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Kalamazoo Bass 30 differs from schematic (plus potential problems)

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  • Kalamazoo Bass 30 differs from schematic (plus potential problems)

    Hello all. I'm in the middle of repairing a Kalamazoo Bass 30, and there are a couple of things I found inside that I don't fully understand. Can someone more experienced provide illumination? I've uploaded a picture of the schematic and marked a number on it corresponding to each of the things I describe below.

    (1) On the schematic there is an RC filter coming off the grid of "unit 1" of V1. (The data sheet for the 6eu7 calls that section, the one that's to the right on the schematic, "unit 1", so I'm rolling with it.) There's a 0.0047uF cap, followed by a 22k resistor and a 500pF cap to ground. Inside the actual amp, though, the 22k resistor comes after the 500pF cap. Has someone changed this from a low-pass to a high-pass filter, or is the order irrelevant here?

    (2) This filter cap has been changed from a 20uF/25V cap to a 50uF/150V cap. How much does this difference matter? Should I take it as an indication that the stock value proved inadequate somehow, or is it likely just an over-spec replacement? I'm considering restoring it to the stock value, but I don't want to do that if there is an underlying problem that necessitated the change, especially if it remains unfixed.

    (3) Off the plate of "unit 2" of V1 (to the left on schematic), there is a 0.022uF coupling cap. It's a ceramic cap, and the top is clearly blackened. What could have caused this cap to experience excessive current and/or voltage? I haven't taken it out of circuit to test it yet. I plan to replace it with a beefier part, but again, I don't want to ignore potential problems with the amp.

    When I purchased the amp, it was working, although to my ears not sounding particularly healthy. Other than the 0.022uF cap mentioned above, the only part in the amp that I've found (so far) that looked dodgy was the first power dissipating / filtering resistor, marked 1k/1W on the schematic. It seemed to have some damage, and tested at 1.4k, so I replaced it. I also replaced the part marked "SA 1045", the dual diode rectifying the PT, as the leads on it seemed very corroded, but I haven't managed to coerce my cheap multimeter into reading a measurement from it. Replacement filter caps are on order; except for the one mentioned in (2) above, they all appear to be stock.

    Click image for larger version

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    Since the amp WAS working, I'm tempted to simply restore the filter cap in (2) to stock, replace the 0.022uF coupling cap in (3), and call it a day. But I'm making a learning experience of this repair, and I want to understand what's going on as well as I can. And I don't want to leave any potential problem without investigation.
    Last edited by The Jonald; 07-26-2017, 09:47 PM.

  • #2
    Invalid attachment.

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    • #3
      Originally posted by Dave H View Post
      Invalid attachment.
      It loads when I click on it. What did I do wrong, how do I make it load here?

      In the meantime here's an alternative link: https://drive.google.com/open?id=0Bx...TFqY05zRUxmUnM

      edit: I tried just adding the attachment again, I believe it should appear now. I don't know what happened.

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      • #4
        Here's another schematic.

        Click image for larger version

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        If it's the 22k and 500p below the Loudness control which are in reverse order then it doesn't matter as they are in series.

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        • #5
          Jonald, welcome to the place.

          As was earlier stated, the order of the series connected components should not make any difference in the performance of the circuit.

          The bias filter cap was probably replaced by someone that didn't have the correct value component, and used something on hand to fix the problem. Is the other filter still the original 20uF@25v value?

          As for the 0.022uF coupling cap, can you post a photo of the blackening? I don't think that I have ever seen a ceramic cap burn before.

          Comment


          • #6
            Originally posted by 52 Bill View Post
            Jonald, welcome to the place.

            As was earlier stated, the order of the series connected components should not make any difference in the performance of the circuit.

            The bias filter cap was probably replaced by someone that didn't have the correct value component, and used something on hand to fix the problem. Is the other filter still the original 20uF@25v value?

            As for the 0.022uF coupling cap, can you post a photo of the blackening? I don't think that I have ever seen a ceramic cap burn before.
            Thank you! Obviously I'm still filling in some gaps in basic knowledge. The other filter cap is indeed 20uF/25V. As far as I can tell, everything else in the amp is original and matches the schematic.

            Click image for larger version

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            Click image for larger version

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            edit: when I got the amp it had Sylvanias (likely original) in all positions except V2, which had some kind of British 6eu7. The lettering is almost gone but I think it's a Mullard. Is it possible that the original Sylvania in V1 blew at some point, burning the coupling cap, and someone swapped the other Sylvania from V2 to V1 and put a replacement tube in V2 because they liked the amp better with the original preamp tube?
            Last edited by The Jonald; 07-26-2017, 10:14 PM.

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            • #7
              The discoloration on that ceramic cap looks to me like like rosin or varnish, not carbon from heat.
              --
              I build and repair guitar amps
              http://amps.monkeymatic.com

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              • #8
                Originally posted by xtian View Post
                The discoloration on that ceramic cap looks to me like like rosin or varnish, not carbon from heat.
                I tried cleaning it with anhydrous isopropanol and not much came off. It measures infinite resistance and zero capacitance.

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                • #9
                  Looks like goop or flux to me too. The other ceramic to the right in the photo looks like it has something on it too.
                  Was it in circuit when you measured it? I would disconnect one leg to use your cap meter. And compare the meter reading to a known good cap of same value.
                  I don't think it has anything to do with the tube, maybe V2 was bad or noisy and got replaced.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


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                  • #10
                    It looks like dripped wax or varnish, etc. to me as well.

                    That doesn't mean that it isn't bad, but it doesn't look like it "burned".

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                    • #11
                      You guys were right about the cap. I couldn't get a capacitance reading (other than zero) on my meter, but I tried it on a different one and it tested okay. And with very aggressive cleaning, whatever substance was on it came off.

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                      • #12
                        I finished replacing the filter caps and checked the bias. Plate voltages were within spec and both output tubes were at about 60-70% plate dissipation. And it sounds good! It has a nice clean tone on guitar with plenty of brightness and punch and good touch sensitivity. The bass control is a bit weak, so I may replace the caps for it, but I'm not sure if that's necessary or would even make it more responsive anyway so I may not bother.

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