Thought I fried my cab, but now I think it's the compressor/preamp tube. Help? MarkBass TA501

Discussion in 'Amps and Cabs [BG]' started by MadModBassist, Oct 15, 2016.


  1. MadModBassist

    MadModBassist

    Jul 16, 2009
    Ohio
    tl;dr How do you replace the compressor tube in a MarkBass TA501? Trying to wiggle it out but it's not budging.

    Entire backstory:
    I've been pushing my Baer ML112 (400W, 8 ohms) a little more than I felt comfortable doing with my MarkBass TA501 (300W, 8 ohms), gain around 2 and the master around noon-ish. The band I've been playing with recently has just been way too loud for its own good and we're in the midst of discussing that currently.

    I went to play with a different group though last night and upon setting up and starting to play, all I was getting was farty-sounding notes. Oh no. Everything was distorting really easily at any volume, so I immediately feared the worst. Figured I really had pushed it too hard.

    However, I knew to listen for warning signs before an actual blowout (audible distortion and other sour noises) and never heard any of that while pushing it. In fact, it sounded really good as I pushed it further. Plus, I should have 100W of safety net, right? I figured as much anyway.

    Meanwhile, I've heard that preamp tubes, though rarely needing replacement, will often have these as warning signs for failure 1. Farty-sounding notes and 2. Loud electronic buzzing/humming/other unpleasant ambient noises, both of which I observed last night. The second one has actually been happening only recently, too.

    Poking around some more, I realized that my amp could very well have its original or close to it preamp tube. It was made in 2007 but I acquired it used in 2014. It was missing a screw on the top when I purchased it so I figured upon opening it now that someone might've monkeyed around in there, but it still has what might be the stock JJ ECC81 compressor tube in it still. I have a replacement one lying around, so I figured I'd give it a shot.

    However, I can't for the life of me yank it out of there. Feels like I'm applying sufficient pressure and I don't want to apply too much for fear of breaking something on the board. I've attached a few pictures so you can see. Hope someone can help out!
    IMG_20161015_234651.jpg IMG_20161015_234604.jpg IMG_20161015_234616.jpg
     
  2. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    Typical cab can take about 1/2 its rated power safely long-term, especially if you use any OD/grind. And yah, there are folks here always saying you'll hear signs of distress well before blowing your cab - good luck with that. A single 12 just isn't going to do "loud" for long :( .

    Them tubes you have to "wiggle" out, pulling straight out on it won't get you anywhere. But I'd try another head or cab first...
     
    SirMjac28 and agedhorse like this.
  3. MadModBassist

    MadModBassist

    Jul 16, 2009
    Ohio
    I was wiggling in my attempts (the tube, not myself!), but to no avail. Maybe I'm not trying hard enough, but it feels like if I try harder I might break something, knowing my luck.

    I'll have to find another head to test the cab with, though it really doesn't seem to be the cab at this point. I powered everything back on tonight and it was fine for about two minutes before the distorting started happening again at any volume.
     
  4. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    Yah, it can take a scary amount of force to get it to move. And still could be a voice coil that rubs when warmed up and/or a cooked midrange. Even if you can borrow a guitar head to try with the cab it might tell you something - just be aware that flat on most guitar heads is bass off, midrange dimed, and treble off.
     
  5. murphy

    murphy Supporting Member

    May 5, 2004
    Canada
    I sometimes have used a wooden chopstick to leverage a tube up
     
  6. R Baer

    R Baer Commercial User

    Jun 5, 2008
    President, Baer Amplification
    Usually a tube can be removed by just wiggling it in a slight circular motion while pulling upwards on it. Sometimes it can take a bit of force, but you shouldn't have to go nuts with it. Always be sure to clean any fingers oils off the tube after handling them. I use those little alcohol wipes from the drug store.

    Best thing would be to swap heads to see if the distortion goes away. If you find it's the cab, it's most likely in the woofer. Those mid drivers are rated for 120w and we've never had one blow yet. The woofers on the other hand get the full brunt of your volume knob. First step would be to take the grill off and make sure all the screws are tight. The hold down screws can loosen up over time and if you get a few loose screws it can cause an air leak and create a problem If you need help on how to do that, I can send you instructions. If you do determine it's a driver, get in touch with us and we can talk about getting it taken care of.
     
    Last edited: Oct 16, 2016
  7. MadModBassist

    MadModBassist

    Jul 16, 2009
    Ohio
    Mr. Baer himself!

    I'm happy to report that it's NOT the cab, but rather, my head. Tested the cab with a friend's MarkBass head and had no problems whatsoever.

    This was after changing the tube in my head, too, so it's not the tube itself.

    Any ideas what it could be instead? A friend of mine mentioned that a good cleaning might be what it needs, but I don't see how dirt could cause audio distortion. It only happens once the amp is on for a few minutes... maybe a cold solder joint?
     
  8. R Baer

    R Baer Commercial User

    Jun 5, 2008
    President, Baer Amplification
    I'm not familiar with that amp at all, but there's probably a number of things that could be going on once the unit heats up. Sounds like it's probably going to need a trip to a qualified service tech to get a once over.
     
    ExaltBass likes this.
  9. This.
     
  10. MadModBassist

    MadModBassist

    Jul 16, 2009
    Ohio
    Yep, that's what it's looking like. Thanks for all the help so far, guys! Very much appreciated!
     
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    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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