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

[Enzo]

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.

[olddawg]

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.

[ATTACH=CONFIG]46524[/ATTACH]

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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 it’s 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.

[The Jonald]

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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Guitar amps are not the polite table radios the tube specs are written for.

I love this.

[Enzo]

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.

[The Jonald]

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.