Kalamazoo Model 2 crackling noise

[Kalamazoo 2]

It's been awhile since I reported, but I have an excuse: I've been busy.

Let's see. I did swap out r7. No change.
I reflowed many but not all connections. No change.

Oh, yeah, the little oscilloscope revealed a lot of RF in the circuits--double-digit megahertz on down--so added grid stoppers. That got rid of the parasitic oscillations and improved the audible gain and sound, but the crackle persisted.
Cleaned up dress lead, moved/rerouted/replaced/reconnected some grounds--eliminating the second input jack.
Sound keeps improving, but the crackle persisted.

I exploited my lack of inspiration by proceeding with fabrication of a new back panel that would support a speaker jack and an IEC mains input with integrated fuse. Made me happy, but of course didn't cure the crackle. I have continued playing the Strat through it and it makes a beautiful sound. Below 6/10 anyway.

Lots of prodding, probing, wiggling and absurd conjecturing led me to conclude that I had pretty much reached the limits of my tools, instrumentation, knowledge and, yes, patience, so I took the little box and schematics down to Paul at Moon. He was almost excited, and now it's in the queue for diagnosis. He said it's most likely some remaining poor joints (the same stubs of leads that y'all had pointed out) and crappy tube sockets (yes, the ones y'all had mentioned and that I had cleaned three times)--preamp and power tube showed (to him) weakness. He's going to suss it and see what's up, and call me. He knows I'd like to do most of the work, understands why and supports it. What a great shop!

In the meantime, I do have the working Epiphone Constellation V bass amp to get loud with. It's a totally different beast and a ton of fun. It's got an interesting tone stack in an otherwise primitive design and a 15" speaker that is surprisingly good. But that's another story. Perhaps it's time to start a thread about that.

Again--I can't say it enough-I am so grateful to you all and I hope you've found some of all this interesting, amusing, or at least, a reason to feel good about yourself.

[Kalamazoo 2]

Holy mackerel, it's mid-August already.

THE KALAMAZOO LIVES, in glorious silence at idle right up almost to 9/10.

Briefly, Paul went through it, did a lot of the same things I did, verified that I hadn't really screwed anything up, cleaned up a bunch of connections and improved grounding, all of which quelled the hum but not the crackle. Then he replaced the little silver mica cap at the input because it was a brand and type he mistrusts, and that fixed it.

It is now a very very quiet amp. It's also a fine loud amp. Clean up to about 8/10, then a smooth round distortion. Turning the guitar volume up gets a richer sound earlier. Tone controls on both amp and guitar have much more effect now than before.

I feel pretty good about myself--it mystified Paul for a good while, and he knows his stuff. I played a Princeton Reverb that he built from scratch, dead quiet all the way to 10 it was, with a reverb that was smooth and mellow, almost unnoticeable below halfway until you turn it off and hear what's missing. So I don't feel bad for being mystified myself. I had looked into most of what he expected to be the cause. You may recall that I had concluded, perhaps wishfully, that the Kalamazoo was a victim of a failed component, and indeed that was the case. I just hadn't taken that far enough.

So now I have a handy, sweet-sounding amp. I will re-do the back panel to my liking, and replace the new pots with still newer ones that will accept the OEM knobs (or replace all with new knobs that match), and tidy a few nagging things, and update BoM and schematic to reflect the new reality. But all that will wait awhile; I'm gonna just play and enjoy this for now.

Well, I am going to keep fooling around with the big Epiphone, as I've had some success in improving it, but that's for another thread...

[Stevem]

I had a Ampeg with that same issue, but it was a customers amp so being at my whits end after 4 hours with no cure I could not just toss it out the window as I might have done if it where mine.

With this one I had narrowed it down to the OT arcing over when the drive signal was high enough, or the cap feeding the PI which is where the issue was stemming from.

After this pain in the butt that it was 30 years ago if I have a similar problem the first thing a do is jump out any signal caps that are not also needed to block out DC.

[Kalamazoo 2]

To be more precise, the cap that failed was on the output side of first gain stage, from plate to ground. Paul ran the amp without it and it worked just fine, but he did install a new one. I was confusing it with some other circuits I've been studying that toss a cap in between grid and plate.

For the most part, my connections were adequate. Paul's, however, are lovely. Inspiring, really.