Hot Pickup+Tube Combo=Nasty Squealing. Please help!

#41
From reading the description of the symptoms, this sounds like a classic case of external pickup microphonics. The pickup is mechanically loose on the bass, and able to vibrate in place. The sound from the amp starts making the pickup vibrate, and it instantly turns into loud screeching feedback.

To test this, try gently holding the pickup with your fingers while you bring the amp up to normal volume. Let go of the pickup, and it starts screeching. Grab the pickup again and it stops. That's your clue.

The fix is more solid mounting of the pickup to the body, with screws, springs, foam, whatever. Pickups need to be relatively solidly mounted to the bass. If they are able to wiggle when the bass is bumped, they are capable of creating nasty feedback.

This is not the same thing as internal pickup microphonics, where wires or coil assemblies are vibrating inside the pickup. That's a possibility, but not likely with a new set of Alumitones.
 
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#43
I PM'd him offering to help him out.
He is a nice guy, my gf's cats like him even tho the house shook.
I'm not going to charge him anything.
I can do it or walk him thru it with my tools.
I wrench on code and cloud infrastructure now, so having a bench session is actually a welcome diversion.

It'll give me an excuse to pull my $$4 preamp out and finish taping the control cavity if he comes back up here.
I also have a couple other pickup swaps I've been meaning to do so there's external motivation for me.

WardEarth said:
Interesting, never heard of this but I could totally see that its something that happens. I think bruce is on track.
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Happened more with customer teched/modded guitars than basses when I was wrenching on gear. Lots of Les Pauls with pickups flopping or easily pushed down shrieking when the guy did the usual 80's stage gymnastics. Cool 90's grunge mods to junk guitars + banging on them like the 2nd coming of the Sex pistols would expose improper pickup mounting.
 
#44
Ahoy again, everybody, and thank you all for taking an interest. I'd like to like and quote a bunch of what you said, but it may be less tedious to just share what I managed to figure out today:
  • I can get that awful squeal fairly easily using any of my basses through this amp (modded '80s Electra with the Lace Aluma-P, stock '90s Ibanez ATK active 5-string, stock '00s Epiphone Thunderbird) with the gain past 2:30 and the master not even past 10:00. I just got there faster with the Electra because it's hotter and I've favored it lately.
  • This squeal literally makes playing through the amp impossible, which I don't know if I've made clear. It's not something I hear between notes; it supersedes anything I play, and the only way to stop it is to switch to standby.
  • The squeal does not respond to turning down the volume knob on the bass. I tried turning down both volume knobs on my least-hot bass (T-bird) when it happened and it didn't stop or even effect the squeal at all.
  • I don't know when I'll get a chance to figure out how to access and tap on the preamp tubes, but I suspect at least one of them is the culprit here, as tapping on the easily accessible power tubes with the amp warmed up produced no squealing sound through the speaker. This seems like an opportunity to get some new preamp tubes that will drive more readily.
  • None of this necessarily rules out problems with this pickup and/or its installation. It seemed weird to me that the pickup came back to me with the middle part of the bass half of the pickup so sharply angled toward the third string (A in standard tuning, but I have this instrument in CGCF), but I figured the tech probably knew better than I did how it should be set (he did a fine job on the new bridge and knobs, as far as I can tell, and readjusted the truss rod without any trouble when I informed him that I don't plan to use this bass in standard tuning); but clearly, the pickup height at the very least was all screwed up (or more accurately, not screwed in). More may be afoot, but this bass has never given my 35w solid state glorified practice amp any trouble.
  • Adjusting pickup height to equalize output between strings to the best of my ability, the bass half of the pickup does wobble a bit; that goes away if I crank it down a bit farther, but I have the treble half cranked down as far as it will go and taking the bass half down farther throws the relative output of the strings out of whack.
  • The incident with the pickup sticking to the string (not just the magnet, the entire side of the bass half of the pickup) was a result of the screw coming completely out; it was barely screwed in at all. I haven't had anything like that happen since I screwed it way down. There's a greater distance between the strings and the Aluma-P than on the bass with the active pickup.
I think that addresses most of the points that most of you made. I'll reread everything and see if there's more to say to any of you individually. For now, I'd just like to say thanks again to everybody willing to help a stranger with little knowledge of electronics. And as his participation in this thread indicates, @somebrains is a stand-up guy; you can deal confidently if you wish to relieve him of any of his many amplifiers, haha!
 
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#46
@somebrains I hope your finger has healed and that that sandwich was worth the wound, haha. And your cats are cool; they probably don't mind the house shaking because that's what our houses in California do anyway.

Thanks for chiming in here and keeping tabs on this. For the deal you gave me on this amp, I figured I wouldn't sweat it if I had to pony up for some tubes sooner rather than later. I liked what I heard and trying out all the permutations of basses and preamps and cabs and whatnot made me confident you weren't sending me home with a lemon. I had expended all my brain power on driving and wheeling and dealing logistics that morning and appreciated your patience with me then as I do now. The guy who bought the Sunn from me earlier that day put it best: amps are like cars, and sometimes you will just get handed a situation regardless of how the previous owner cared for the thing. Frankly, a new preamp tube sounds like the easiest and least expensive thing that could be causing this, so if that diagnosis seems right, it could have been worse.
 
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#47
JKos said:
I'm so confused about who's talking about what here. The OP says turning the bass down doesn't stop the squeal. Somebrains comes in and says it does. Huh?

- John
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@somebrains and I are talking about two different things. This squealing didn't happen when we tried any of my basses or his through this amp at his place when I bought it. If this thing had squealed like this when I was there, I probably wouldn't have bought it. I don't know much about electronics but I know what I would rather not deal with out of the gate, haha.
 
#48
JKos said:
I'm so confused about who's talking about what here. The OP says turning the bass down doesn't stop the squeal. Somebrains comes in and says it does. Huh?

- John
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It's bc that was my amp that I sold the OP.
We ran thru my basses bc I wasn't sure what was going on with his Westone.
2 J's and a Spector are what I played thru that amp all the time, I had him play my basses thru it.
Basically it's me speaking out loud troubleshooting the process bc I'm wondering if I can help him fix that Westone bc that P pickup was troublesome when he was at my place.....and it's actually a good sounding setup.
Really hot but articulate.

Monterey Bay-ss said:
@somebrains I hope your finger has healed and that that sandwich was worth the wound, haha. And your cats are cool; they probably don't mind the house shaking because that's what our houses in California do anyway.

Thanks for chiming in here and keeping tabs on this.
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Yeah, no sweat man.
Cats are doing well and I burned the other finger on the BBQ this weekend.

I would grab a pencil, heat up the amp, tap the preamp tubes one by one.
Keep a finger on the standby, preamp to 3pm and master at noon.
I did dive back there when I plugged the T310 into it and banged on it some but that aluminum bar back there makes it a funny angle when I'm going over it while it's under my desk.

It's a good idea to pull and reseat the tubes just to cross that off the list. Someguys like to put on a surgical glove to keep fingerprints off the tubes.

Come back up sometime and we will wrench on those basses.
 
#49
ctmullins said:
... Was the pickup purchased new, or second-hand?

...

I suspect it’s the amp (but I’m not that savvy about tube amps). Does it display the same behavior with a different bass, after having adjusted the input gain to match?
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Brand new Alumitone pickup (and new, not-the-cheapest-option pots as well). And as I determined today (and edited my original post to read), my other basses do also set off this squealing with the gain up. It was, regrettably, not as hard as I thought to get that nasty squeal even with my relatively tame Thunderbird.
 
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#50
JKos said:
I'm so confused about who's talking about what here. The OP says turning the bass down doesn't stop the squeal. Somebrains comes in and says it does. Huh?

- John
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I think the scratchy pot was the stock one on my active 5-string. The pots on the Electra with the Aluma-P are brand new, and I did not skimp on them. Not surprised you're confused. @somebrains and I played a lot of things through a lot of other things. The sale was almost secondary to trying ALL THE THINGS, haha. Now I want to buy his Spector but he'd be nuts to let it go at a price I could afford, haha again.
 
#52
JimmyM said:
Well if they all do it and it won't stop even when you turn the bass volume down, then you've almost certainly got a seriously microphonic tube in there.
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That's what I'm thinking at this point, and I'm grateful that the power tubes seem okay, as it sounds like preamp tubes are a relatively easy and inexpensive fix.
 
#55
lowplaces said:
Amp squeal

Squeals

From this amp debugging page..

Tube Amplifier Debugging Page
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I was sure that when I opened this it would just link to video or audio of this terrible sound, haha. The input jack seemed a bit touchy when I first got the amp home but threading the cable through the handle on top seemed to take care of that; that would be an even quicker and easier fix than a preamp tube, which seems most likely. I'll have to figure out how to tap on those and confirm that the issue lies there.
 
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#58
lowplaces said:
MESA/Boogie®

Scroll down and download the manual for your amp if you don't have a hard copy or have downloaded it already.

There's a section on Tube info and diagnosing tube noise.
Click to expand...

Bookmarked it a while ago! Mesa (and Peavey, and I imagine many others) are pretty great about this.
 
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#59
Tap the preamp tubes with a pencil one at a time when its hot with a hand on the standby.

My concern has always been that you don't kill the 15L in the combo.

That thing sounds so sweet.
 
#60
somebrains said:
Tap the preamp tubes with a pencil one at a time when its hot with a hand on the standby.

My concern has always been that you don't kill the 15L in the combo.

That thing sounds so sweet.
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Your concern and mine both, haha. Any tips on how to reach the preamp tubes? I can see where they are on the schematic but can't find the glass when I start poking around between the power tubes from the back.
 
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