I want controlled feedback from my amp but all I get is whistling

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sizzlemeister, Feb 26, 2015.

  1. sizzlemeister

    sizzlemeister Member

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    AMP IN QUESTION: SF Fender Bassman with 1x12 4 ohm

    MODS: The output section is AA864, except the bias circuit wasn't fully rendered to AA864 specs, it still operates as a bias balance. The bias cap is a 47uf/100v. I'm going to have to sort this out, although it does bias fine, cold-ish, but fine.

    The bass channel has been modified to be guitar friendly, although I forgot what that was exactly. I can pull out the chassis and take pics if necessary.

    PROBLEM: Everytime I get close to the amp to attempt controlled feedback, all I get is whistling.

    At this point I assume there is something not right with the circuit because my old Bassman and Bandmaster were sublime at controlled feedback.

    I'm using either a VFE The Scream or Skreddy Lunar Module (or both) to help invoke feedback.

    MORE: Whatever it is you need to help me fix this, let me know! I will take pics, measure things, whatever.
     
  2. RiftAmps

    RiftAmps Member

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    Whistling? Like big microphonic feedback or subtle wind noises?

    Check V1 + V2, try replacement tubes there first
     
  3. sizzlemeister

    sizzlemeister Member

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    Neither type of whistling. It's a higher-pitched whistle that doesn't vary in frequency or amplitude (relative to the volume of the amp - meaning it isn't louder than what the amp is set at). It's more like it wants to feedback, but the pitch is high, and instead engages in the whistle, and it doesn't "fade in and out" as you decrease or increase your distance from the speaker.

    I will try swapping preamp tubes. Thanks for the suggestion!
     
  4. Bruce Gerard

    Bruce Gerard Member

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    Have you tried another guitar on it to see if the problem persists?

    If you mute the strings with your hand - does the oscillation stop?

    How close is the instrument to the amp when the oscillation starts?

    Unplug the speaker & use an extension cab at least 4 ft away and turned AWAY from the amp / instrument. See if the amp still oscillates when the guitar is near the amp. If it does, the pickups are inductively coupling to the amp and shielding of the amp or circuits will be necessary to counter the problem.
     
  5. TimmyP

    TimmyP Member

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    I think it's likely either:

    1) The proximity of the pickups and the output stage. A number of amps will do this, especially with single coils. Adjust your position relative to the head. If no joy, put the cabinet on a stand and the head on the floor.

    2) The proximity of the pickups and the voice coil - the radiated magnetic field is 'hitting the pickups harder' than the acoustical vibrations are 'hitting the guitar', so the magnetic field feeds back first. Adjust your position relative to the speaker. If no joy, play louder (and make your mixperson mad), so the acoustical feedback happens first .
     
  6. J M Fahey

    J M Fahey Member

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    Looks like a guitar and excess gain problem, rather than an amp problem.
     
  7. sizzlemeister

    sizzlemeister Member

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    TimmyP and Bruce are definitely in the ballpark, if not close to home.

    So, with three Teles, here's what happens:

    Tele 1 - main axe - whistling, or high-pitched, aggressive oscillation, sets in nearly immediately. If I change my orientation to the amp (the Bassman head is built into a combo), I can get feedback with the volume above 5, but the whistling is there as well - it's a fight.

    This guitar has a standard Telecaster configuration, but with hot pickups that have over-sized pole pieces (a set of THESE).

    Tele 2 - backup tele. Some whistling, but no where near as severe. With the amp volume at 5 I get controlled feedback.

    This guitar has a standard Telecaster configuration, with a custom wound Strat pickup 8.5k neck pickup and CV 50 pickup 9k in the bridge.

    Tele 3 - a p90 Black Dove. This performs as expected (meaning I get controlled feedback).

    So, is there a way to make Tele 1 perform better.
     
    Last edited: Feb 27, 2015
  8. TimmyP

    TimmyP Member

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    See what happens when you roll back the tone control on each of those guitars that whistle - it could be that the pickups in those have an excessively wide bandwidth. Also try a cable of another brand to see if that changes the response enough to solve the problem.
     
  9. Ronsonic

    Ronsonic Member

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    Try pressing on a front corner of the tele bridge to see if that kills the squeel.

    Tele's are a bit infamous for this. The mounting screws are near the back of the bridge plate and the front can vibrate causing squealing. There's a metal plate on the bottom of the p'up that can get loose and not all tele pickups were potted.

    I suspect your bridge.
     
  10. sizzlemeister

    sizzlemeister Member

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    I'll try the tone, but with regards to the cable, I have two from different vendors that I used and the results were the same.

    With regards to the bridge; it's amazing the details we neglect to consider important! Tele 1 has a Hip Shot B bender installed with the G bender and drop D options. Sorry, it never occurred to me to mention that detail.
     
  11. tlpruitt

    tlpruitt Member

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    I agree. Teles can squeal with high gain and loud amps for all the reasons listed here. Bridge plate not mounted firmly and completely flat against the body and/or the metal plate on the bottom of the bridge pickup being loose are common causes.
     
  12. Geetarpicker

    Geetarpicker Member

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    As long as you know your muting technique isn't the issue its probably a pickup potting or lack of it problem. That said some folks really resist potting pickups in wax as some claim it can hurt the tone. If the squeal is very high pitched it truly sounds like a pickup issue but like mentioned you should first check other details. You might even try removing the pickup from the guitar, and by using alligator leads attach it to the guitar cable and amp and see if the feedback is similar. At that point you have isolated the issue down to the pickup itself, and if it still feedbacks back...there you go. I will mention most pickups will be just a bit more microphonic when installed in the guitar (especially the deeper bridge pickup route area) even if everything is tight. Still, if the pickup squeals easily when removed from the guitar you have to first fix that.

    I to often use controlled feedback (with either a Trainwreck Express amp, or with various vintage Fender amps by adding gain with pedals) and in my case ALL my guitars have potted pickups. Sometimes a pickup may only need a little wax added between the coil slugs and the bottom plate if it uses such a plate, but in many cases the coil needs full potting.

    Like I mention some folks don't like the tonal difference potting can have on a pickup. Still, I have some Lindy Fralin Strat pickups that are fully potted and sound great. Actually the Fralins come potted, but I typically have to redo the potting to really keep things controlled with them.

    I have noticed that sometimes you can get oscillation directly off the amp head itself, so you might also try putting the amp head away from the cab (like on the floor not stacked) and recheck for feedback. Also keep in mind that the less treble you can stand in your EQ settings the less problems you may have. But if you have to EQ the amp totally muddy to stop feedback issues, well it's probably time to investigate the pickups...
     
  13. corn husk bag

    corn husk bag Silver Supporting Member

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    Ken,
    Thanks for your input hear. I still love going and listening to your tones on your youtube channel. You are a monster player!



    Still pull out the cd regularly and give it a listen!
    Kind Regards,
    Steve
     
  14. sizzlemeister

    sizzlemeister Member

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    This has been an enlightening thread. I am somewhat disappointed as these pickups are supposed to be wax potted. Still have to investigate all of the possibilities, though, but I'm not entirely convinced it's a guitar issue.
     
    Last edited: Feb 28, 2015
  15. walterw

    walterw Gold Supporting Member

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    potted or not, they're still tele pickups, maybe with metal bottom plates, mounted in metal bridges that themselves may not be solidly resting on the face of the guitar.

    all that metal creates sound with any vibration, and that can lead to the uncontrollable squeal at higher gain.

    (as i understand it, part of the impetus for the strat's original use of plastic pickup covers mounted in a plastic pickguard was to avoid this issue.)
     
  16. sizzlemeister

    sizzlemeister Member

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    Thanks, walterw, I think that helps narrow it down, along with the input of the other folks in the thread.

    Here are the results of further experimentation:

    First, the two standard Teles only: both produced equal results this time, so I have to assume I must of had the second Tele's tone pot down when I was experimenting with it the other day.

    The reason I say that is because with the Bassman, following TimmyP's suggestion, I discovered that if I rolled back the tone, the whistling reduced relative to the amount of tone rolled off. If I rolled the tone all the way back, the whistling went away completely and I was able to get my feedback the way I wanted it. This occurred for both guitars AND for both the neck and bridge pickups.

    I also have a modified Musicmaster Bass amp here (12" speaker, upgraded output and power transformers, etc.), putting out about 15 watts (thanks to the mods). Cranking that, and with only a slight tone reduction on both guitars, I got excellent controlled feedback.

    FWIW, pushing/touching/manipulating the bridge plates and the pickups had zero effect. I put together both guitars myself so this is not surprising (a gap or loose bridge plate would drive me nuts), but certainly worth trying. I am still open to the idea that the bottom of the pickups may contribute to unwanted oscillations, but I should note that, again, both sets of pickups produced pretty much the exact same results between both amplifiers with the only difference being that the hotter set of pickups had a more aggressive whistling.

    To keep this short(ish), I wound up changing out the 12ax7s in slots V1 and V3 of the Bassman. This improved the situation, and allowed me to get controllable feedback at around 4 on the volume on each channel IF I rolled the tone back more than half-way on the Teles. I still should try changing V2 on the Bassman, but I think I need to get a good set of tubes to make it worth while at this point.

    I believe, now, that there is a combination of elements working against my goal, here: tubes, pickups, and I'm going to add the speaker to this, because I think I don't like the Jensen I have in the Bassman (okay, maybe not the speaker, but it gives me a reason to swap it for something else).

    I've eliminated the bridge plate (and Hip Shot) as a factor. The whistling occurs with both bridge and neck pickups in both guitars so there needs to be something common to both designs that is a contributor - and I know the fat pole piece pickups are wax potted, along with the Strat pickup. I can't remember if the cv50 pickup is potted or not.

    Other than a preamp tube upgrade and a new speaker (wink, wink), what I'm left with is thinking my Teles' tone pots are too large a value, but they're both 250k, which is on the lower end. This might not be the way to go. I've read elsewhere that you can pack foam in the pickup cavities to help reduce squeal. Might that help?
     
  17. walterw

    walterw Gold Supporting Member

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    aha!

    yeah, i've seen this before, my old marshall 2203 did it; the pickups are triggering a whistling oscillation in the amp, the brighter the pickup sound the quicker it kicks in (which is why you can cut it on or off with the guitar tone knob). as i recall, single coils would trigger it but humbuckers really wouldn't.

    i think the fix in my case ended up being shielding the signal from the input jack to the first preamp tube, something like that.
     
  18. sizzlemeister

    sizzlemeister Member

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    I got it. In other words, I need a shielded cable to run from the input jacks to the tubes (plural since it's a Bassman), and ground just one end of the shielding. Right?
     
  19. walterw

    walterw Gold Supporting Member

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    maybe!

    it was many years ago, i'm not the one who actually did the repair, and it's a totally different amp than what you have.

    it's just that i remember the issue seemed to be right "up front", around the input jack/first tube/preamp gain pot area; we might have "shotgunned" a few of those parts including the pot before it went away.
     
  20. sizzlemeister

    sizzlemeister Member

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    I did a little research on parasitic oscillations. I'm not sure if this circumstance applies, but it seems like a good place to start.

    I suspect, since both channels experience the same phenomena, that the issue is post preamp stages. Maybe in the PI, maybe right at the power amp section.

    Some possible solutions: check lead dress, snubber caps, walterw's solution above and a variant I saw, I think from Stokes, where you wrap a wire around one of the others from the PI tube (I have to look up details, I mention it off-the-cuff in case someone recognizes it).

    Anyone have any further insight?
     

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