers to pull a tube to inspect it, using their fingers. Finger oil, or fried-chicken grease, or dirt or whatever was on their fingers, will transfer to the glass of the tube, and begin to insulate it, reducing heat transfer. That makes the tube run a bit hotter, reducing its life. Instead, I always use a clean cloth, a couple paper towels, or something to keep oil from transferring. I've even used the tail of my T-shirt.

Another problem I saw, was our lead singer use to toss all her cables into the back of her Mesa Boogie Mk II combo. One of those cables hit the tip of a pre-amp tube, on the nipple of the glass where they sealed it, and cracked it a tiny bit causing a pin-hole to open up, and vented the tube's gas. The next time she powered up the tube cooked and the amp was dead. I had to do triage on the amp after the show, and saw the little nick. She was told to put her cables somewhere else. (My personal solution is to put them in my gear trunk in a cloth bag that keeps them from getting tangled and the tips from getting damaged.)

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+1 For greasy fingers; I got a pretty blank look from a guy I used to play with when I explained this one to him. –  DRL Jan 16 '11 at 22:11
LOL... I used to play with people who never thought about eating fried chicken or BBQ ribs, or corn on the cob. Their fretboards were disgusting. Repairing/cleaning a fretboard like that starts with a liberal dose of lemon-oil, followed by gently scraping with a single-edged razor blade just to get the stuff against the fret, then finer and finer sandpaper and steel wool. They'd be amazed how pretty the neck was when the gunk was removed. –  Anonymous Jan 16 '11 at 22:15
@the Tin Man: someone I once played it had the crumiest fretboard. Not only that but he must have add super-acidic sweats cause all his frets were orange and green. The strings were a greenish blue and felt oily to the touch. I was afraid to let him play my guitar for any length of time. –  Anonymous Mar 26 '11 at 19:18
@bleakcabal, I am an assh*le when it comes to people touching my guitars and amps; I paid good money for them, and I've usually done a lot of work on them making them how I want to feel and/or sound. It only takes a few seconds for someone to screw up the tuning, drop the guitar, break a string, change all your knobs, etc. And then there's the jerks who set beer pitchers on other people's amps... GRRRRrrrrr.... –  Anonymous Mar 26 '11 at 21:09
I threaten death and dismemberment to my wife and cat if I find my current rig damaged in any way--and it's not even anything that great :D. –  Jduv Mar 27 '11 at 5:47
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