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Characteristics of a dying tube.

Discussion in 'Amps and Cabs [BG]' started by Chunk-O-Funk, Sep 24, 2005.

  1. I did a bit of searching on when you need to replace pre-amp tubes and have found that you do not need to replace them unless they are going bad. Loosing gain, microphonic.

    So my question is, basically how does this sound. Loosing gain I understand, and the term microphonic to me means the same thing or maybe a small sound with no bottom, also, how does it occur. Is it gradual over time or can it happen suddenly.

    The reason I ask is I took my rig out of storage and set it up at a rehearsal space me and my brother (a drummer) rented. When playing I would suddenly loose volume, down to about half I suspect and then it would come back. It happened a few times. My first thoughts was a bad instrument cord but wiggling it around did not cause the problem, I did not have a spare with me. So right now a dying tube is just a thought, I have used this set up for a few years (5?) when gigging regularly, two to four nights a week.

    So the problem I have, could it be a tube? If not, what are the characteristics of a dying pre-amp tube?

  2. tadawson


    Aug 24, 2005
    Lewisville, TX
    "Microphonic" typically means that the tube picks up excessive mechanical noises and inserts them into it's output. IE, lightly tap it and get a thunk in the output - good tubes don't do that, or do it very little. I can't say that I have ever heard of a tube dropping volume like that and coming back, but I suppose it might be possible if an internal connection has failed and is disconnecting itself/reconnecting with vibration and heat. Intermittent volume loss is typically something more mechanical - bad pot, cord (like you tested) or a loose connection internally.

    - Tim
  3. Thanks Tim,

    Micropohonic: picks up noise much like a microphone, Got it. I'm glad I got up today! ;)

    Hopefully it is just the cord, if not I have two components to check out. Alembic F-1X or my world 2.1 power amp. I had two basses so that rules out a bass or onboard preamp problem.
  4. luknfur


    Jan 14, 2004

    I just had an Ecc83 drive tube go out and it was making a sizzle/hiss-like like sound when switching off standby into Hi volt setting. Initially the sound was only on switching over but that quickly became intermittant. When I first got the amp the 12ax7 that came with it was dead and no sound.

    This is my first experience with tube amps and one thing apparent about them is to 1) always have known working spares and 2) any time there's a problem with the amp, swap the tubes out first to rule them out cuase they seem to be the weakest link.
  5. urje


    Sep 13, 2005
    i had a problem with my (used) valve head when i just got it, a bit similar i think. but the thing was, the volume didn't drop to half, but to a lot less than half. and i don't remember it came back to normal, but perhaps that was because i didn't wait long enough. anyway, i had a spare valve, so i just switched one after another, and the second valve seemed to be the problem.
  6. I heard when they start going bad your hear a glass/crystal sound with every note you play. I was reading some manual and it said it. So.. if you hear something like that...
  7. Allow me to point something out:

    It's not the fault of tubes in general that yours came with a cheap defective tube. They've been working just fine without having to have spares for everything for about the past 70+ years.

    MOST of the time, tubes are the LAST thing to be the actual problem. With heat, use, and age, capacitors and resistors are much more prone to failure and drift.
  8. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Are you using an active bass? After a while my bass battery ran outta juice, and I experienced problems very similar to yours and I screwed around with my amp for a while before getting the same problem on another amp and determining it was the bass.
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    This is in fact true. Maybe less so than it used to be in 1960, due to some marginal tube manufacturers.

    However, the ONE thing in favor of swapping tubes as a possible fix is that a tube MIGHT be the problem, and you can fairly easily make the swap. Easier on some units, harder on others. Usually super-simple on an old Fender, for instance.

    it is far less likely that you can identify and swap any other potentially bad electronic parts, especially during a break at the gig....

    So if you have spare tube(s), and the tubes are accessible, at least for preamp tubes there isn't anything really against swapping. About the worst that can happen (aside from screwing something up as you make the swap) is that it won't fix the problem.

    If it does, you are back in biz. If not, you aren't any worse off.
  10. luknfur


    Jan 14, 2004

    Yep, probably so. Still, intelligent trouble shooting begins with the simplest, cheapest, quickest, least invasive, and most reversable probable solutions first which puts backup tubes at the top of the list with fuses. At the least, if it's not a tube or a fuse, you're probably done playing for a while. The way I figure it, even if I buy a tube and that's not the problem, I still have a backup tube I really need anyway. Different tubes and different states of tube wear put out different sounds so it's not like backup tubes need to be redundant. Tubes are also a low skill fix and if you can change a fuse, you can probably change a tube and you don't need a tech or to be without the amp.

    Initially I called Andy at THD and he said they've had virtually no problems with the amp. Replacing the tube resolved the problem for the time being but that doesn't mean there may not be something else that is burning out tubes. Time will tell. But it was a cheap Eastern Euro tube and the DOA tube wasn't even marked so probaby junk. But a junk tube that works it better than some high dollar tube that doesn't (and visa versa probably)

    Regardless, I have a lot to learn about tube amps and all accurate info is appreciated.
  11. zombywoof5050


    Dec 20, 2001
    You said you took the amp out of storage. How long was it in storage? You should not let an amp sit for too long without turning it on occasionally, because it can make the caps dry out and fail later when you turn the amp on. Even if you no longer use an amp, you should turn it on and play through it probably once or twice a month to keep the caps in good condition. Depending on how long you had it in storage, maybe some caps are going out?
  12. protoz


    Nov 30, 2000
    A friend of mind had that happen to him after he bought a used 5150. Volume would just cut out randomly and he would power it down and let it cool before it would work again.

    Just take it to a tech and explain the problem or just buy new tubes. That's what solved his problem at least.