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Thread: Looking for a Noise Theory...

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    Looking for a Noise Theory...

    Disclosure: I do not have a scope.

    I just finished an amp build & let the buyer hear it the other night. He loves it. I decided to make a few changes to "clean up" the insides. After changes, was testing with my guitar. I know the pickups are microphonic, too much so for my taste. So I was testing my amp out. I usually give it an "extreme" test before letting go, just in case the buyer tries something "crazy." I'm also fairly close to the amp while testing.

    First, it's a single-channel 2x6L6 Fender "Normal" channel. I'm playing it through my 15" Weber bass speaker (which has more than enough treble for guitar musing). My guitar is a Tele w. Duncan singles in it. So, I had the treble on 10 & cut control (added) on "minimum treble cut", and the volume pretty much maxed too. Bass was on 0. I was getting a high-pitched whistling noise, almost like microphonic squeal. I could dial it out with the 2 controls or the tone control on my guitar.

    Testing of the amp ensued. Weird findings:
    I ended up undoing all of my changes trying to get rid of it except for upping the coupling caps to the 6L6s, which are now at .022. The squeal is still there with the extremes of controls. I can also induce it at a slightly lower volume by keeping a finger close to the 6L6 closest to the front panel. I swapped the tube positions & it stayed with the socket. I haven't tried a different set of tubes yet.

    If I unplug tbe guitar & cable from the amp I can run all the knobs on 10 (V, T, B, Cut) with no squealing or tube cacophony if I tap the tubes. No strange changes in the control knobs if turned that indicate amp-based oscillation to my ears. Just normal changes that come with adjusting knobs. No redplating without signal that may show oscillation. I'll do some voltage checks for leaky caps later today.

    Guitar otherwise sounds good on all pickup combinations, controls work smoothly after some cleaning, nothing out if the ordinary at more "normal" settings... Didn't notice any issues theother night while testing woth buyer's guitars & speaker cab (1x12" closed-back Celestion) other than there was not much bass, probably because I trimmed it out in my own testing cuz I have a bass speaker & a modded Tele.

    So it seems to me that SOMEthing related to my guitar is setting off something. A different guitar with a P90 in the neck does it too, though not as badly. The humbucker doesn't do it. I know I'll need to pot my Tele pickup(s) & I'll get around to it. And I know that "nobody plays with the Treble on Max & the Cut on Minimum!" But since it seems to only happen with a guitar plugged in, & with microphonic pickups, can I reasonably conclude my guitars are the problem? And why I can trigger it by getting a finger close to the 6L6 is beyond me; it's new to me!

    This is more a "fun discussion" to me, though if anyone has any quick&dirty theories on what's going on I'll hear em.

    Jusrin

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    And why I can trigger it by getting a finger close to the 6L6 is beyond me; it's new to me!
    To me this indicates that you don't have acoustic feedback (caused by microphonic PUs) but rather electrical feedback caused by capacitive coupling between the 6L6 plate (carrying high signal voltage) and the PUs.

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    Last edited by Helmholtz; 01-16-2020 at 03:22 PM.
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    Lifetime Member Enzo's Avatar
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    Agree, and it doesn't have to be loud to do it.

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    Clarification: no problems if I have the controls on 10 AND I'm touching some grounded part of the guitar. If I am inducing the squeal by touching the tube AND then I touch a metal part of the guitar, the squeal stops.

    There's mo squeal right now if I just leave the guitar sitting full up with amp cobtrols on 10. If I touch the pickup bobbin without touching the strings, the picked-up noise through the amp gets louder. It then goes back to normal if I touch metal. If I touch just the plastic bobbin AND get my hand close to the 6L6, it squeals until I again touch metal on the guitar.

    So is something here wrong with my pickup or guitar groundibg?

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    To Helmholtz & Enzo,

    Any ideas how to fix this if your theory is correct?

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    It doesn't even require sound or a speaker to excite this kind of positive feedback.
    I will be influenced by PU/guitar shielding and PU phase and of course distance between power tubes and guitar/player. As well as total (high frequency) gain as capacitve coupling favors high frquencies.

    Any grounded metal shield somewhere between power tubes and PUs should help.
    As the power tube plates are the strongest field emitters in an amp, its most effective to shield the power tubes with a grounded metal plate.

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    Last edited by Helmholtz; 01-16-2020 at 03:21 PM.
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    Member uneumann's Avatar
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    Quote Originally Posted by Helmholtz View Post
    To me this indicates that you don't have acoustic feedback (caused by microphonic PUs) but rather electric feedback caused by capacitive coupling between the 6L6 plate (carrying high signal voltage) and the PUs.
    Could be FB to the guitar PUs - but if you move some distance from the amp, it should stop. If it doesn't stop, it's likely internal in the amp. I had a similar experience a while back - a "guitar dependent" oscillation. Turned out to be the cable shielding and grounding between input jack and 1st stage grid was picking up signal from elsewhere (output tubes maybe - never did find out). With no guitar input, the jack was shorted so the problem didn't occur. Different guitars/cables put different impedances on the input so it impacts the oscillation. I recall it was also a Tele that was "worst case". Anyhow - something to consider.

    Two ways to deal with it - shield the source (plate leads and PI outputs are both big producers of radiated signal) or shield the receiver (the input stage wiring).

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    Bent Member Chuck H's Avatar
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    Justin. I know you know what microphonic pickups are. And I know you know what acoustic feedback is. You mentioned that it happened with single coils on two different guitars, but not humbuckers. Hmmmm.?. I had a similar thing happen when I was building/designing my attenuator. Singles squeal, humbuckers don't. It was the EMF interaction with an air coil inductor in my design that was interacting with single coils. Humbuckers would "buck" the problem. I won't go into how I fixed it here because I don't think it's applicable to your situation. But I think you may have a similar problem. I wouldn't expect this trouble with an OT, but I could be wrong about that. You mentioned that it didn't happen with the customers test gear (guitar and cabinet). But then that test may not have been as extreme as your private test as far as settings. So...

    If you can, try a different speaker cabinet than your 15". My first suggestion was going to be to ground the speaker frame to it's - terminal. But this amp will be used with different cabinets so that's not a universal solution. Still, try another speaker anyway just for kicks. And try the 15" speaker again after alligator clipping the frame to it's - terminal.

    There may be some other resonance dependent and non EM cancelling demon at work too. Maybe a sensitive capacitor.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    I've had a few amps for repair where when the shielded back panel is removed they exhibit guitar-specific electrical feedback. One Fender in particular played like a theremin with hand proximity to the output tubes, but only with the volume turned down. Full up or at zero, no noise. With a different guitar, no noise at all.

    I also has a standalone tube reverb that did this and I kept coming back to that for ages before finding it was the recovery tube coupling cap at fault. Why that only fed back with certain guitars is a mystery.

    Do you have foil screening in the enclosure or back panel?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    I've had a few amps for repair where when the shielded back panel is removed they exhibit guitar-specific electrical feedback. One Fender in particular played like a theremin with hand proximity to the output tubes, but only with the volume turned down. Full up or at zero, no noise. With a different guitar, no noise at all.

    I also has a standalone tube reverb that did this and I kept coming back to that for ages before finding it was the recovery tube coupling cap at fault. Why that only fed back with certain guitars is a mystery.

    Do you have foil screening in the enclosure or back panel?
    Ah! This is a good observation. If these test results are with the amp outside the cabinet that could make the difference. I've tested for this before with open chassis by just placing large pot lid on top of the open end. There's usually enough contact to make for a partial shield.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Two ways to deal with it - shield the source (plate leads and PI outputs are both big producers of radiated signal) or shield the receiver (the input stage wiring).
    I think the main source would be the power tube plates themselves as the rest of the circuit is typically shieded by the chassis.
    The tubes are outside the chassis and the plates have a large radiating area.

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    Last edited by Helmholtz; 01-15-2020 at 08:30 PM.
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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Any grounded metal shield somewhere between power tubes and PUs should help.
    As the power tube plates are the strongest field emitters in an amp, its most effective to shield the power tubes with a grounded metal plate.
    Like this. Not for heatsinking.


    Click image for larger version. 

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Not for heatsinking.
    Right, but nevertheless shields the ecaps from the tubes' heat radiation by acting as an infrared reflector.

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    Last edited by Helmholtz; 01-15-2020 at 08:56 PM.
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    Hey All,

    Thanks for all the theories/advice. I'll try what's easy & quick, since this is a recycled organ chassis with true PTP wiring & limited space on the chassis. The cab is something I kinda threw together. Yes, all of this is with the amp outta the shell, so it's not like anyone will be fingering the power tubes while it's together.

    Chassis is steel; guessing anty sheild I install should also be steel & not aluminum? Or at least magnetic... Might be able to find some window screen to use, but kinda at the tail end of this so... I think as long as my amp isn't gonna explode itself I'll leave it. Cuz "nobody uses it at those settings anyway," right?

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Mine is laid out nothing like that... ;P
    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Chassis is steel; guessing anty sheild I install should also be steel & not aluminum? Or at least magnetic.
    As I suspect capacitive/electric and not magnetic coupling, any conductive material should provide adequate shielding.

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    Last edited by Helmholtz; 01-15-2020 at 09:14 PM.
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    A few random observations...

    Setting it up to squeal but then setting the guitar across the room (maybe 10' away) & no squeal. I feel like it's ground-related, though, as I can get it to screech by grabbing the tube & then if I grab the strings/controls/bridge it stops. I'm picking up a loud buzz too, mainly through the bridge pickup, that also ceases when I grab the strings. So I still feel like there's something there... And why the pickup transmits more noise when I get my hands near it or grab the bobbin is kinda annoying.

    I'm gonna use shielded cable for the Cut control runs; as for the rest of the amp, it's tight & there aren't really any long signal runs due to the tube layout. There's not really enough room to get any shielded wire in there.

    Thanks for all the ideas, & some of these I'll have to file for "future use" on build #5. Otherwise, it's clean up my mod-mess & finish the cosmetics for this guy. And explore my guitar wiring & probe my grounds.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    I feel like it's ground-related, though, as I can get it to screech by grabbing the tube & then if I grab the strings/controls/bridge it stops. I'm picking up a loud buzz too, mainly through the bridge pickup, that also ceases when I grab the strings.
    Grabbing the tube increases coupling capacitance between the plate and the PU (the human body is a conductor!) while grabbing the strings grounds your body and thus reduces coupling.
    BTW, grabbing the other power tube should not produce squeal as its plate voltage is out of phase.

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    Last edited by Helmholtz; 01-15-2020 at 11:21 PM.
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    Ah, that makes sense.

    I stuck a bit of insulation in the input jack to defeat the shorting feature, that enabled noise. I also both used shielded wire for my Cut runs, then used regular with a shield of copper tape around it. No success. Put a grounded steel shield between pickup & power tubes, no luck.

    I may pull out my Bassman & compare some things, using same speaker. Kinda wish I'd bought a scope way back when I hd money instead of buying more amps...

    Justin.

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Senior Member Pedro Vecino's Avatar
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    I don't know if I understood correctly but I would check the path and physical orientation of the OT input wires. Even disconnecting the feedback lifting a pin on the input resistor to the circuit. Sometimes rare things happen in that area.

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    Update.

    So I dug my Jet with the crappy Celestion out of the closet & rigged it up with a series of jumper wires & such. Holy $#!+ this amp was brite! After looking at about 4 different tone stacks, and 18 working hours later, I finally got pissed off enough to just make the damn thing a straight-up Fender 50W Normal channel w. a Mid control & permanently set Cut across the PI.

    I'll just say I have a useful tone stack with lots of spread & this little tird gets loud AF. Tweaked a few caps but otherwise, yeah. That was easy... Three years later. Oh well, at least I'll get a decent amount of money on this one. If I average it out over the 3 years, I think I'm up to maybe $1.50/hour.

    But the damn thing WORKS.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    ...I'll get a decent amount of money on this one. If I average it out over the 3 years, I think I'm up to maybe $1.50/hour.
    I think the most I ever made was about $5/hour. The least I ever made was probably an actual negative figure. It probably averages out to about $1.50/hour. Welcome to the industry! You've made it

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    I've had a few amps for repair where when the shielded back panel is removed they exhibit guitar-specific electrical feedback. One Fender in particular played like a theremin with hand proximity to the output tubes, but only with the volume turned down. Full up or at zero, no noise. With a different guitar, no noise at all.

    I also has a standalone tube reverb that did this and I kept coming back to that for ages before finding it was the recovery tube coupling cap at fault. Why that only fed back with certain guitars is a mystery.

    Do you have foil screening in the enclosure or back panel?
    I like that, and amp that plays like a Thermin ! I could use that at one of my gigs when we are playing the crazy part of Led Zeppelin's "Whole Lotta Love" !

    Usually I just get some feedback and harmonics,and run up the repeats and level on my digital delay, and use my whammy bar to warp the sound further.

    This would be a step up, no Joke !

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    Update.

    So I dug my Jet with the crappy Celestion out of the closet & rigged it up with a series of jumper wires & such. Holy $#!+ this amp was brite! After looking at about 4 different tone stacks, and 18 working hours later, I finally got pissed off enough to just make the damn thing a straight-up Fender 50W Normal channel w. a Mid control & permanently set Cut across the PI.

    I'll just say I have a useful tone stack with lots of spread & this little tird gets loud AF. Tweaked a few caps but otherwise, yeah. That was easy... Three years later. Oh well, at least I'll get a decent amount of money on this one. If I average it out over the 3 years, I think I'm up to maybe $1.50/hour.

    But the damn thing WORKS.

    Justin
    $1.50 and hour for doing what you love. Sounds like a fair exchange in my book !

    I've run into the same amp tone tuning problems as you before, and I've only just kicked the habit of tuning my project amps with a single Jensen test speaker. I always have to go back and change something later on when I match the amp with a much bigger "real" cabinet that has darker speakers. The single 12" Jensen C12Q on my bench is convenient as it's in a small cabinet with a small footprint, and not too loud for testing, but it also is a bit too bright for a rock amp. So I stopped being a bit lazy, and now hookup the project amp to a Well worn Twin Celestion cabinet, before I consider the tone changes done.

    Good Luck with that new Beast Justin !

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