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Help diagnose Ashdown CTM-15 issue

Discussion in 'Amps and Cabs [BG]' started by overbybr, Jan 23, 2017.

  1. overbybr

    overbybr Supporting Member Supporting Member

    I have an ongoing issue with my CTM-15 where the amp is producing beautiful sound and then it just cuts out. Not completely, it's still producing sound, but at a significantly reduced volume. It seems to take an hour of playing before it starts being a problem.

    Here's what I've tried so far before coming to the conclusion that it's in the amp.

    Cables - tried 4 different instrument cables (bass straight to amp), and I was able to replicate the problem each time. I checked the cables with a multimeter too, all OK.

    Bass - I tried 3 different basses (active/passive), and was able to replicate the issue with each instrument.

    Amp - I changed out the preamp tube over Christmas, and when I have practice on Tuesday, I'll swap out the tube to see if that's the issue. These amps have an analog input meter on the face, and when the volume drops the signal meter also goes to hovering around -20dB.

    I'll welcome other thoughts while I wait to change out the preamp tube.
  2. overbybr

    overbybr Supporting Member Supporting Member

    I changed out the preamp tube to one I know is good, and the same scenario occurred. I then went on to validate power and speaker cables by switching to a different tube amp, zero sound issues. Any thoughts from the community?
  3. overbybr

    overbybr Supporting Member Supporting Member

  4. Does the amp have an effects loop or pre out/power in sockets. If so try linking these with a signal cable and see if that helps.
  5. overbybr

    overbybr Supporting Member Supporting Member

    Excellent tip. However, no pre/power sockets or fx loop sockets. Just instrument in and 4/8 Ohm speaker out.
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I would contact Ashdown customer support and ask them about it. It might be a known issue that they can deal with.

    A couple of comments about your amp. This amp has a solid state preamp and a tube based power amp. What you are calling the preamp tube is actually a phase inverter, part of the power amp. The meter shows the power output, not the input like on a lot of amps. This allows you to see how hard you are pushing the amp.

    Something is overheating and becoming intermittent. There are a number of possibilities. If you have a spare set of power tubes you could try swapping them in. The only way to properly diagnose it is to hook up some test equipment and measure what is going on when the problem occurs.

    Here is a gut shot. You can see the preamp in the lower right, there are two transistors and two operational amplifier chips; the lower left side is the power amp.

    overbybr and AstroSonic like this.
  7. overbybr

    overbybr Supporting Member Supporting Member

    Thanks! I'll reach out to them. I still have the original power tubes, I'll try those tomorrow night and see what happens.

    beans-on-toast likes this.
  8. overbybr

    overbybr Supporting Member Supporting Member

    Update. I switched out the new Genelax Gold Lions for the old Chinese EL84 power amps and no cutouts after a 3hr practice. While that's awesome, I really liked the tone those power tubes gave me. Any ideas as to why those new tubes would fart out like that?
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    More than likely, one of the tubes has a problem. When it gets hot, something internal to the tube is malfunctioning. This is rare but it happens. You may be able to return them under warranty. It is important to order a matched set. If you can return them, provide an explanation that details how the tube becomes intemittent after an hour or so. Unless they test the tubes under those conditions, the problem will not be seen.

    There is another possibility. Compare the pins on the old and new tubes. The diameter of the pins can vary from one make to another. If the new tubes has smaller sized pins, the contacts with the tube socket terminals may not be as tight as they should be. If the new tubes slide in and pull out of the socket a little too easily in comparison to the old ones, that is an indication that the contacts are not tight enough. The solution is to re-tension the tube sockets. This involves using a small tool (needle or jewelers screwdriver) to carefully bend the socket terminals closed. Google re-tensioning a tube socket for how-to's.
  10. overbybr

    overbybr Supporting Member Supporting Member

    Thanks. I did order a matched set, and hopefully they'll take them back, but who knows. I'll check pin diameter this weekend and see if there is a significant difference, but they both installed/removed with about the same force.

    beans-on-toast likes this.

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