Weird Tone Problem

Discussion in 'Amp-a-ridifiers' started by RetiredNSquired, Dec 29, 2019.

  1. RetiredNSquired

    RetiredNSquired Squier-holic

    Age:
    64
    Jun 20, 2018
    Canton, Ga
    My MP15 has developed a tone issue, and not sure how to pursue a fix. Hoping some of the amp-savvy folks here can point me in the right direction.
    When I first power the amp up and play, everything sounds great. Nice chimey highs, tight bass response, just what I like.
    However, once it's warmed up for 15-20 minutes, the tone seems to go really bassy/boomy. To my ears at least, not so much that it loses top end, but the bass frequencies get overwhelming. Seems particularly noticeable when I'm playing my LP (ish) with humbuckers.
    My first thought is maybe theres a cap in the tone circuit thats failing as things get warmer, causing the passed frequencies to go wonky.
    I know I've seen a schematic for (I believe) the Laney version and thought I'd saved it, but can't find it now.
    Just looking for any ideas or advice from folks I trust! Thanks in advance for any input.
     
    Angry Possum likes this.
  2. Triple Jim

    Triple Jim Squier-holic

    PM me your email address and I'll send you a good .png of the schematic. The forum will cut down the resolution if I post it here.
     
    drewcp and RetiredNSquired like this.
  3. RetiredNSquired

    RetiredNSquired Squier-holic

    Age:
    64
    Jun 20, 2018
    Canton, Ga
    Done, and thanks!
     
    Triple Jim likes this.
  4. RetiredNSquired

    RetiredNSquired Squier-holic

    Age:
    64
    Jun 20, 2018
    Canton, Ga
    Well, I finally got around to checking into this, been in and out of town a LOT over the holidays (new grandbaby takes precedence over amp probs!) It has also developed a hiss/crackle while nothing was being played. Went through all the tubes, doing a "Tap Test" with it powered on, using a chopstick to tap each tube. The last 12AX7 on the right is HIGHLY microphonic, practically roaring thru the speaker when tapped. So I know that is A problem if not the only one, so I'll attack that first. Gonna order a new set of JJ 12AX7s, replace them as a set, and go from there.
     
    Angry Possum, Triple Jim and Mr Bones like this.
  5. Triple Jim

    Triple Jim Squier-holic

    It's probably better to replace them one at a time, starting with the known bad one. If you do all three and happen to get a bad JJ, you might be lead to believe there is another problem.
     
  6. TimTheViking

    TimTheViking Squier-holic

    Is it the last on the right from this orientation? If so, it's the first tube in the gain stage if I'm not mistaken. So any issues will be amplified through the next gain stage before hitting the inverter. What you can try until the new tubes arrive is to switch it with the second gain stage tube (middle one) or the inverter. That may mask the microphonic problem until you can replace the bad one. You can also watch the tubes as you power it on and look for flashes.
    BTW - If it's under warranty Monoprice will replace the entire amp. I had a microphonic tube but didn't really squeal until it was in the first position. It was originally in the inverter position. I put it in the first position before sending it back to make sure Monoprice could hear the problem. I'm not sure they even tested it as the new one was shipped out the day after mine arrived. So they either have a very efficient defect checking process (which I doubt) or they simply checked for physical damage before shipping a new on. No complaints with their customer service.

    [​IMG]
     
  7. Triple Jim

    Triple Jim Squier-holic

    At some point I expect Monoprice will exclude tubes from the warranty. I suspect other manufacturers already do this, but I don't know for sure. It would be expected for them to replace an amp that was shipped with a bad tube, but if it was working fine and then developed the problem, it seems a bit much to expect them to replace the whole amp.
     
    Angry Possum and RetiredNSquired like this.
  8. RetiredNSquired

    RetiredNSquired Squier-holic

    Age:
    64
    Jun 20, 2018
    Canton, Ga
    Had it for 1 1/2 yrs, so I doubt I can expect warranty. Definitely gonna try the swap around, see what results. Thanks!
     
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