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help troubleshooting a tube amp.....

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bryan k, Jul 24, 2006.

  1. bryan k

    bryan k Member

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    heres my situation. I have a Carvin legacy 100 tube head, that acted real funny (and still is) this past weekend. At higher volumes (3 or higher) this thing is LOUD! i mean reall loud! so let me expain what im experiencing......

    i turned it on ( guitar is pluged stright into the amp, no pedals, nothing in the effects loop), and as i turned up the volume, past 3 or 4, the volume didnt increse all that much, the only thing that happened was a "choking' sound and more increased clipping/un-wanted distortion. The higher i turned up the volume...the more it started to "choke". At volume 5, usually this thing is rattleing windows, and rumbling walls.......but i could hold a normal conversation with my friends, and we were talking louder than the amp! and it was on volume 5! All it did was just "fart' out, 'choke" and clip like crazy the more i turned up the volume. and this happens to both the clean and dirty channel.

    so i changed all the cables (both guitar and speaker cables), nothing improved. i switched out the v1,V2, and V3 preamp tubes.......nothing changed. i looked inside, and the power tubes appeard to look fine....no red glow, and no flickering. BUT i did notice a very small amount of black burnt marks (very small) on the inside of the glass of my V7 and V8 power tubes near the upper portion of the plates. But they still appeard/looked to react as normal.

    what could be causing this? maybe the V5 phase inverter needs replaceing/ or maybe the power tubes are in need of replaceing? or is it something totaly differnt?
     
  2. Blue Strat

    Blue Strat Member

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    No one knows. The only way to find out is trying a few things. Be sure your speakers are plugged into the correct speaker jack and be sure that the speakers are ok. Substituting a known good speaker cab would help.

    The phase inverter, power tubes, or numerous other things could be the source of the problem. You probably have a "spare" 12AX7 to try for the phase inverter. After that, one or more power tubes could be shot. Check your bias if possible. This can point to power tube or power supply problems.
     
  3. bryan k

    bryan k Member

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    yeah, i did switch the cab...and its not that, its the head that has the problem. I do have a new quad of power tubes on the way, so what ill do is change them out/bias the head, and stick a new phase inverter in there.......................and prey that is the problem.

    it could very well be the power tubes, they are about 2+ years old. when i purchased them, i biased the head myself....and set the bias around 36-37 (for el34). and the amp has worked fine for 2 years , up untill now.

    if it is in fact a bad power tube.......is this anormal characteristic of what happens when a tube goes bad?
     
  4. hasserl

    hasserl Member

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    It can be, it depends on the failure mode. Sometimes the failue will result in a blown fuse and the amp will go dead.

    If you replaced V1 - 3 try taking using those tubes in the V5 slot. Don't worry about V4 right now as it is just the reverb tube and you could take it out completely and the amp would work fine (it just wouldn't have any reverb).
     
  5. bryan k

    bryan k Member

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    well i replace all 5 preamp tubes, including the PI tube.........

    nothing changed. I guess all i can do now is wait for the new power tubes to arrive....and pop them in, re-bias it....and see what happens.

    i just find it strange that the fuse hasnt poped out or something.......usually if a power tube gos dead......doesnt the fuse pop out and the amp shut off?

    oh well....maybe im looking to much into this, and just should wait for the new powert ubes and take it one step at a time.......
     
  6. hasserl

    hasserl Member

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    Taking a full reading of al the voltages will help alot.
     
  7. bob-i

    bob-i Member

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    Sounds to me like a power supply problem, possibly a bad filter cap. If replacing the power tubes doesn't help that'd be my next check.
     
  8. bryan k

    bryan k Member

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    and that is what im afraid off........something i will have to send to a tech, or back to the factory for repairs............

    im always a 'do-it-yourself" kinda guy......and it will bug me to know that there is something wrong with the amp, that i cant fix myself! if its tubes....no problem, i can handle tubes and biasing........

    anything other than that.........GGRRrrrrr......someone else is going to to be all up in her, invading her personal space!
     
  9. deeval

    deeval Supporting Member

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    I had an output trans,go out and do the same thing.
     
  10. Blue Strat

    Blue Strat Member

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    Power tubes only take out a fuse if one or more is shorted. You can easily have very weak power tubes and a low volume, high distortion situation.

    I'm betting you have another problem, but you'd have to take it to a tech to find out. Replacing power tubes is you best option right now.
     
  11. bryan k

    bryan k Member

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    i did notice that one of my power tubes isnt glowing as bright as the rest, which the other 3 glow exactly the same. the filament on the top of the plates is very dim, but the other 3 are nice and bright................

    that wouldnt be an issue would it?

    i still belive that it may not be tube realated, and have something to do with the output transformer...............but seeing as how i know nothing about fixing amps, OTHER than replacing tubes.........this is far as the troubleshooting i can go..................if its not tube realated, it has to go out and get servised by someone.
     
  12. Blue Strat

    Blue Strat Member

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    The only visible indications of bad tubes are no filament glow or a getter (shiny part on the glass) has turned white. A bias probe can often give indications of weak tubes or power supply problems. If you're a "do it yourselfer" you MUST have a bias probe.
     
  13. bryan k

    bryan k Member

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    yeah, ive got one.......im just gonna wait for my new tubes (should be arriving today from doug preston, that guy ships so fast!) to arrive, and im gonna put them in and bias em up......i figured it would be best to troubleshoot with a freash quad of new tubes..............
     
  14. Blue Strat

    Blue Strat Member

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    Why not troubleshoot with the possibly bad tubes, now? See if all the tubes are still biased as when you set the bias when you installed them. That alone can give you an indication.

    Also, if you have the option to measure plate voltage, this can be VERY illustrative.

    BTW, there are other tube dealers who ship quickly (I know one of them personally ;)
     
  15. donnyjaguar

    donnyjaguar Member

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    I'm of the mind that you can't properly set output tube bias without dismantling the amplifier. In my experience component values change so much over time that you really need to check things out in the PA section before attempting to bias the amp. A perfectly matched set of output tubes will look unbalanced if one of your Sgrid resistors is whipped. These usually sit atop the tube socket and get subjected to all the heat the amplifier gives off thus ensuring their finite lifespan. The cathode resistors of the phase splitter can wander in value too and any imbalance here will limit maximum power. Many of these issues are buried in the negative feedback of the amplifier and there's much benefit to disabling this circuit and having a look-see on the 'scope. This is especially true if you are putting in a brand new set of output tubes - these things are expensive!!!

    That said, I obviously don't use a bias probe. :)

    I'm an engineering tech by training and got the reputation of being able to repair recurrent problems in equipment. Its not enough to replace one component and "fix" a problem when the fault is often lying deeper. I see this a lot in musical instrument "repairs". That said, I don't have a problem with a tech using the "wrong" values in an emergency repair if consideration is given to the effect it will have in the long term.

    I guess if your equipment is relatively new and has lead the life of Riley a bias probe will work fine. But that sure doesn't describe the gear that crosses my bench. Mind you, I seem to get the jobs others won't touch. :)

    DJ
     
  16. bryan k

    bryan k Member

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    LOL.....i know, KNOS is a great company..........ive been to the site alot......, but i am faithfull to doug, for 'current production' tubes. ive been buying from him since he opened his internet tube store. I have been to KNOS for NOS stuff.......

    you are right about poking around inside with the tubes that are in there now. I have a bias probe, and a decent DMM......but my extent of using these was just to bias power tubes. ive never measured plate voltage before...........i wouldnt even know where to start.

    when i bias my amp, i just stick in the bias probe...........and turn the bias pot till it reads around 36-40 mA on the DMM. I know i should get a plate voltage reading first.........but i never learned how to do that, so i just jumped in and went for it!
     
  17. bryan k

    bryan k Member

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    well...........its going back to the factory today for a full look over and repair job.

    i replaced the power tubes, bias it up......and still same problem. i looked around inside.....all the caps seems to look ok, all wiring is ok, no burn marks anywhere.....no craks...etc.....

    so i can only assume its the power transformer................

    well, i thank everyone here that helped troubleshoot the problem, you guys rock!
     
  18. hasserl

    hasserl Member

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    Bryan, don't assume the worst. Often times you cannot tell just be looking at a component, there might not be visual signs, or you may not realize what is there if there was.

    It's best to get it into someone who has the knowledge and equipment to check it out properly. The bad part of it is the cost and the downtime. Hopefully neither one will be too bad. Transformers are pretty stout, I wouldn't expect that is your problem. More likely a cold solder joint has failed after repeated heat/cold cycles. Maybe a failed cap or resistor.

    Let us know the damages when you get it back.
     

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