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When Tubes Go Bad...

Discussion in 'Amp Central Station' started by basdenchris, Mar 21, 2019.

  1. basdenchris

    basdenchris TDPRI Member

    May 30, 2015
    Minot, ND
    I'm sure this topic has been discussed at great length in fragments all over the forums, but I can't seem to find a good thread that centers around it.

    Though I feel like I've been a gear-addict for ages, until about a year ago, I wasn't gigging regularly. My amp(s) didn't get cranked up ever, let alone for four hours a night two nights a week, so wear and tear wasn't really a concern. It has occurred to me recently that I actually have never experienced bad tubes. I don't know what to look for or what they sound like when they're starting to go.

    In my searches for information on this topic, I've come across descriptions of microphonic tubes, and I've found a couple preamp tubes that make noise when tapped lightly. But other than that, I'm not really sure what to look for or what to expect from my tubes as they get older. Would diminished headroom be a potential symptom of bad preamp/power tubes?
  2. alnico357

    alnico357 Tele-Afflicted

    Jun 2, 2015
    Maybe they don't sound as good. The last tube to go bad on me was a 6L6 that shorted and blew out a 5 watt power resistor. I use tubes for years with no problem, especially preamp tubes. I think for my own part, the great majority of my tube replacements over the last 50 years have been an exercise in "tube sniffing." My DRRI had new 6V6s a year ago. Otherwise I'm using the same tubes that came in the amp in 2010. A small amount microphonics upon a gentle tap is not something I sorry about.
    Shuster likes this.
  3. beninma

    beninma Friend of Leo's

    Mar 17, 2017
    I just changed the V1 preamp tube on my Orange amp last night after talking to tech support.

    These amps you can't actually tap the tubes unless you reassembled the amp and left all the protective features off. The preamp tubes have spring loaded covers that would probably reduce microphonics and shield the tube from vibration damage, and the power tubes have both the spring loaded cover and a huge protective cage.

    Oh well.. they had said maybe go ahead and change it. It sounds different/better with a new tube so whatever.

    I'm not going to make a habit of changing them for the sake of changing them or trying to get better sound but in my case that channel has no EQ so if the tube changes the EQ it's about the only thing you can do. And I had reason to suspect that tube might be suspect and not sounding it's best.
  4. Musekatcher

    Musekatcher Friend of Leo's

    Jan 15, 2013
    meridianam altum centralis
    Good question. I read where a lot of folks share about how much better their amps sound after having them re-tubed. As is customary, these shop visits usually include re-biasing, new capacitors, and sometimes repaired resistors. I also read how folks A/B power tubes, and declare one better than the other, but without re-biasing. Do tubes detectably deteriorate without loosing functionality? Tube health is measured, such as Gm, and by other values. Should we hear a difference in a low tesing and a high testing tube, if both are functional and not noisy?
  5. Matthias

    Matthias Friend of Leo's Gold Supporter

    Jun 30, 2003
    Third rock from the Sun
    A tube has no guaranteed life. Hence the three month warranty most have... Some last a few months, others decades depending on the tube itself rather than usage. I had a power tube go in my Blues Junior in under a year of light use... But my other tube amps have soldiered on fine.
  6. mgreene

    mgreene Tele-Afflicted

    Jan 27, 2010
    south carolina
    The first and most obvious clue is that the silver getter flashing (in most modern tubes) discolors or disappears all together. This does not mean that the tube is dead - it may work fine for a long time and sound OK. But it is an indication that the tube has had a long service life.

    In my experience - when preamp tubes go bad they can present many different symptoms. Not all of them mean that the tube is worn out. Some tubes are bad form the factory. Pre tubes may not give signal, pass reduced signal - low volume - or they may make funny noises, I have had tube that squealed and recently one that gave a low-frequency tone that sounded like a bad filter cap.

    Power tubes often just sound bad. AS above, worn flashing is an indicator that a tube is going.

    I have always kept a couple of old tube testers to test emission on suspect tubes - see if some one you konw has one. (IF Silverface sees this he will tell you that an emission tester is not a thorough test - but it is a good indicator) :)
  7. L.A. Mike

    L.A. Mike Tele-Afflicted

    Mar 1, 2014
    somewhere over a rainbow (Ex-L.A. resident)
    Most preamp tubes will make a sound if you tap them. It sort of sounds like a drum. That's ok. If you tap them and you hear a bunch of static or swishing sounds or screeching then you need to replace it.
    Power tubes rarely make noise when they go bad. They just start sounding dull. Sort of like when your strings go dead. You might notice some humming, maybe. But that's not common.
    Preamp tubes fail more often than power tubes.
    If you hear noise or the amp sounds odd, maybe the tube sockets are just corroded. Especially if it's an older amp with old tubes.
    Just pull the tubes in and out of their sockets a couple of times. That may do it.
    If you have an effects loop in the amp, take a 1/4" plug and insert it and remove it a bunch of times. Corrosion in there can affect some amps even if you don't use the loop.
    Don't forget to rotate your pots on a regular basis. Sometimes dirty pots can cause weird noises, especially if their position never gets changed. Turn them all the way in both directions a couple of times. I do this every couple of months. Like when I change strings or batteries.

    If you have extra money there are tube sellers who will gladly accept your money.
    basdenchris likes this.
  8. Obelisk

    Obelisk Tele-Afflicted

    Apr 1, 2013
    NW USA
    A microphonic tube is always pretty obvious. When a tube starts howling because of vibration, then you have tor trace the source. Best to use a Sharpie or chopstick to lightly tap the tubes to see they will be induced into oscillation. A broken power tube is often perceived as a broken speaker. Changing brands of the first preamp tube can give different voicing to an amp. As long as you are turning your amps on a regular basis(like a few times a year)the caps should stay charged up. I have a a couple of dozen amps that I should start considering to offload. I always play them in rotation every month or so to check how things are working. I leave them on for awhile to keep the caps charged...I am going to go downstairs do that today.

    This is all really good advice. I definitely move the pots when I am doing an amp turn on and off. I don't how many times I have saved cash from having some ECL contact cleaner around. A lot broken audio gear is merely a dirty pot or a dirty tube socket. Dirt accumulates even in the cleanest of interior spaces. A dirty input or speaker jack can cause problems. This sort of basic maintenance takes little skill to do.
    basdenchris likes this.
  9. golfnut

    golfnut Friend of Leo's

    Jan 15, 2008
    I've been playing 2 to 3 nights every weekend for the past 6 months. I had an NOS mullard rectifier tube that I paid a lot of money for that didn't last 3 months before it went. Was at a gig and the fuse blew. Put in another fuse and it blew. So I changed back to my current production sovtek rectifier that had been in the amp for a few years. That was about 3 months ago and its been fine ever since. So much for NOS. The only other tube I had go on me was a 6v6 that blew up and took a resistor out.
    Other than that I usually tube roll enough that I haven't worn out tubes where I've noticed a tone degradation.
  10. Steve 78

    Steve 78 Friend of Leo's Silver Supporter

    Oct 27, 2015
    Melbourne, Austraila
    I'm no tube expert, so grain of salt, etc. When I was rehearsing/gigging regularly with a dimed amp I went through a few tubes (mostly power tubes. I think I only had to replace pres once). When they went bad it was really obvious. Big drop in volume or loud obnoxious noise. If the change is so subtle that I couldn't really notice it, then, well, it's not a problem is it? If you're worried, try swapping the tubes out with new ones and see if things improve. (Biasing may be a factor, but I was using a Mesa Boogie which didn't require biasing.)
    basdenchris likes this.
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