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Need help with Kalamazoo Model 1

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  • #16
    This thing is turning into a real PITA!

    I isolated the input jack. No effect.
    I removed completely the Herzog line out. No effect.
    I tightened the contacts on the tube sockets. No effect.
    I tested the tubes again, this time in another amp. They work fine.
    I didn't have a spare rectifier tube, so I replaced it with a couple of 4007s. I didn't measure the voltages before doing this, but it seems to have bumped them up substantially.

    They read as follows:

    V1 - pin 6, 149 / pin 1, 154
    My plate resistors are 180K.

    V2 - pin 7, 372

    C4 - a, 372 / b, 372 / c, 255

    I changed R12 (between C4a and b) to a 3.3K. Odd that there was no voltage drop at all after the 3K3. Should that be a 1 watt resistor? All other resistors are stock values, and all read at or near their stated values.

    There was a change when I put in the solid state rectifier. Whereas before I could get a fairly normal clean sound at low volume if I played lightly, now I have to turn it way up to get any sound at all. And there's no clean sound at all. But the thing where the sound disappears when I hit it hard and then returns slowly continues.

    I really hate to change out those power sply. caps. It's gonna be a pain. But that's about the only thing left. Anymore ideas to save me from this chore?

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    • #17
      Well, I finally got around to switching out those power sply caps. It helped slightly, but didn't fix the problem.

      I've run out of things to try. My only thought is the OT. But I have no idea how to evaluate it. I know that OTs can still work when they've gone bad. I've never heard one sound quite like this though. But then I've only encountered one bad tranny ever.

      Short of replacing it, is there any way I can know if the OT is working as it should or is the source of the problem? The PT is in the clear, I assume - seems if it works at all it would be fine, right?

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      • #18
        Originally posted by Boy Howdy View Post
        This thing is turning into a real PITA!

        I isolated the input jack. No effect.
        I removed completely the Herzog line out. No effect.
        I tightened the contacts on the tube sockets. No effect.
        I tested the tubes again, this time in another amp. They work fine.
        I didn't have a spare rectifier tube, so I replaced it with a couple of 4007s. I didn't measure the voltages before doing this, but it seems to have bumped them up substantially.

        They read as follows:

        V1 - pin 6, 149 / pin 1, 154
        My plate resistors are 180K.

        V2 - pin 7, 372

        C4 - a, 372 / b, 372 / c, 255

        I changed R12 (between C4a and b) to a 3.3K. Odd that there was no voltage drop at all after the 3K3. Should that be a 1 watt resistor? All other resistors are stock values, and all read at or near their stated values.

        There was a change when I put in the solid state rectifier. Whereas before I could get a fairly normal clean sound at low volume if I played lightly, now I have to turn it way up to get any sound at all. And there's no clean sound at all. But the thing where the sound disappears when I hit it hard and then returns slowly continues.

        I really hate to change out those power sply. caps. It's gonna be a pain. But that's about the only thing left. Anymore ideas to save me from this chore?
        if there is no voltage difference on R12 something is wrong,i mean,its 2mA for the preamp tube plus something for the screen grid of the power tube,multiply that for the resistor value and you have at least some volts,not zero.

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        • #19
          Measure the cathode voltage of each tube & report back.

          Simply measuring the plate voltage is not enough info.

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          • #20
            I fixed it. Found the problem. Ain't it always just the dumbest thing?

            When I went to put in the Herzog line out I had to make room for it. So I ripped out a master volume and a preamp out jack that I had mounted on a piece of aluminum angle attached to one side of the chassis. In my haste, I guess I removed my stage two grid load resistor (R 9).

            I coulda sworn I went through the amp more than once checking it with the schematic. How could I have missed it? Especially since I had specific plans to modify the value of that 330K. I was never able to turn the amp up over halfway because it would just have way too much gain and would sound really muddy. (I tried a 220K and a 180K, ended up at 200K, FYI.)

            So anyway that fixed it. I've got a big mess to tidy up now, after tearing through the amp like Sherman through Georgia. But at least I'm out of the woods and can finally sleep nights.

            The voltages are looking normal now. I'm getting the voltage drop after R12 I wasn't getting before.

            V1 - pin 6, now 118 (was 149)
            V1 - pin 1, now 122 (was 154)
            My plate resistors are 180K.

            V2 - pin 7, now 293 (was 372)

            C4a, now 313 (was 372)
            C4b, now 294 (was 372)
            C4c, now 200 (was 255)

            One last thing: I did the tone control mod from that Kalamazoo Field Guide site many moons ago. At some point, for whatever reason, I disconnected the .0047 mF, leaving just the 500 pico in place. I find that sounds much better. Try it, model one owners.

            http://www.netads.com/~meo/Guitar/Am...mod-1-sch.jpeg

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