Kalamazoo Bass 30 60hzHMMMMMMMMMM

[Rafael]

Working on a '65 Kalamazoo Bass 30 that has the funky flip out control panel. Love the amp, hate the hum: This thing sounds like a bee hive when you turn the volume above 12 oclock. Nice tone, but ugh!

What I'm wondering is if anyone has experience with this amp and reducing the hum and if so, what areas to concentrate: I don't know if it's the fact that the power switch and all the volume/tone controls go in and out of the same multipin connector, or if it's just a bad design, and I need to do something drastci like raise the potential of the heaters, or should they be quiet and I should be looking for a defective component?

Thanks in advance!

[Enzo]

Bad design? Bad design comes to your mind before the fact the amp is 40 years old? Are your filters still the originals? Is the hum 60Hz or 120Hz? They come from different sources. If the volume control affects the volume of the hum, then the source of that hum is before that control. Have we tried different tubes in it? Have we loosened and tightened the input jacks so they get a better ground contact to the chassis? Are the nuts mounting the pots tight and snug? are there intact ground connections between the tilt out assembly chassis and the rest of the system?

[bbartilson]

should they be quiet and I should be looking for a defective component?

I have one of those amps. Most assuredly there's something wrong with yours. The one I have had old dual element paper filter caps, and one of them had failed dead short. If your amp has those original caps, just replace them all.

Then, go about fixing the problem if it still exists. Mine is VERY quiet with the proper filtration.

-Bill

[Rafael]

Bad design? Bad design comes to your mind before the fact the amp is 40 years old? Are your filters still the originals? Is the hum 60Hz or 120Hz? They come from different sources. If the volume control affects the volume of the hum, then the source of that hum is before that control. Have we tried different tubes in it? Have we loosened and tightened the input jacks so they get a better ground contact to the chassis? Are the nuts mounting the pots tight and snug? are there intact ground connections between the tilt out assembly chassis and the rest of the system?

Well, I _did_ sort of look at it to make sure there was nothing obvious, and the fact that it uses a funky multi pin connector to run several signal lines back and forth from the amp to the flip out panel had me suspecting that was the source of the hum, therefor my 'bad design' conclusion.

I've attached a pic of the inside of the amp chassis, and it looks to be recently rehabbed. As far as I can tell the solder joints look good, the tubes swapped for known working, everything seems snug. As you mention, though, I do suspect something prior to the output section as the noise is volume control dependent, so I'm looking earlier on in the circuit: either there isn't enough gain in the design to overcome the hum or the spaghetti from the flip out to the chassis is picking up something.

And regarding bbartilson's post: notwithstanding your experience, I've seen a website dedicated to these amps

http://www.netads.com/~meo/Guitar/Amps/Kalamazoo/

and they have all kinds of stuff like DC bias on the tube filaments, etc. so I think I'm not alone in trying to tame the hum on this bad-boy. BTW it sounds GREAT! other than the noise.

More examination goes on tonite, and maybe I'll see something I missed so far. Thanks for all input!

[bbartilson]

And regarding bbartilson's post: notwithstanding your experience, I've seen a website dedicated to these amps

http://www.netads.com/~meo/Guitar/Amps/Kalamazoo/

and they have all kinds of stuff like DC bias on the tube filaments, etc. so I think I'm not alone in trying to tame the hum on this bad-boy.

It's excellent that you searched for and found an additional web resource.

Your initial post suggested that you thought the design was bad. You gave no info regarding any mods or repairs the amp had seen. Regardless of any of that, troubleshooting is troubleshooting. The power supply filters are effective or not. The grounds to and from the front panel (as Enzo suggested) are good or they aren't. Neither post is meant to insult your experience or skill - they are simply the shortest route to a fix for your amp. Had you provided this information (like the picture you posted) you may have gotten a very different response. Keep in mind that we don't know each other. At this point I'm still trying to help you.

Your second post implies that I don't know what I'm talking about. Since any child who can reach the keyboard can post on the internet, I tend not to make assumptions, positive or negative, about the folks I find here. Which means that my comments are made without any assumption regarding the amp or your abilities. As far as I know, you're 13 and you've never held a soldering iron, and may not know what one is. That won't stop me from helping you.

Posting about some website as though it's authoritative data and impuning my experience as a result is a different matter.

Now, if you had actually read and retained the information in the link you posted, you would find this info on the impressions and experiences page:

"Replacing and upgrading the power supply caps will help drop the hum levels a great deal."

Which is exactly as you were directed.

As for the 'All kinds of stuff', they suggest:

* DC bias on heater wires
* Moving the on/off switch
* Adding cathode resistor bypass caps
* Lead dress (where the wires go)
* Ground loops (more wiring)

These are acceptable if somewhat overkill for an amp of this sort. I certainly would not approach them from top to bottom as they are presented. Regardless, I'm not sure that five mods (three of which involve basic wiring practice) constitutes 'all kinds of stuff'. AC filaments, DC filaments, and AC filaments with DC offset have all been tried, with varying levels of success in varying circuits. Unless the hum you complain of is VERY small, you need to fix the big hum before these will yield a noticeable gain.

All of these mods are suggested only AFTER you have verified that the power supply works correctly, and the hum is still a problem beyond that point.

Now knowing that the P/S supply caps have been replaced, and we have a picture of the unit in question:

Let's have DC voltages, and AC ripple readings for each of the P/S sections, unless I am to assume that you know what level of ripple is okay and what isn't.

The fact that I have a bone stock unit without any of these mods was meant to illustrate that they (the mods) may not be necessary. I was simply trying to save you work. Remember that your initial post questioned *the design* of the amplifier. There are certainly better textbook ways to go about it, but mine, for one, is not problematic in the least, without any of these mods. Both Enzo and I were hoping you would repair the obvious before resorting to modification that may or may not help the complaint. In short, let our experience save you time and effort. Or not. Still your choice.

Suggestions from the picture:

1. Verify that the grounds all measure very low from one to another. The brown wires from the front panel end being the more or less 'central' ground, but the caps that carry the B+ are on the other side of the chassis. So from the terminal strip where the light green wire is connected to the two terminals where all the brown wires are connected is the biggest concern.

2. Your output jack is creating a ground loop if it is not insulated from the chassis. It is not the most likely source of trouble, but it is a notable difference from my unit. Insulate the jack from chassis, or remove it temporarily to insure it's not part of the problem.

3. The fact that you can turn it down helps a great deal - it's before the volume control, or associated with the front panel. Double check the panel to chassis grounds individually. Each is important.

4. The 4.7k resistor from C to D is wrapped around other wires and soldered poorly. Not terribly suspect, but worth correcting.

5. OH. And disconnect that stupid bias balance pot. That alone could be most of your problem. Replace it with a couple of 50 (or 47) ohm sandblocks. 2 watts oughta be more than fine.

Beyond that, let's see what you have to report regarding supply voltages and ripple.

Best,
Bill

[Rafael]

Your second post implies that I don't know what I'm talking about.

Oh no! What I was saying was that even though you're amp works great, they all don't. If I didn't think I had a problem, I wouldn't be asking for help. The only thing I was trying to indicate by the 'authority' of the link I posted was that, like all things electronic, some work right and some don't. If you took that as impugning your experience, that was not my intent.

Now, if you had actually read and retained the information in the link you posted

Now THAT'S a dig... ;-)

Let's have DC voltages, and AC ripple readings for each of the P/S sections, unless I am to assume that you know what level of ripple is okay and what isn't.

I can't give you ripple as I my main test equipment is whether a battery stings my tongue but I will check DC voltages and report back...

The fact that I have a bone stock unit without any of these mods was meant to illustrate that they (the mods) may not be necessary. I was simply trying to save you work.

I understand, and like you said, if I'd have posted the picture with my first post, I could have done the same for you and Enzo...

Suggestions from the picture:

A picture really IS worth a thousand words...I'll look into all your suggestions:

1] Easy enough, I do have a DVM
2] The hum was the same before I hacked on that funky output jack so I don't think that's it, but just in case I'll disconnect until everything else is disqualified as a cause.
3] see 1] above
4] I'll take a look at that: there does seem to be a lot of rosin on those joints....
5] got it.

AC filaments, DC filaments, and AC filaments with DC offset have all been tried, with varying levels of success in varying circuits. Unless the hum you complain of is VERY small, you need to fix the big hum before these will yield a noticeable gain.

Agreed...that's why I jumped to the conclusion that it was something regarding the signal path too and from the front panel:BTW you can see the shielded cable I'm assuming the last person to work on this amp used [a bit of it runs from the multi-pin connector] is this the original cable?

So, sorry for any misunderstanding: I'm taking everyone's suggestions seriously and appreciate all of the remote diagnosis. Every exchange increases my knowledge of both how these things work and the processes used to track down problems. Far from being a 13 year old without a soldering iron, I am still a newbie to this particular aspect of electronics. After over four decades I'm rekindling one of my first hobbies: A/C theory.

May none of your solder joints be cold.

[bbartilson]

So I think we're on the same page now. That's good.

Measuring ripple requires nothing more than the DVM you already own. Put simply:

Measure the DC supply. Then, measure the same supply with your meter on AC volts. That is the ripple on the supply.

300V of B+ with 3V of AC component is 1% ripple. Simple as that. So we need a DC reading and AC reading for each supply. Include the bias supply.

The wiring on your multi-pin is identical to mine - including the cheesy grey shielded wire.
The same stock grey wire is used for signals on the other side of the connector going out to the front panel.

While I'm at it...

What is up with that adjustable bias circuit? I can't tell for sure, but it looks as though the pot is connected to a resistor which to connect to the hot side of the bias supply, and the wiper connects to ground. Is that right?

Seems to me that one would connect the one end to ground, and the wiper would go to the grids of the output tubes. But it's really not clear to me how it's wired. I'm going cross-eyed trying to suss it out, but it looks way wrong to me... If I can make any sense of it, I'll post again. I might actually take mine out so I have a real example to look at. Unless I'm missing something basic, there's something very wrong with the way that circuit is wired.

-B

[Rafael]

So I think we're on the same page now. That's good.

While I'm at it...

What is up with that adjustable bias circuit? I can't tell for sure, but it looks as though the pot is connected to a resistor which to connect to the hot side of the bias supply, and the wiper connects to ground. Is that right?

%< snip!

Unless I'm missing something basic, there's something very wrong with the way that circuit is wired.

-B

Actually there is another pot at the upper right corner of the pic, hooked up as a balance between the two 7591 cathodes. That one is one my schematic. The other pot near the power supply doesn't appear to be stock [at least it isn't on the schematic I have] and without the amp in front of me I'm doing the same as you: sussing it out from the pic, and it appears you're correct: there is a resistor connecting to the pot then to ground. I don't see how disconnecting it would harm anything, do you? I won't be near the amp for a couple of days but at that time I'll make my measurements and try disconnecting that pot.

[Rafael]

Bad design? Bad design comes to your mind before the fact the amp is 40 years old? Are your filters still the originals? Is the hum 60Hz or 120Hz? They come from different sources. If the volume control affects the volume of the hum, then the source of that hum is before that control. Have we tried different tubes in it? Have we loosened and tightened the input jacks so they get a better ground contact to the chassis? Are the nuts mounting the pots tight and snug? are there intact ground connections between the tilt out assembly chassis and the rest of the system?

1] Filter caps have been replaced. See photo I attached to another post
2] the hum at low volume is 60Hz [not bad] but above half volume it becomes 120Hz [loud especially at full volume]
3] Changed tubes, no effect
4] All connections checked for tightness: OK
5] All grounds [including tilt out] test out at <.01ohm

[Rafael]

Measuring ripple requires nothing more than the DVM you already own.

%< snip!

While I'm at it...

What is up with that adjustable bias circuit? I can't tell for sure, but it looks as though the pot is connected to a resistor which to connect to the hot side of the bias supply, and the wiper connects to ground. Is that right?
-B

Voltage readings at PS caps [all voltages with no signal present]:
A: 397v w/ 18VAC
B: 354v w/ 1.1VAC
C: 286v w/ .005VAC
D: 283v w/ .001VAC
BIAS: -89v without pot, -12~18v with pot

The amp doesn't operate without the pot/resistor on the bias supply. That seems like a lot of bias, but I admit I have little knowledge in this area. Only heavy strumming causes a BRAPT from the output. I checked values on all the PS caps and they check out to withing tolerance.

One thing I notice is that even without anything connected to the input jack [which is shorting] there still is a lot of noise above say 2'oclock on the volume control.

I didn't have time but I had half a mind to just jump straight across the front panel altogether [from the .02u cap on pre output to grid/470k resistor of the phase splitter]. I know that would be full volume [] but I thought it would isolate the front panel?

[bbartilson]

Voltage readings at PS caps [all voltages with no signal present]:
A: 397v w/ 18VAC
B: 354v w/ 1.1VAC
C: 286v w/ .005VAC
D: 283v w/ .001VAC
BIAS: -89v without pot, -12~18v with pot

First off, sorry for leaving you hanging for a few days.

All excellent readings, and about what I would expect, with the exception of one. The bias supply needs to be measured for ripple as well. And we need to get the bias supply under control - as I think it is both your problem and the cause of your symptom at this point.

The way it seems to be wired, the pot is dragging the bias supply down to some value you feel is acceptable. I don't have a problem with you installing adjustable bias, but I'm pretty sure it's wired wrong.

If you want to adjust the value going to the grids, that's fine. Disconnect the red wire from the hot where it is now, and connect it to the wiper of the pot. One end of the pot can remain as it is, connected to the same point where the red wire was, and the other end (not the wiper) should go to ground.

My guess is that by lowering the bias voltage to what you feel is an acceptable level at the output of the supply, the subsequent load is leaving you with an inordinate amount of ripple on that supply. It's only half wave rectified, and shouldn't have to produce any current under normal circumstances. With your pot and resistor wired the way they are, you're simply loading the supply beyond it's capability, and likely introducing a great deal of hum. Even if you are not, it's the wrong way to go about the adjustment.

Secondly, you claim the amp 'doesn't operate' with the bias supply as designed. You need to be a lot more elaborate regarding exactly what that means. Again, my example has no mods, and the bias is just fine. So I'm left with questions.

1. What do you measure at the tubes themselves under both circumstances? (With or without your bias circuit in operation)
2. What exactly do you mean by the amp 'doesn't operate'.
3. If you changed the cathode balance pot to fixed resistors, what voltage do you measure across them?

The amp doesn't operate without the pot/resistor on the bias supply. That seems like a lot of bias, but I admit I have little knowledge in this area. Only heavy strumming causes a BRAPT from the output. I checked values on all the PS caps and they check out to withing tolerance.

Yes, it does seem like a lot of bias. Given your ripple readings, I suspect the supply caps are not a problem.

One thing I notice is that even without anything connected to the input jack [which is shorting] there still is a lot of noise above say 2'oclock on the volume control.

Gotta be more specific here. White noise? I would expect that. Lots of hum? Probably part of the original symptom which we have not yet discovered the cause of.

I didn't have time but I had half a mind to just jump straight across the front panel altogether [from the .02u cap on pre output to grid/470k resistor of the phase splitter]. I know that would be full volume [] but I thought it would isolate the front panel?

I don't see anything wrong with trying that. The amp has limited gain wide open, so it certainly won't hurt.

So you don't have a scope, or a signal generator? Is your renewed interest in electronics strong enough to pursue some inexpensive test equipment additions? Not that you'll need to in order to fix this amp - I'm just curious.

Oh. And anyone that reads this - what are you guys using to post little snipets of schematics? I've no scanner, and I draw like a child of four in MS paint. Are there any decent inexpensive solutions that will output a .png for me?

Best,
-Bill

[funky54]

Hi, I have been reading this thread over and over trying to understand. This is all over my head. My bass 30 is doing the same thing. I replaced the power caps, but am unqualified to go any further..(Might as well be 13 with my first soldering gun) Can anyone recommend a qualified person who could repair my head? I live in central Florida.

[gmoon]

I've got the same amp, and had the same problem. I can't recommend a tech, but here's my solution...

Despite some of the above comments/arguments, the most effective fix for ME was to reroute the power switch / indicator / fuse wires away from the MOLEX connector on the chassis, and away from the flipout panel. This fixed 95% of the hum; much of which was 120 hz.

I drilled a hole in the chassis by the PT for the new connections. The switch and fuse are now mounted on the front of the cab (temp; the cab is pretty rough and needs new tolex and grille.) I (re)used a 5-pin connector so the chassis can be removed easily (photo.)

This amp has other grounding "oddities"--the OT was grounded at the same point as the first preamp stage, for instance. And all the filter caps and center taps for the transformer were grounded together in a bunch. I sorted these out...

Another issue: MANY of the soldered connections were tinged with a green liquid corrosion. I don't know if someone used acid-core solder at one point; or if the wiring is some odd bi-metal that causes the problem. I had to clean and resolder at a dozen points.

These other fixes eliminated whatever hum remained. Some of these I tried before rerouting the panel, as well as replacing the filter caps, replacing the rectifier diodes, etc. (all the usual stuff first.) Some I did / undid, then redid after the rerouting the panel...

I have the original speakers, but one is shot (I'll get that reconed at some point.) On a whim, I dropped in a couple 10'' Rola Alnico speakers. The amp sound GREAT. It nails SRV. It's clean-ish overall, but the clean is so good (I.E., not sterile clean.) And it doesn't take much of a bump with a stompbox to drive it...

[rf7]

Just had a Bass 50 with massive 60Hz hum. I even moved the power switch off of the front panel, which helped a bit. I ended up putting it back on the front panel, though. This thread had the answer via SGM:

http://music-electronics-forum.com/t30781/

If you install a 3 prong cord, you must lift the ground from one side of the Heaters and install balancing 100 ohm resistors to ground from the Heater lines. That reduced the 60Hz hum by at least 95%.

[rf7]

And getting the Power Switch and Lamp off the front panel got rid of the rest of the 60Hz hum.