MT15 Help!!!

Discussion in 'Amplifiers' started by Jess O'Toole, Aug 30, 2019.

  1. Jess O'Toole

    Jess O'Toole New Member

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    Hey all, I've had my MT15 for a few months now and it's suddenly stopped working properly. It is super quiet even at full volume, and as a tube amp novice I have no idea how to fix this problem!
    It started very early on with slight ducking in volume, but now it won't come back up at all and I'm in need of a fix ASAP.
    I've tried swapping my cables and when I run my guitar and leads through other amps it works as usual.
     
  2. alantig

    alantig Zombie Four, DFZ

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    I'm no expert, but it sounds like a tube issue to me. Someone more tube-wise than me can tell you how to debug that.
     
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  3. elvis

    elvis Hamfisted String Banger

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    Atlantig is correct.

    With a tube amp, I strongly recommend having a full change of tubes in a drawer. Tubes are very similar to incandescent lightbulbs, and about as reliable. At the very least, have two new 12AX7 ready to swap in.

    How to diagnose:
    Presume that the power tubes are fine. For the moment. They usually go out with a bang.
    Is the sound affected on only one channel or both? Some preamp tubes are for only channel one or channel two, some are for both.
    Are you using the FX loop? There is usually a tube dedicated to that.
    Based on the answer to these, you can take a new 12AX7 and change out the likely problem tubes one at a time and see if you can find a bad one.
    OR, just do the shotgun approach and change every one one at a time.
    My experience is that the PI (phase inverter, the closest preamp tube to the power tubes) is most likely to go bad. So I always start there.
    Usually Ch1 and 2 both go through V1. V2 is generally channel 2 only. V3 and V4 can be an odd assortment of functions including FX loop (both channels).

    You COULD have 2 bad preamp tubes, but this is fairly unlikely.

    It COULD be the power tubes, but usually you would either get nothing at all or it would be kind of OK, as the push-pull circuit actually works with one bad tube a lot of the time, just not quite as loud and awesome sounding.

    Other causes: Bad solder joint on a plug. This can happen for a lot of reasons, the best one being from bumping or dropping the amp with a cable installed. Less likely with a new amp.
     
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  4. LSchefman

    LSchefman Hears Tones

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    Jess, most tube amp issues are just tube-related, and very easy to fix.

    There’s a lot of good tube issue info in most Mesa manuals, and you can download them.
     
  5. Logan21063

    Logan21063 New Member

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    Maybe get some help of a technician near by you. Sound like a tube issue
     
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  6. Jess O'Toole

    Jess O'Toole New Member

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    This is really excellent help, a huge thank you to all! I am a little torn between trying myself and taking it into a shop, with the likes of the 'Mesa' manuals do you think an amateur could fix this? Or worth spending a little more?
    I am reasonably competent with hardware and wiring in general, just a novice with all things tube/ valve.
     
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  7. Jess O'Toole

    Jess O'Toole New Member

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    Thanks I'll take a look at that now!
     
  8. Boogie

    Boogie Zombie Two, DFZ

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    Dive in! We all started where you are. You’ll be a pro in no time.
     
  9. elvis

    elvis Hamfisted String Banger

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    Changing preamp tubes is super easy, anyone can do it. Just be careful not to bend the pins. Of course we've all bent the pins...

    Changing power tubes means adjusting bias, so that is more involved.
     
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  10. Jess O'Toole

    Jess O'Toole New Member

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    Okay so it took me a while to get to it, but I opened up the amp and it looked alot like the power tubes were the issue, so I replaced them with new ones and checked for the bias with the handy ports on the back.

    All looked good, fired the amp up and let it warm up- I plugged in and started to play on clean, sounded good but already getting fuzzy and breaking up with louder playing. I switch to the lead channel and it sounds awesome... For about a minute then drops way down in volume again losing loads of gain, slight smell of burning almost.

    The preamp tubes all look fine and now I have no idea how to fix it :( can someone help? I only got 3 gigs out of it so far and I'm gutted.
     
  11. CandidPicker

    CandidPicker Open - Eared

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    Jess,

    The burning smell is not a good sign. At this point, I'd bring it to an amp tech who can diagnose and test the issue. Not saying you've done all you could yourself, but when a tube replacement/home bias fails, something else is happening...

    Tube replacement/bench test can run anywhere from $30 to $85 plus cost of tubes. Plan on bringing in a new set of good tubes just in case to curtail repair costs...
     
  12. DreamTheaterRules

    DreamTheaterRules Navin R. Johnson

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    Did you buy it new or used?
     
  13. shimmilou

    shimmilou Established in 1963

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    If one or both of your power tubes actually were bad, it is possible that a bad tube caused the loss of a screen grid resistor. At this point, having a look inside at the circuitry might be the best next step.
     
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  14. CandidPicker

    CandidPicker Open - Eared

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    THIS is why I love this forum. One can learn things you never knew before. (TBH, I've NO idea what a screen grid resistor is, but it sounds like a resistor attached to the tube socket point that helps prevent damaging voltages from doing additional damage if a tube goes bad. Plz correct me if I'm wrong; useful knowledge helps people do their job more effectively.)
     
  15. bluenova

    bluenova Electrified since '84

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    What made you think it was the power tubes? I’d try swapping out preamp tubes...
     
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  16. elvis

    elvis Hamfisted String Banger

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    Usually impossible to diagnose tubes by appearance. Red glow or not, they may be good or bad. If they get a crack and lose vacuum they will turn white. Otherwise, no way to know without a tester or swapping.
     
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  17. Boogie

    Boogie Zombie Two, DFZ

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    When in doubt, or out of time/patience, swap ‘em all. Once it’s working normally you can regression test (one at a time).

    To support Shimmi’s comment, take a peek at those resistors like the ones circled in this pic. If they look like Spock shot them with a phaser, you need to replace them before harmony returns to MT-15ville.

    [​IMG]
     
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  18. shimmilou

    shimmilou Established in 1963

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    You are very close in your understanding. There are 5 elements in a typical output tube (thus the name Pentode). 1) Plate, 2) Suppressor grid, 3) Screen grid, 4) Control grid, 5) Cathode. This drawing is a schematic representation of a Pentode 6L6 output tube. The high voltage power supplies are connected to the tubes, the highest voltage to the Plate, and a slightly lower voltage to the screen grid, the screen grid resistor (SGR) is used to lower the voltage to that grid. If the tube fails, it can sometimes take out the SGR, and if the SGR fails, the replacement tube will not function properly. Contrary to popular belief, the SGR is not designed to protect the amp in any way. Many modern amp designs have added HT fuses specifically for protection in case of an output tube failure.

    [​IMG]

    As pointed out, we are speculating based on what the OP posted. I would also like to know why the output tubes were suspect.
     
    #18 shimmilou, Dec 8, 2019
    Last edited: Dec 8, 2019
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  19. shimmilou

    shimmilou Established in 1963

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    Interesting. I notice that 2 (inner) of the 4 output tubes use a standard 470 ohm SGR, while the other 2 (outer) use 2.7K ohm. Am I seeing that correctly?
     
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  20. Boogie

    Boogie Zombie Two, DFZ

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    No, you see that correctly. The outer two are part of the Simul-Class power stage where the class A section resides. I’ve had EL34s in there for years but can take 6L6s without an issue or rebiasing.
     
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