Kalamazoo Model 2 crackling noise

[Kalamazoo 2]

Last fall I dug out my mid-sixties Kalamazoo Model 2 and set about bringing it back to life. First project of this kind, so I replaced all the caps, tubes, and speaker with quality modern ones and it worked great. After some hours of use, it developed some unpleasant highs that fixed with correcting the polarity of a cap. Brilliant. Terrific little amp. A hum and hiss came up so I renewed all the resistors with metal film and new pots and all was well--except for a gain-stealing crackle that came up some hours of playing after the resistor renewal.

I am baffled--I've reflowed a lot of solder joints, mine and factory. I've rolled tubes. I've cleaned. I've chopsticks and wire-wiggled. Any ideas would are appreciated.

Here's a schematic, courtesy of Miles O'neal, whose website has been invaluable.

KalamazooM2Schem.jpeg

The narrative above is likely over-concise, but it's a place to start. I can go on (and on) and have pictures and even a spreadsheet of measured voltages.

Thanks in advance, and thanks for a forum of folks who have already helped so much--what a great resource.

Stephen
Austin, Texas, dead amp capital of the world

[maxkracht]

it developed some unpleasant highs that fixed with correcting the polarity of a cap.

Did you replace the cap or just flip it? Could be damaged.

Crackle can be a lot of things, photos of your amp might be helpful. The tube sockets in those aren't the greatest, but you should be able to clean and retention. Even a new tube can be crackly, if you have spares you might try swapping one at a time. New resistors can also be bad, or possibly underrated for the job. Could also have a mistake with a resistor value causing a tube to have problems.

[Kalamazoo 2]

Did you replace the cap or just flip it? Could be damaged.

New cap. Other direction. Was magic for awhile.

... photos of your amp might be helpful.

Pretty much as it came from factory, except for grounded AC main I installed decades ago:



When photo was taken, I'd already moved the power switch off the tone pot and the fuse to the back of the chassis.



You can zoom and see the knobs.

The tube sockets in those aren't the greatest..

Indeed. Have cleaned, twice, and tightened the clips. There were no signs of overheating or arcing on the sockets.

Even a new tube can be crackly, if you have spares you might try swapping one at a time.

Swapped in old tubes, ALL of which were good, still crackled. Had the new JJ PT, tremolo and PT, and NOS 6x4 tested, all good.

New resistors can also be bad, or possibly underrated for the job. Could also have a mistake with a resistor value causing a tube to have problems.

All new R's are nominal or better wattage, all checked with DVOM before installing, and have been checked since. Also replaced one that I hit with the soldering iron, which did check out okay.

A few more notes on the crackle:

The crackle is a constant background rattling-cellophane noise, with random louder clicks (pops?). To me, it sounds a lot like dragging a conductor across a surface, making and breaking. But I've found nothing loose at all.

It is not audible until amp is warm. It first showed up after I'd been playing for a couple of hours. It is barely audible until gain is above 3/10, and is barely annoying until 5/10. At 6.5/10, where the sweet spot begins, it is full irritating and I can tell it is stealing gain. Atop the hump at 7.5, it is extremely unpleasant. Any volume, it affects the sound even though a good loud strum overwhelms it.

Repeated comparison has revealed declining voltage (expressed as percent of rectifier output) over a few weeks. This fact reinforces my suspicion that crackle might be coming from a deteriorating component, perhaps one of the few I've not replaced or perhaps one I've damaged.

[maxkracht]

Noise getting louder with the gain pot usually indicates a problem before the pot. Updated photos, after your most recent changes, would help. Your chart of voltages would too. The shorting contact on the input is bent out of place, so there could be other issues with the input jacks. Almost looks like it could touch the tip when a plug is inserted. Preamp is grounded via the input jack so those need to be tight and free of rust or you will have problems. The 10uf cathode bypass cap should be replaced. Sometimes those black and red sprague caps can be leaky, easy enough to test or just replace them if you have extras. Anything further without updated picture or voltages is just a guess.

The solder joint on the fuse holder looks pretty rough as well. Those old power cords tend to get corroded. Sometimes you can cut it back a few inches to get to fresh copper, sometimes you can't. Good idea to pretin before soldering back in place. I'm also not keen on the idea of using the fuse holder as a strain relief for the power cord.

[Kalamazoo 2]

Noise getting louder with the gain pot usually indicates a problem before the pot.

That's what I thought, and is why I started at that end of things. Ended up replacing all the passive components.

Updated photos, after your most recent changes, would help.

Hope so!


You might can see I've replaced that input jack and made sure it's well-grounded to chassis. And yeah, that shorting contact was useless. Some of those Spragues were a little soft on the ends; they're all gone, indeed all the caps in this picture are new. I replaced that bulging cardboard can triple cap with some F&T electrolytics. Fuse holder looks a lot better now, but you can't see it under the heat shrink. New (ish) power cord that I had to chop back quite a ways to find shiny wire. Fuse holder as strain relief will be gone as soon as I fab a bracket for the IEC receptacle.

I am not crazy about the way Gibson used the rectifier socket as a terminal strip for so many connections:



My little IR thermo showed me that the rectifier was the hottest thing in there, until hard playing at volume warmed up the power tube, which didn't really surprise me. Neither was really hot--I'm not terribly confident in the accuracy of the device, but reckon it can indicate relative temps adequately.

Your chart of voltages would too.

chart 1.jpg

Sorry, that's so small. It shows voltages at different points along B+ according to the schematic and a recent measurement, expressed as percentage of PT AC output. I wanted a scale-free measure so I could compare my amp's circuit behavior regardless of input voltage or PT performance. It's pretty close, which implies that changes in absolute voltages over time might be explained by PT output voltage change.

And below shows the decline in PT output voltage. The last three observations are only about three weeks apart.

chart 2.jpg

I don't know that reduced PT output causes the noise, but I can imagine that whatever is causing the reduced output might could cause some noise.

[Stevem]

1)
Remove all the tubes and with a needle nose squeeze all the tube pin receivers together from the bottom side of the socket so they grip better.

2)
Many of those terminal strips use there mounting legs as grounds and the ground connection comes thru old Rusty and many times loose rivets.
Drill the rivets out from the top side of the chassis and use machine screws washers and nuts to retain them .
6/32" hardware will work.

How many volts is on the output tube cathode and how many volts is that cathode bypass cap you changed out?

[maxkracht]

Using the old component leads and attaching with j hooks is potentially fine, but you have to be diligent about cleaning the dirt/oxide off of the old leads before soldering. Some of them look like they weren't fully cleaned and might have iffy connections. You also have potential for creating a cold solder joint at the tube socket when doing this if there isn't much distance between the new joint and the old.

Hard to tell from the camera angle how precarious things are. Might be fine, but getting some heatshrink over some of the flying leads where components are somewhat close might be good.

I forgot these amps don't have a grid leak resistor on the input stage, might be a nice addition if you have a spare 1M resistor.

[maxkracht]

Steve is right about those rivets sometimes being a problem, but looks like you're only using the one at the trem oscillator. Could just run a wire from that lug to your nearest ground point to see if that's a problem, or move the two resistors.

[Kalamazoo 2]

1) Remove all the tubes and with a needle nose squeeze all the tube pin receivers together from the bottom side of the socket so they grip better.

Did that awhile back and I've checked a couple more times since.

2) Many of those terminal strips use there mounting legs as grounds and the ground connection comes thru old Rusty and many times loose rivets...

Moved parasitic cap ground off one of those.
Moved output tube grid leak resistor off another.
Only one remaining is the ground for the cap and resistor farm of the tremolo PSO. Tremolo works very well, adds no noise.
Nevertheless, I'll move those to the buss to see what happens.

How many volts is on the output tube cathode and how many volts is that cathode bypass cap you changed out?

Output tube cathode voltage is 7.7 v on schematic, has measured 7.3-7.5v consistently.

The cathode resistor bypass cap on the tremolo triad (V2) is a 10uF 25v electrolytic.
Voltage there measures 2.2v.

Note that this cap/resistor/ground is shared by preamp stage two (V1a) cathode. I'll add that the amp works with tremolo tube (V2) removed--and crackle persists.

Thanks so much.

Stephen

[Kalamazoo 2]

Question:
What happens when PS filter cap's capacitance rating is too high?

Background:
O'Neal's guide to the KM2 suggested increasing the PS filter cap from a 10/10/20 to 20/30/40, also suggesting 40uF was a maximum according to RCA 6x4 rectifier tube specs. Closest I could find were 22/30/47 F&T 450v so that's what's in there.

Malcom Gladwell asks:
Might this be the problem?

[maxkracht]

Going over the rectifiers max spec can shorten its lifespan. People do it all the time but better to stay within limits. MOD and nichicon make 40uf caps. This only applies to the first cap off the rectifier, you can go higher with others.