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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 07:12 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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    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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    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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    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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    I'm working on one of these right now, and have found the same as the OP: replacing parts (especially old Arrow Radio coupling caps that were causing red plating and awful oscillations) has improved the tone/volume while increasing the 60 cycle hum. I jumpered the AC mains inside the chassis and cut the AC wires running to the switch and pilot lamp in the control panel, and that virtually eliminated the 60 cycle hum.

    For a permanent solution, I'll either (a) install an AC switch on the main chassis and cut a hole in the back cover to access it plus run some low voltage DC to the panel for an LED pilot lamp, or (b) run low voltage DC to the existing panel switch to run an LED in the panel and wire it back to the main chassis to a relay for the AC mains.

    Replacing the shielded wire from chassis to panel and separating them from the AC lines might help, but I'd rather get the AC out of the control panel altogether.

    EDIT: given that the fuse is also on the control panel, I may instead run a separate mains cord from somewhere near the mains in the chassis, follow a path well away from the signal wires, enter the control panel near the fuse. Then omit the pot switch and replace the pilot lamp with a push-button, lighted switch. This may also involve putting a small sheet metal barrier around the mains section of the control panel. This seems no more (or less) work than other solutions and it keeps the amp relatively stock. The amp sounds pretty good.

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    Last edited by Iron Works; 11-19-2019 at 02:36 AM.

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    My Gad I hate that design... I don't know if you have the same layout as a Gibson Thor (I think it's just the 50W version of your 30), but... I had one of those a few years ago. Same problem - take care of the maintenance; all is great except for an unbearable hum. I got so frustrated I just gutted it & put a different preamp in it & got rid of the umbilical control panel. I followed EVERY good ground practice & hum practice I could find. Eventually as a last-ditch effort I was screwing around one night. I was moving physical ground points around with a jumper wire & one was flapping around in the breeze. As the wire moved closer & farther from the powet tranny it would come & go. So I figured out how to route that ground wire by the PT to buck the hum.

    I chalked it up to just a really noisy PT radiating noise. They do have the preamp tube right between the two trannies & all those controls flapping about? Eventually I was able to knock it down by 95% by running a single ground wire halfway around the inside of the chassis but still connecting the same physical points. Which goes against "good practice," but it worked. I hope you don't have that same issue, but don't rule it out as an option.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
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    That's interesting. I wonder if they are 2 approaches to resolve/reduce the same insane 60 Hz hum. My thought was that having the AC mains running alongside the (albeit shielded) signal wires to the control panel and then - worse yet - having the mains in the control panel running through the switch portion of the treble pot and to the pilot light was just inviting hum into the signal path. I did notice that the hum was strongly associated with the treble control pot.

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    Last edited by Iron Works; 11-19-2019 at 02:24 AM.

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    Depending on how the controls are wired, it wouldn't surprise me if they boosted certain frequencies a good bit. I'm not up on how they work enough to give any help there. But I do think having the mains go the way it does is kinda... Shielded or not. I don't trust shielding to do a jpb that is easily rectified by another means.

    But this amp was built by an old company in a time when manufacturers intentionally tried to be innovative & creative rather than copy the competition. Some things just turn out to work better than others I guess...

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
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    I just want to note here that Justin "discovered" a technique that was actually used by a major amp manufacturer. Mesa! Mesa had an amp model (I can't remember which though) that had an unterminated lead off the filament supply to one of the preamp tubes. Just a three to four inch lead with the end clipped flush and (I assume) moved around to "buck" hum (as Justin accurately put it). A brilliant idea IMHO . There are a S ton of aspects to a design and idealizing all of them is damned hard even for an experienced manufacturer like Mesa. Best practice SCHMEST practice. Justin used a solution that was employed by (perhaps) the greatest patent holder in amp manufacturing. That's not cheating, that's just clever.

    EDIT: In fact I tried to add to Justin's "reputation" points based on his post, but I was thwarted by a notification stating I had to spread them around more before giving them to him again. I guess I just want Justin to know this.

    Churck

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    Last edited by Chuck H; 11-19-2019 at 05:54 AM.
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    Lifetime Member Enzo's Avatar
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    A short piece of wire is called a gimmick capacitor.

    I remember a Mesa that had a short piece of bare wire soldered to a socket pin. I mean like one inch long. Just bare wire sticking straight up. And indeed, it was there for hum abatement. I can imagine a young tech, spots that and feels superior, and snips it off. Wondering why the guys made the amp sloppily. And then have no idea where the hum came from.

    Noise reduction by cancellation is certainly a valid solution to hum, and darn clever.

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    I've seen the skinny wire wrapped tightly around thhe signal wires in later SF Fenders before. Never seen a "gimmick cap" though. I ended up using about a foot of wire instead of the usual "shortest route possible." Had to reposition a ground on a Bassman 50 once & it killed a thuddy attack, which I guess was an oscillation

    And, thanks Churck, I got your message!

    There are a ton of tricks out there; a lot are listed in those Golden-Age tube books. All of which say keep your input tube as far as possible from trannies & don't run long signal leads next to power supplies... And a Gibson says D'oh!

    Justin

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    Some of those 'gimmick' caps are etched right into the circuit board. Usually a kind of squared spiral looking pad that goes nowhere.

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    Lifetime Member Enzo's Avatar
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    One of my favorites of those is the Peavey Classic 30. And you can see it on the board layout drawing. Over by V1, the input tube, you can see in the traces a bunch of parallel lines in the traces. Connects to pins 1 and 2. Definitely done on purpose, but not on the schematic.

    You also can see traces used as shields in some circuits. A circuit might result in two long traces carrying signal and running parallel along the board. There will be a third traces between them. COnnected to ground somewhere, that third trace really goes nowhere, but it acts as a shield between the two signal traces. I cannot recall a good example at the moment.

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