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Carvin guitar tube combo problem

Discussion in 'Miscellaneous [BG]' started by Johnny Mack, Sep 5, 2008.

  1. hey there, this is regarding a guitar friend's carvin combo amp. i do not know the model, but know it is about 6-8 years old, a two channel 2-12 tube combo amp. anyway, recently his amp has not been putting out low end for his guitar, there is a noticable difference in that it is not so full sounding anymore. i looked at the back and there are 6 smaller tubes, and i think 4 bigger tubes. the second small tube does not glow as orange as the others while the first glows somewhere between the second and the rest. there are no noticable differences in the tubes, even the large ones other than that.

    my questions are:
    are the small tubes for the preamp?
    do all 6 control both channels?
    with this information, does it seem that 1 or 2 tubes need replaced?
    does one faulty or loose tube (kinda scared to jostle it around too much) affect tone in that it just reduces a certain frequency but otherwise sounds normal?
    am i completely missing something else (it is not the guitar) like a bad pot or something?

    any and all input is hugely appreciated.
  2. RickenBoogie


    Jul 22, 2007
    Dallas, TX
    Since it's not a bass amp, and this is a bass amp forum, your chances of getting hits are slim. Also, since you and your freind seem to not be really tech-minded, the best bet would be to take it in to a good repair shop. Good luck with it.
  3. thumpbass1


    Jul 4, 2004
    The small tubes are the preamp tubes, and the large ones are for the power amp section.
    As for the second preamp tube not glowing like the rest doesn't mean that the tube is bad. A drop in bass response could be a result of a few things, including capacitors that don't recover a charge as well as they should. Tell your friend to take his amp to a decent tech. Unless you know how to work on a tube amp, as the capacitors can store an electrical charge that needs to be bled off or left to dissipate over time to avoid a potentially lethal shock, it's best to let a tech look at it. I'm hoping it's something simple that requires a mundane preamp tube swap, pot cleaning, or easy repair. It could also be time for the power tubes to be replaced along with a bias job, a good tech should be able to pinpoint the problem. Tube amps for guitar or bass follow pretty much the same circuit paths and designs with a few different tweaks here and there between the two.
  4. fireincairo


    Jul 8, 2008
    In my experience, the way a tube glows doesn't necessarily mean it's a bad tube. Being a guitarist for many years, I will give you a quick way to troubleshoot for tube issues:

    He should get a decent 12ax7 preamp tube and go through each of the tubes in the preamp swapping it out and trying the amp again to see if he can find a bad tube. If none of the tubes change the tone of the amp, it could be a powe tube.

    The problem as you described it actually sounds like it may be a tired, old set of power tubes that need replacing. Since your friend is unsure of himself, he should take it to a tech to get the power tubes replaced. If he wants, he can buy a set of matched power tubes and simply toss them in there to see if his bass response and volume comes back. Then he will know for sure it was the power tubes. Then, he can take it to a tech to have them bias the amp, which isn't that hard. A weber biasrite meter will cost about the same, but will last a lifetime. He will need to do some research to learn how to drain the caps in order to avoid getting a jolt (which could conceivably kill).

    If it's something beyond the tubes, just have a tech take a look. It really sounds like a power tube issue, though.
  5. fireincairo


    Jul 8, 2008
    That's what I'm thinking it probably is. Whenever I come across loss of volume or power, it seems to end up being the power tubes.
  6. interesting, thanks for the tips people. im pretty good with instruments and cabs, but not so much amps. ill have him take it up to the shop. id hate to have him buy tubes and find out it is something else. thanks again.
  7. Most of the small tubes are likely 12ax7 for the various preamp gain stages. There is likely one 12au7 to drive the power stage. The four large tubes are power tubes, either 6L6 or EL34 depending on the model.

    What you describe may be a tube problem or it may be something else. Lack of bass response could be an output transformer going or it could be some interstage capacitors drying up due to heat exposure over time. Carvin used to use some electrolytics in the signal path (not a good idea, especially in a hot tube amp) and those could have dried up and affected your response.

    If you don't know your way around a tube amp, really you need to give it to a tech. Or send it to Carvin. No joke, you can really hurt yourself or the amp. The B+ rail is 400 Volts DC. I've been bit by 400 VDC and it's not fun. Even if you don't hurt yourself, bias is very important in a tube amp. Carvin's all have adjustable bias which will most certainly need to be tweaked after a tube change.
  8. No, it doesn't. I have a pretty good idea of what's going on, but he needs to hear it from the tech who will fix the amp.

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