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Thread: KMD GV-100 distorted output

  1. #1
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    KMD GV-100 distorted output

    I think this is a GV 100s but there's nothing on it saying what it is. It has a pair of 6L6s that have 694vdc on the plates, 338v screens, 22v on grids and about 75vdc on the cathodes..
    Do these voltages look OK for this amp?
    I'm posting this faded KMD schematic along with the musicman 2165 because it's a very similar circuit.
    I believe the problem is in the output section. Feeding the pre amp to another amp does a strange thing. None of the controls work thru the other amp but the sound is full and clean. What's up with that? Plugging into the power amp in the sound is distorted (Im assuming I'm doing this right, but might not be,see pictures of effects return) Referring to my photo the right jack I'm thinking is the power amp in. When plugged there and switched to channel 1 the sound is distorted. When switched to ch 2 there's very very little sound in comparison.

    This amp was here a few months ago. It needed one of the TIP41s replaced T4 or T5. There were some ICs in the pre that were shorted. Left here sounding good. Now it's back with a distorted sounding output.

    Thanks
    pete
    Click image for larger version. 

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  2. #2
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    Please recheck whether bass and treble work when running the send to another amp. They are the only controls before the send jack.
    This is a more readable schematic:
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    They definitely are not doing anything.
    Thanks
    NIce easy to read schematic!

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    Oh boy,,,,, I was wrong about the tone controls. When in channel 1 the tone controls do work!!! When in ch 2 none of them do, which makes sense.
    Does this confirm that it is in the power amp that's the problem?

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    Here's some further info that might help. With a 1KHz sine wave in the input the output at full volume is not even measuring a full volt AC!
    Power amp not working?
    Also, the voltages on 6L6s are 695v on plates 334v on screens 22v on the grids and about 70v on the cathodes.
    Do these looks correct?

    I've already tried swapping in a different output transformer.

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    Also, Line out is working well, pre amp sounds good, all controls work.

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    I pulled the fly back diodes off of the plate of the 6L6s and the parallel cap, no change.
    Decided to change the TIP41 that I didn't change the last time it was here. Pulled the power amp board, re-soldered the diodes and caps touched up some solder joint around the tubes, installed the TIP41 and now it sounds much better. I'm getting 25vac across my 8ohm load for about 78watts. I dialed the pot until it sounded best and this what I have for voltages on the 6L6s:
    Plates 655vdc
    Screens 327vdc
    Grids 22.9vdc
    Cathode 55vdc one
    and 65vdc

    This seems odd to me that the cathodes are not equal. Any thoughts?

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    They don't balance out all that perfect. What are the idle DC voltages across those 3.9ohm resistors at the emitters of T3 and T4?
    In the music man, they are biased for around 25mV across each. (25mV min., 55mV max.). If they are both within that window, it is considered close enough.

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    The voltages are very different across the 3.9R resistors. One has 41mV and the other is 2.3mV. So I seems that only one tube is doing all the work.

    What else could be causing this?

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    Just change out the other TIP41, now T3 and T4 are new and my voltages are much closer, 38mV and 34mV.

    I wonder if this transistor went bad because it wasn't seated absolutely flush against the mica to the chassis. The leads are bent over and the metal body of the transistor fits up against the mica and the chassis. I remember it being difficult to position the body flat against chassis when I replaced it before.

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