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

[Enzo]

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.

[g1]

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

[Justin Thomas]

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

[Enzo]

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.

[Chuck H]

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

[Justin Thomas]

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

[Iron Works]

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.

[Justin Thomas]

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

[Iron Works]

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.

[The Jonald]

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.

[olddawg]

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.

[Enzo]

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.

[The Jonald]

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.

[olddawg]

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.

[The Jonald]

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.

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.