Why do I get this spontaneous Tremolo? (And how to get rid of it?)

Discussion in 'Shock Brother's DIY Amps' started by voskarp, May 24, 2020.

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  1. voskarp

    voskarp Tele-Holic

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    This homemade "AC4" where I've combined the preamps from two different amps has a weird quirk. When the volume knob of the EF86 channel is set over 3/4 a tremolo effect starts to set in as the note fades (sample 1). Sample 2 is playing though channel two (ECC83) at low volume, with the EF86 channel volume set on max.





    I believe this effect started when I vent from version 1 to v.2. (In v.1 the gain from the EF86 channel was very low, so maybe it "was there" but never showed because it comes with high gain.)

    AC4v.1.png

    •AC4v.2c.png

    This is not a huge problem since there is no reason I need to dime the gain of the EF86 channel, it gets crunchy enough at about 1/2 anyway. But if I can get rid of it, that would be the best.
     
  2. Lucius Paisley

    Lucius Paisley Tele-Meister

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    I could follow those diagrams, but I'm not going to.

    Instead, I'm going to ask, are you sure everything is wired properly? Because that second sound sample is 1) awful, and 2) definitely sounds more like the signal cutting out than tremolo.
     
  3. voskarp

    voskarp Tele-Holic

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    Ha ha, yes. But with EF86 channel volume down a bit it sounds great. It's also just recorded with a phone next to the speaker.

    I'm quite sure I've wired it as the schematic is, since I've checked it a couple of times.

    Here are sound samples when the effect is not provoked: https://www.tdpri.com/threads/ac4-project.1006802/page-3
     
    Last edited: May 24, 2020
  4. Lucius Paisley

    Lucius Paisley Tele-Meister

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    I imagine the wiring was the first thing you checked, but sometimes we panic and forget. Also, with that much wiring inside, you hope against all hopes that it isn't the wiring or that you find the problem immediately. Pulling that all apart and having to start over is by no means the least stressful situation. Especially if the result is the same.
     
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  5. eddnix

    eddnix Tele-Meister

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    I kinda like it. It reminds me a bit of the old Vox Repeat Percussion effect. Don't think I could carry a whole set off with it though.
     
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  6. voskarp

    voskarp Tele-Holic

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    :D

    Yes, it can be kind of a cool effect, but I’d rather have it as a separate effect pedal.

    The “best” thing about the effect is that if you set it up just right, you always get the attack from the note and then the tremolo starts.
     
    Last edited: May 24, 2020
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  7. tubelectron

    tubelectron Tele-Afflicted

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    Me too, I find it a good sounding effect I would call it "Ending Tremolo"... :cool:

    If I could discover the cause, I would make it switchable or footswitchable ! :cool:

    I'd say it's an unwanted low frequency coupling of some sort, or a relaxation effect... o_O

    -tbln
     
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  8. voskarp

    voskarp Tele-Holic

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    OK, but what does that mean?
     
  9. EsquireBoy

    EsquireBoy Tele-Afflicted

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    It reminds me of the bias tremolo of my 6G2 Brown Princeton that has this quality too: the more you push the amp, the less tremolo you get at the initial attack.
    Maybe it would make sense to investigate around that?
     
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  10. tubegeek

    tubegeek Tele-Afflicted

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    In practical terms, it means you (maybe?) don't have enough power supply decoupling between several gain stages, and the power supply gets unstable.

    Notice how on most amp schematics, only one or two stages will be fed by the same spot in the power supply. Then more stages will have an R & C filter before they get a stage or two hung off them.

    That 2nd schematic looks ok though, with B+ 2 and B+ 3.
     
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  11. voskarp

    voskarp Tele-Holic

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    I’ll look at that schematic.
     
  12. voskarp

    voskarp Tele-Holic

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    Maybe I better move the EF86 to B+3, so it doesn't share the same power supply as the second half of the ECC83. I guess I'd have to reduce the R25 in that case.
     
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  13. tubegeek

    tubegeek Tele-Afflicted

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    Worth a try. Maybe 10K maybe don't bother.
     
  14. tubelectron

    tubelectron Tele-Afflicted

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    It means that this unwanted coupling or relaxation effect transforms your amplifier stage in an oscillator stage - here it is a LFO (Low Frequency Oscillator - exactly like a tremolo oscillator)

    Well, it's always difficult to tell the cause at distance... I don't see anything wrong in both of your schematics that could do that LFO behavior.

    I would suspect the screen of the EF86 though, with its 5M resistor plus SW2 to be part of a kind of LFO RC network, but I may be wrong, OK ?

    It may be interesting to test a resistor divider instead of a so high serial resistor value which, coupled to C2A or C2B, may be responsible of the relaxation effect.

    This screen connection can be very sensitive to induction or capacitive couplings, and if you have deported SW2 on the front face with some distance, it can be a possible source of oscillation.

    Several other ideas / tests, amp open and on, primarily :

    1 - with your amp at the "LFO issue" setting, try to move with a pen the SW2 connections, the EF86 screen connection : any changes in frequency or sudden stop ?

    2 - with your amp at the "LFO issue" setting, remove the EF86 : does the unwanted trem effect still continues ?

    3 - same as above, but remove the 12AX7 instead of the EF86 : same question ?

    4 - switching SW2 changes the LFO frequency ? Or makes the issue disappear ?

    5 - try to GND directly the grid of the EF86 at the nearest GND point : the problem ceases ?

    6 - shunt the 5M resistor with a 1 to 2M2 (not critical) : the frequency varies ? The problem ceases ?

    Or maybe there is some connections somewhere in your wiring that are too close, or something like this ? We do not have pictures of you wiring work to have an idea, @voskarp.

    Let us know ! ;)

    -tbln
     
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  15. voskarp

    voskarp Tele-Holic

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    I moved the EF86 power supply to B+3 (and changed R25 to 10k), and voilà! -tremolo effect gone!

    Many thanks for all the other suggestions too, I'm glad I could solve this without tearing the whole amp apart! :)
     
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  16. tubegeek

    tubegeek Tele-Afflicted

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    Niiiice!
     
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