Amp randomly fizzling out... Brand new tubes!!

Discussion in 'Amps and Cabs' started by rootbeersoup, Feb 23, 2012.

  1. rootbeersoup

    rootbeersoup Member

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    So I've owned this Marshall Haze 40 for the past few years (Disclaimer: I bought it used originally). I recently had brand new tubes put in and biased by a tech (changed the EL34s to KT77s if that helps)

    Now all of the sudden the amp will fizzle out after a bit of playing 9sometimes 10 minutes, sometimes an hour). Everything will be fine and then it just fizzles out until there is nothing but loud HISSSSS. Earlier today, in fact, when I turned it on and took it off standby for 30 seconds, there was no sound whatsoever! Usually I can just turn it off and turn it back on a minute later and it'll be fine, but I know that's not exactly the best way to solve the problem.

    Is it possible the tech messed something up? The amp was fully functional even with completely shot tubes and never gave me this problem before. I'm hoping this is a speaker problem since I have a new one coming in anyway. Wish I could get audio/video of it but it's too unpredictable to catch.
     
  2. dorianadams

    dorianadams Member

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    sometimes new tubes can be DOA.cold solder joint on tube socket?Bias resistor? Take it back to the tech.
     
  3. Blue Strat

    Blue Strat Member

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    +1. Try popping the original tubes back in and see what happens.

    Even before that, remove and reinsert each tube one by one.
     
  4. Structo

    Structo Member

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    Intermittent problems can be hard to chase down but if you think about it step by step sometimes you can discover the culprit.

    I think this is a connection problem, as the amp heats up a solder joint or tube socket is losing connection.
    I don't know anything about the Haze amps but I assume they are PCB?
    There have been cases and a well known one on a newer production Fender amp that had a power resistor that got so hot it would melt the solder every time the amp got to a certain temperature.

    Not saying your amp is having the same problem.

    Usually on a tube amp the first thing to do is to substitute each tube, one at a time with a known good tube.

    So if the amp has several 12ax7's, sub a good one in each socket one at a time and play the amp.

    When you get to the power tubes replace them both.

    In doing a brief Google about this amp people have problems with:

    Effects loop. (sometimes the switching jacks get dirty or corroded, take a plug and plug it in and out several times in each jack.

    Dirty/ corroded tube sockets. (this can be dangerous if you are not sure how to drain the caps. If you use a tiny screw driver to re-tension the socket pins make sure the handle is insulated to protect you from a shock.
    Depending on what type sockets they are, make the bits that grab the tube pins a bit smaller to make better contact.
    Some power tubes have larger or smaller pins, this can cause the sockets to get stretched out making bad contact.

    It could be a problem with the bias circuit as well.
    If the voltage is not steady then the gain of the tube can jump around.

    You would need to open the amp and measure high voltages to check this.

    Try the suggestions above and if you can't get it going take it to a qualified tech.
     
  5. TDJMB

    TDJMB Gold Supporting Member

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    Years ago, I put new tubes from MB in a Mesa Boogie. The amp would occasionally make a farting, blaaatt sound. It took a year, embarassing moments at gigs, and multiple trips to an amp tech to figure out it was the new tubes. So I'd start with the last change that was made - which, in this case, is the tubes.
     
  6. rootbeersoup

    rootbeersoup Member

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    So it just happened again... This time it was ~30 of playing time all fine and functional, and then it starting fading out. Volume dropped and the sound was more distorted but it took about 20 second for it to completely fade out into the loud hum/crackling. I turned it off or 30 seconds, turned it on in standby mode for another 30 seconds and now everything's fine again

    The fact that it took so long (20 seconds or so) to fade out after 30 minutes of playing makes me think this is a bad preamp tube. Should I just order one and replace it myself? I don't think those need biasing but I'm not sure
     
  7. Blue Strat

    Blue Strat Member

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    Tubes are virtually never intermittent. CONNECTIONS are often intermittent.

    Remove and reinstall all the preamp tubes, one by one, and see if that helps.
     
  8. rootbeersoup

    rootbeersoup Member

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    Ok before I go about doing this (I'm a complete amp novice, don't know much about them) are there any precautions I need to take? I know the procedure for taking out/installing tubes but is this something that is dangerous?
     
  9. Blue Strat

    Blue Strat Member

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    No. Just be gentle. You don't want to break the tubes.

    Pull out one preamp tube, then put it back in before you try any others. Note the pin orientation when you take it out because there's only one way you can put it back in. Careful not to bend the pins.
     
  10. windwalke9649

    windwalke9649 Member

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    It sounds like a component has a bad solder connection. It's happening after 9-10 minutes because thats how long its taking the amp to get to a hot enough for whatever is happening to happen. If you need to ask what precautions to take; you should bring it a tech.

    I dont think its the tubes, like the other poster said; tubes dont work intermittantly, they work, or dont. But components like a resistor or cap thats leaking voltage does.
     
  11. Davidg1974

    Davidg1974 Member

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    If it didnt do this before u changed the tubes I can almost gurantee it is the new tubes. What brand are they and where did u get them? I had almost the exact same problem with my amp and I also thought it must be something heating up but I had a tube going into runaway that I finally saw. next time it does it look at your power tubes and see if any are getting hotter-redder than the rest . I got some new JJs from eurotubes and my amp sounds 10 million times better and no more volume loss or cutting out. Change the tubes-try your old ones first before buying any and change them all then one at a time to find the culprit.Good Luck cause I know this incredibly frustrating.
     
  12. rootbeersoup

    rootbeersoup Member

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    The tubes are three EHX 12AX7s in the preamp and two JJ KT77s in the power section. Ordered them from Amazon, probably through another seller

    I did the uninstall/reinstall with the preamp tubes last night but I haven't found the time to properly test it out yet. Played it for 10 minutes with no problem but it usually happens after 30 or so.

    The one thing that sucks even more is you pretty much have to dismantle the entire amp in order to get to the tubes
     
  13. rootbeersoup

    rootbeersoup Member

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    Update: The amp now produces no sound whatsoever. The tubes warm up just fine, you can feel that subtle vibration they make, but there is no sound at all.

    Time to call the tech... God I wish I never bought this amp
     
  14. rootbeersoup

    rootbeersoup Member

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    Just an update for anyone curious, I brought it back to the tech and he said it was one of the power tubes running away. He also replaced the fuse with a higher value because this particular amp doesn't send voltage to the tubes until you take it off standby mode, or something like that lol.
     

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