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Thread: Playing quietly, a hum then no sound

  1. #1
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    Default Playing quietly, a hum then no sound

    I was playing quietly at bedroom level on my D60 when I heard a hum start. I stopped playing, the hum quieted slightly for a second then came back and the sound just dropped to nothing.

    I plugged my guitar in direct, no sound at all from the speakers at any volume level, not even a hiss or hum. Nothing. I checked the fuses and the DC Fuse is blown. Where do I start here?

    The valves are all quite young with less than 30 hours on them. This doesn't preclude valve failure of course, given the alternatives I kind of hope it is.

    From reading mesa forums, they suggest replacing the fuse, pulling all the tubes then powering on and checking the fuse. Then add valves one by one powering on each time, until you find the valve that causes the fuse to blow. Do I do this with the D60?

  2. #2
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    lol 293 views and not one response

    Well, I pulled the amp out of the head shell, and checked for anything obviously damaged. Everything looked and smelled fine. Doing some reading revealed that preamp tubes aren't going to blow the HT fuse, only a big current draw (ie power tube) will do that.

    Next step was replace the DC fuse and without the power tubes in measure the DC voltage at pin 5 - this measured at -48.3v on both sockets which I believe is fine.

    Inspecting both power tubes, they appeared pretty normal but the silver coating inside the top of one looked like it was receding up the walls. I put the original 2 power tubes in and fired it up, and it seemed ok just ticking over. I darkened the room and began to play, suddenly one tube lit up bright blue and the sound thinned right out. The problem tube was the one with the receding silver coating. I hit the standby switch and pulled the tubes. Replaced with a matched pair of electro-harmonix KT88's and I'm back in business. Just have to check the bias now.

  3. #3

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    Quote Originally Posted by phonix View Post
    lol 293 views and not one response

    Well, I pulled the amp out of the head shell, and checked for anything obviously damaged. Everything looked and smelled fine. Doing some reading revealed that preamp tubes aren't going to blow the HT fuse, only a big current draw (ie power tube) will do that.

    Next step was replace the DC fuse and without the power tubes in measure the DC voltage at pin 5 - this measured at -48.3v on both sockets which I believe is fine.

    Inspecting both power tubes, they appeared pretty normal but the silver coating inside the top of one looked like it was receding up the walls. I put the original 2 power tubes in and fired it up, and it seemed ok just ticking over. I darkened the room and began to play, suddenly one tube lit up bright blue and the sound thinned right out. The problem tube was the one with the receding silver coating. I hit the standby switch and pulled the tubes. Replaced with a matched pair of electro-harmonix KT88's and I'm back in business. Just have to check the bias now.
    Ha ! That sounds like the way to diagnose and repair an amp like us amateurs tend to do; I would have done exactly the same Good for you !

    Giga

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    Thanks, it was good to hear it again!
    Last edited by phonix; 01-04-2014 at 12:04 PM.

  5. #5
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    Thumbs up

    Sorry, I just saw this. My suggestion would've been power tubes based on the fuse blowing. But....you got it handled on your own!

    Ultralead
    D120H
    Sig X
    Fat Bottom 4x12
    D412
    Valvulator I

  6. #6
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    Glad to see you can hear it again. I spent several hours in the last few days playing my D120 - what a sound ...
    The fact that Stephen appears from nowhere, is sentenced to death in an emotional public trial without anyone interfering at all and is then altogether forgotten as if nothing happened, is already suspicious.

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