Kalamazoo Model 2 crackling noise

[nuke]

And there it is. (Maybe..)

I've thought about a SS rectifier just because 6x4 aren't very common, nor are the substitutes, and that socket is a bowl of ramen. Full-wave bridge rectifiers are cheap and abundant. Well, easy enough to pursue this angle. Guess I got some work to do.

Thank you.

Stephen

I wouldn't do a full-wave-bridge, just a pair of silicon diode rectifiers, applied the same in the circuit as the two diodes in the 6X4. 20uf is plenty for this one as the input filter from the rectifier.

[Stevem]

I have had new coupling caps crackle on me when the lead on one end had a failing internal connection.

Pretty easy to tell when a lead rocks and rolls around too much.

[Phil_S]

If you continue to face challenges that you aren't getting past, you might try to find Miles O'Neal. My last exchange with him was quite a long time ago, maybe 15-20 years. It's my recollection he's located in Round Rock, There is an email address on his Kalamazoo web page, but who knows if it still works?

[Kalamazoo 2]

Just a quick update:

I removed the 6x4 and soldered a pair of diodes across the appropriate terminals. Voltages downstream were 10+ volts higher than with the tube, as expected. Amp was a bit louder, timbre was blangy metallic, sudden attack but pretty clear. Oh yeah, the crackle was muted a bit, but still there.

So I knew I wanted to return to the 6x4, especially since I'd met The Amp Guy at Moon Music--a truly wonderful small, fantastic mostly guitar and amp store, where I got my Strat. He's a fan of 6x4, but gave me a couple of diodes for my testing. Turns out he stocks plenty of components so now I don't have to order everything. So I got some MOD caps with lesser Faradayitiness and stacked then neatly on some vertical terminal strips. Tidy. Made me happy. While I was in there, I cleaned up some connections and made some notes on a cleaner star/buss ground scheme, and went back and forth on all the things I'd have done differently from Gibson. Also thoroughly hosed the tube sockets with isopropanol. Clipped the diodes--which I will eventually install in series between PT AC and 6x4, safety first--plugged in the tube and, of course, it still crackles but I got my lovely sound back and nothing caught fire. The crackle itself is barely audible when playing with gain below 6.5/10 but it is sucking gain or something like that and messing up the sound, particularly as I turn up the gain. Muddies chords.

We're about down to a cold solder somewhere or the PT is flaking.. or both. I went back through this thread to see if I'd neglected anything. What stood out was my use of old oxidized leads rather than going all the way back to the terminals. I've been avoiding redoing those connections because it won't be easy and I've been concerned that I'd damage a socket or something. I mentioned that the factory used the rectifier socket as a terminal strip--only half the connections there have anything to do with the rectifier, so if I go after connections, I intend to move all those off to new locations and at the same time improve the ground layout. Yeah, it hums, but you can't hear it ten feet away and it does not get louder as gain is increased. Heck, I can't hear it at four feet. This approach would be a significant rewiring and so I approach it warily. While I overthink all this, I'll proceed with installing the IEC receptacle, tidying the AC side, and adding a speaker jack, cable and plug.

Now you're updated. And also thanked. I am very grateful to each of you for your support.

[Stevem]

Your crackling issue I believe is the OT arcing I am willing to bet at this point.

[Kalamazoo 2]

Your crackling issue I believe is the OT arcing I am willing to bet at this point.

Hmmm. It is an ugly old thing. The paper wrapper is wrinkled, almost looks wet, or like it got wet--it's not and it hasn't. More oily or waxy. I'll take a picture later, have to mow the lawn before it rains. I'll check resistance, though I don't have a spec, and for shorts.

Thanks.

[Stevem]

A resistance test is meaningless if it's intermitanly arcing.

[nuke]

A resistance test is meaningless if it's intermitanly arcing.

True.

However, a megger (hi-pot ohmeter) is pretty inexpensive these days. About $100 on Amazon straight from the China ecosystem. I've got one that tests resistance and insulation at 250v, 500v, and 1000v. Runs on batteries, just has a step-up voltage booster and a digital read out.

[Kalamazoo 2]

A resistance test is meaningless if it's intermitanly arcing.

Yes. Nevertheless, if it's arcing, resistance could also be out of spec and eligible for replacement regardless.

Thanks for the reminder. Truly appreciated.

[Kalamazoo 2]

The old OT:

[Kalamazoo 2]

...However, a megger (hi-pot ohmeter) is pretty inexpensive these days. About $100...

Although I love an excuse to buy another tool--and I'm aware of more than a few occasions in the past where I could have used such an instrument--I'm also aware that I can get a suitable Hammond for far less than that and have multiple output impedances.

Thanks for pointing out yet another tool to obsess about.

[maxkracht]

If you watch it in a dark room you might be able to spot the arcing to confirm a bad OT, you would also likely be able to smell it. A bit more scientific, you can do this if you have a spare neon and a 9v battery. https://www.premierguitar.com/the-super ... mer-tester If you don't have a neon on hand and don't mind getting a bit destructive, the illuminated power switch of a regular power strip is usually neon.

[Kalamazoo 2]

If you watch it in a dark room you might be able to spot the arcing to confirm a bad OT, you would also likely be able to smell it. A bit more scientific, you can do this if you have a spare neon and a 9v battery. https://www.premierguitar.com/the-super ... mer-tester If you don't have a neon on hand and don't mind getting a bit destructive, the illuminated power switch of a regular power strip is usually neon.

That PG article was interesting. That test method essentially works like a Kettering ignition system--good ol' points and coil, with a neon bulb instead of spark plug. Nice. I'll likely try it out. Thanks Max.

Meanwhile, I bought a cheap little oscilloscope, figuring the noise would be obvious even on such a dinky device. And it sure seems it did. The pattern synched with the noise from the speaker as I worked my way back from the input jack. Everywhere I went I got a fuzzy jumpy pattern on the scope that increased in voltage when I turned up the gain, but it was most dramatic at PT output. Further, the two legs of the AC yielded very different traces that remained synched with the noise. I don't think this is right.

I still feel like I should test the 120 VAC upstream of the PT as I suppose there could be really dirty AC at the plug or the pole even though my shop was built in 2005 with green-wire grounding. I have tried both circuits in the building with identical results already, so I suspect this is not the cause. I'm gonna declare this progress and call it a night.

[Kalamazoo 2]

The Wonderful World of Oscilloscopy is a fearsome place. Pretty clear that I have a lot to learn here. Also pretty clear that a "real" o-scope would be easier to learn on than this little boxful of menus, but I've largely sussed the display and buttons. Less clear to me is what probing different spots in the circuit tells me.

From One True Ground (rg) to different places in the circuitry clearly showed the really low-amplitude (<100mv) 120hz hum. Cute. Also failed to show much noise from input jack down to the tube side of the second gain stage plate load resistor. Very distinct. The B+ side of that resistor, R7 100k, shows little noise.

I don't know if it's proper/meaningful, but I put probe on the the B+ side of R7 and ground lead on the other and saw a real messy waveform on the display. Did the same on R4 100k B+ stage 1 Plate Resistor and it was not noisy. BTW, all DC voltages vs ground are nominal according to DVM.

Is R7 failing under load? Shushy static-y pop snap kinda sounds that way, right?

"Stephen, why don't you swap that R7 for a new one and see, huh? Do you feel lucky? Well, do ya, punk?"

[Stevem]

You just jarred my memory.

I once had a combo on the bench that was working but poorly and would crackle when playing loud chords or somewhat loud note below A on the low E string.

When I finally move enough wires to view all the power supply dropping resistors one of them ( for the screen if I recall) was burned up some and cracked in half, but yet still working .