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Thread: Repairs, hum, and the noise floor on a Kalamazoo Bass 30

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    Repairs, hum, and the noise floor on a Kalamazoo Bass 30

    Hello all. I've been repairing/restoring/rebuilding a Kalamazoo Bass 30. Schematics are attached. The following is a list of all the issues I noted in the amp and how they were fixed. After every fix, the power output and the tone of the amp improved dramatically, but so too has the output of 60-cycle hum increased. It's most noticeable with the amp volume and treble at or near maximum, and diminishes as either or both of those controls is adjusted down (if a guitar is plugged in, it's also responsive to the guitar's volume and tone controls, as of course it would be).

    1) The output was weak and anemic, possibly due to leaky caps in power supply / bias supply; they were all original, and the caps for the bias supply and the cathode of the preamp were the bumblebee type and had crumbled with age. The electrolytics were replaced; the two HV rectifier diodes (actually two in a three-lead package) were failing and were replaced; the one for the bias supply was a more modern part and was left undisturbed.

    2) Bias to the output tubes was low, running the tubes at near their max dissipation rating, which I suspect was according to design. All resistors in the power supply and power section were replaced, and a 50k trimpot was added (wired as a varistor) in series with the 47k resistor in the bias supply. That plus the bias balance pot allowed biasing of both output tubes to 70% max dissipation (at about -21 V which is correct according to the datasheet).

    4) With the rebuilt power supply and power section, plate voltages on the output tubes were over 450 and screen voltages over 440 (the maximum rating), so the 1k/1W resistor was changed to a 1.5k and that dropped plate voltages to about 450 and screen voltages to about 435.

    Plate voltages on the preamp are about 185, and those on the balanced driver for the output tubes (the second 6eu7) about 195. On the phase inverter (cathodyne, second part of first 6eu7) the plate voltage is about 260, cathode voltage about 60, and grid voltage about 5. This, I believe, means at idle it sits at the rated maximum of 55 volts negative-grid bias.

    I think maybe in the 60s Gibson weren't so worried about keeping operating conditions strictly within the maximum specified ratings for the tubes.

    3) There was hiss and crackle, so all plate resistors were replaced, as well as all resistors in the signal path, and all solder joints were reflowed. The grid-leak resistor on the preamp, marked 470k, was found to be a 47k, and replaced with a 390k, which was the closest value I had on hand.

    4) The bass control was wonky, with bass increasing from zero to about 50% on the control then decreasing. One of the resistors, I think the 220k from the wiper of the bass pot to the wiper of the treble pot, was an 800k instead of the value on the schematic. It was replaced with the proper value, and the control's function was restored.

    5) The volume, treble, and bass pots all had issues with smooth operation, particularly at the bottom end of the throw, and so they and the rotary power switch were replaced. The "death cap" and polarity switch were removed and a 3-prong cord grounded to the chassis installed.

    I've checked every side of every solder joint, every tube socket connection, every cable and wire, and every ground connection, and they all read 0.1 ohms or less. The few resistors I haven't replaced all read within spec. I've checked the capacitors for leakage and have found none I can measure. The power and output transformers aren't shorted and give reasonable values for resistance on each winding.

    That's what I've done, and what looks to me to be correct.

    However, as I said at the top, every change I've made, while it's improved the output, tone, and overall performance of the amp, has also increased the hum. It's predominantly 60 cycle. And it's responsive to volume and tone controls. I'm wondering if the hum is just a natural part of an amplifier of this design, and I didn't notice it so much before because the amp was limping along in such an anemic state, or if I have missed or caused a problem somewhere which, if fixed, would remove it. I don't want to get into a wild-goose-chase reworking the grounds, rerouting wires, or moving controls in an attempt to progressively reduce it.

    I will note that the center taps of the heaters and the HV windings are grounded to the chassis, at the same point and separate from anything else. I wonder if lifting the heaters from ground and installing a virtual center-tap, and/or moving the HV CT to share a ground with the first filter cap, would tame the hum.

    Does everything look correct to y'all? See anything I missed? In every respect but the hum, it's probably working better than it did from the factory. I'm trying to convince myself that it's okay to close up the chassis, which is very much not fun to work on, and move on to other projects. I thought I would ask for your thoughts before I do so.

    edit: I have a scope, and I just received new high-voltage probes for it, so I plan on poking around a bit with that to investigate, but I've relatively little practice at using a scope.
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    Last edited by The Jonald; 01-14-2018 at 06:12 AM.

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    Lifetime Member Enzo's Avatar
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    I think maybe in the 60s Gibson weren't so worried about keeping operating conditions strictly within the maximum specified ratings for the tubes.
    No one did, any more than race car drivers worry about the tire pressure specs and what not in the glove box manual. Guitar amps are not the polite table radios the tube specs are written for.

    As you do things to the amp that bring back its gain, then any noise it makes will be amplified more.

    You can easily have more than one source of hum going. Get the scope going and determine which is 120Hz and which is 60Hz.

    I see no reason to unwire the heater CT, then make up a virtual center tap to replace it. The whole idea of the two resistors was to make something when there was no center tap.

    Moving the HV CT to the filter cap can't hurt. But your filter caps don't appear to ground at the same place as it is now.

    Your drawing shows input jacks, but I see a AMP connector which looks to take the input jacks elsewhere.

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    Quote Originally Posted by Enzo View Post
    Your drawing shows input jacks, but I see a AMP connector which looks to take the input jacks elsewhere.
    The cabinet design is unusual. The rotary power switch, fuse, neon lamp (120 volt, unfortunately), and tone and volume controls are on the front panel. Everything else is in the chassis, which has a 6-pin molex connector. One connection runs from the input to the preamp, one into and one out of the tone stack, and the other three are shields/grounds. So there are wires with mains power running past the front panel, and though there is shielded wire on the panel and running to and from the molex connector, there are ground loops.

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    edit:

    Quote Originally Posted by Enzo View Post
    Guitar amps are not the polite table radios the tube specs are written for.
    I love this.

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    Quote Originally Posted by The Jonald View Post
    The chassis design is unusual. The rotary power switch, fuse, neon lamp (120 volt, unfortunately), and tone and volume controls are on the front panel. Everything else is in the chassis, which has a 6-pin molex connector. One connection runs from the input to the preamp, one into and one out of the tone stack, and the other three are shields/grounds. So there are wires with mains power running past the front panel, and though there is shielded wire on the panel and running to and from the molex connector, there are ground loops.

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    edit:



    I love this.
    It does apear that a lot of components are just hanging out into space on that control board. I would temporarily disconnect the power lamp, and jump the power switch at the main board. Then hardwired a known good guitar cord straight into the main board and hardwired the volume and tone controls to max. Plug a guitar in with that front panel mess completely removed. Still have hum? Trouble shoot it. When itís clean hook up the input jack. No noise? Hook up the other panel stuff. Consider mounting the components on the main board and connect the controls by shielded 3 conductor cables. Concider replacing the lamp with an LED and run DC to it.

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    Lifetime Member Enzo's Avatar
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    olddawg gets it. I am actually familiar with the amp, I brought it up to suggest that that remote preamp/jack board may be part of the problem. Hum doesn't like a lot of wires.

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    Quote Originally Posted by olddawg View Post
    It does apear that a lot of components are just hanging out into space on that control board.
    It's not ideal. The old pots had terminal strips soldered to the back of them, but I don't have an iron that will remove them, or attach them to the new pots or to the panel. The connections are good, though, wrapped and soldered solidly.

    Quote Originally Posted by Enzo
    I am actually familiar with the amp, I brought it up to suggest that that remote preamp/jack board may be part of the problem.
    It's definitely part of the problem. I'm not sure there's much I can do about it, without completely redoing the control board, and/or moving things to the chassis. Enzo, do you own one of these amps or have you heard one? Did it hum, and if not, was it modified from the stock layout? Mine is restored to pretty much factory spec except for the addition of a bias trimpot. It's a neat amp but I don't really want to start major surgery on it. If the hum is due to the design and layout, which I suspect it is, then I'm willing to live with it. I just wanted to make sure I didn't do anything wrong or miss anything easy.

    I'll put it on a scope soon, tonight if I can, and report back if I find anything interesting.

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    Old Timer olddawg's Avatar
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    What will you be using the amp for? Generally hum is exaggerated by gain. It may be unusable with a boost pedal. You could also try building a shield around the controls, but it might cause a ground loop. The best way to figure it out imho is still to spend 10 minutes and take the front panel completely out of circuit. Once the main board is clean then you can reconnect the other stuff one at a time starting with the input jack. Things to consider:

    Keep the lead dress as short as possible.
    Keep anything that has line AC well away from your signal path.
    Use shielded wire going to the pots (The shield may only need to be grounded on one end to avoid ground loops).
    Consider mounting the control components on some perf board or something on stand-offs on the main board.
    Twist your signal wire dress
    Use wire ties to position your lead dress in the optimal location to reduce hum.

    None of this is particularly time consuming or expensive, maybe just a little tedious.

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    Quote Originally Posted by olddawg View Post
    What will you be using the amp for?
    I've been using it to learn and practice working on tube amps (I thought it was a neat amp, it was cheap, and the tubes that were in it are worth a good chunk of what I paid for it). I understand the approach you suggest, to disconnect the front panel, troubleshoot, then reintroduce items on it. That would of course be the best thing to do, but it is as you say tedious, and I am concerned that I'm fighting what is essentially a bad design on the hum front, and that a lot of work would be required to combat it: more shielding, more shielded wiring, moving wiring, moving components to a perfboard inside the chassis (and making sure they don't get hum from somewhere in there, it's a bit cramped), moving controls... This may be a good time to learn how to use my scope and see exactly what's going on.

    By the way, thanks to you and Enzo for your replies. I've learned a lot from this community and I appreciate all the people who take time to help me and others learn.

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    Lifetime Member Enzo's Avatar
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    I would be trying to isolate the source of the hum, and removing that preamp panel is a place to start. Jump the power switch wires and unplug the preamp, what does that do to the hum.

    A lot of forensic work is tedious. Imagine what it would be like to spend all day every day running DNA tests? Or closer to home, all day every day plugging 12AX7s into a tube tester.


    I might make up a metal shield for the rest of that preamp panel.

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    Old Timer olddawg's Avatar
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    Probably the easiest way to shield the controls on the control board would be to get a couple of small, inexpensive metal project boxes (or one larger one if the cabinet architecture will allow it) and mount them with a hole drilled for the control pot shafts.

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    I shortened the leads for all the components on the front panel as much as possible and grounded the sheath for the piece of coax running in from the input jacks (visible in the picture of the panel) on one of the input jacks instead of the back of the volume pot and the hum is much better. It's not too bad even with everything cranked. Amp's working quite well now.

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