Amp Started Sounding Terrible. Help.

Discussion in 'The Workbench' started by mAx___, Mar 26, 2019.

  1. mAx___

    mAx___ Active Member

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    100w Major build. Played all weekend, loved the sound, better than ever after changing mic positions.
    Tonight after work it still sounded good, then, almost without realising, playing a B note on the sixth string with the neck pickup had this horrible, bassy, muddy, boomy sound. Couldn't believe my ears.

    I checked the circuit, poked around with a wooden stick, found no loose connections in the preamp and pots. Turned the Mains back ON and when I turned Standby ON there was a loud spark but the amp was still working and there was no smoke. Turned it off and when I checked underneath the board I saw the wire that goes from the rectifier diodes to the first electrolytic hanging by a thread. Replaced the wire thinking it was going to fix the problem. But it didn't. The muddy bass is still there. Highs were afffected less but overall the sound of the amp is horrible compared to its usual self.

    What should I try next?
     
  2. neikeel

    neikeel Well-Known Member

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    Probably a short inside the filter cap - check that first.
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The fuse should have blown long before the wire burned. There should be some kind of wiring error causing this to happen.
    Impedance mismatch would make the high voltage arc inside the amp usually at the tube sockets.
     
  4. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    One would think why was the "wire hanging by a thread". What condition caused that? Was the "wire hanging location" where the arc happened, possibly burning most of that wire, or where did the arc happen?

    And are the diodes still in serviceable condition, and what are the voltage readings after the diodes. Is there arcing on the power tubes?
     
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  5. chuckharmonjr

    chuckharmonjr Well-Known Member

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    Kinda sounds like you managed to let the magic smoke out
     
  6. mAx___

    mAx___ Active Member

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    I recently had changed cap values. In lifting the board I might have weakend the wire somehow. Yes, it’s where the arc happened. Have yet to measure the voltages. No arcing on the power tubes.
     
    Last edited: Mar 26, 2019
  7. mAx___

    mAx___ Active Member

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    Thanks. Should I replace the cap and find out or is there another method to check if it is shorted?
     
  8. neikeel

    neikeel Well-Known Member

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    Isolate cap from wiring an measure resistance to ground and between terminals, anything between terminals in M ohms it is dead and if you get low resistance or short to ground with either terminal it is dead.
     
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  9. mAx___

    mAx___ Active Member

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    Thanks. Both filter caps tested OK. Voltages are normal. Bias is normal. Lifted the whole board to check for any faulty connections underneath and found none. Replaced valves, tried a different guitar. Nothing. The problem persists. I'm at a total loss. :(

    Sound sample. Treble 10, Mids and Bass 0.

     
    Last edited: Mar 26, 2019
  10. NewReligion

    NewReligion Well-Known Member VIP Member

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    This exactly.
     
  11. NewReligion

    NewReligion Well-Known Member VIP Member

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    Does this occur on both Channel I & Channel II?

    As Guitar Rocker stated, there appears to be an issue in the rectification circuit. (Where AC is converted to DC). When in doubt change it out. Be Careful Here! This is where people get lit up!

    Then go back and visually inspect the circuit in a pattern to ensure you cover everything. This works for me often. Try a plastic dental mirror (Wal-Mart) & powerful light (phone) to see beneath caps, resistors, board etc...

    Check all capacitors while powered on to see if DC is passing through as their job is to stop DC from passing through. If you find DC passing through after powered on for 30 seconds replace the cap.

    Bass & Mids on O with Treble on 10 is unheard of. I would scrutinize the tone stack and Pots.

    Exchange all tubes or get one power tube and one preamp tube to “roll” through the sockets. If you find the Poeer Tudes are bad replace and bias. Usually you lose bass when tubes start wearing out.

    I will scratch my head on this one and check back in. Listen to Guitar Rocker. The man knows his circuits.

    David
     
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  12. mAx___

    mAx___ Active Member

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    Thanks David. Yes, the problem happens in both channels.
    I checked voltages in the diodes and they were normal. I have some extra ones, would it help to replace the diodes in the circuit just in case?
    Bias voltage seems normal too. Can I assume the capacitors in the rectification circuit are OK? I don't have replacements for those, would have to order some if it's worth it.

    I'll check for DC passing through all capacitors as you suggested. With reference to ground?
    To be sure, I should be reading 0 direct current on one end of the capacitor, correct? If not, I should replace.
     
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  13. NewReligion

    NewReligion Well-Known Member VIP Member

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    Correct on the cap inspections. Remember. When in doubt, change it out.

    Helpful hints.

    1.) When removing components remove standoff nuts. Loosen the pots for removal allowing the PCB to be raised at a 45* angle for easy access to the bottom of the PCB (Prop up with correct length of chop stick which have many uses). Clip old components from the top leaving room to push legs from the top through the bottom as stock components are bent. If you try to pull it through the top you will eventually delaminate a trace(s).

    2.) Though it would be nice to know how to isolate the area having 2 amps with effect loops helps. I pull power tubes on one amp to effectively remove and protect the OT. I run that amp out of loop send to return of the second amp. And reverse the process. This will tell you which half of the amp the issue is located.

    3.) Be SAFE. This can be a lot of fun and take notes as you will learn much along the way.

    Welcome to amp mod 101. :)

    I hope this helps.

    David
     
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  14. mAx___

    mAx___ Active Member

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    Great advice, thanks again David.
    Another question on the caps inspection: I'm assuming it includes preamp caps too, or is it only power supply and bias caps?
     
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  15. NewReligion

    NewReligion Well-Known Member VIP Member

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    Yes check your coupling caps while you are in there. They are early in the circuit. They will affect the frequency exponentially.

    David
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Just throwing this out there. It would help to know your measurements and where they were taken. Everyone is flying blind.
     
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  17. NewReligion

    NewReligion Well-Known Member VIP Member

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    While you are at it. Pick up some evaporating non oil electronic cleaner. Use it on all pots rotate a few times, tube sockets- insert a couple time and all jacks.

    Inspect (look) the power tube sockets for proof of carbon from arcing.
     
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  18. mAx___

    mAx___ Active Member

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    You are right. Will post more detailed voltages tonight.
     
  19. mAx___

    mAx___ Active Member

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    Late night report:

    I inspected all the caps and they all checked fine. To remove all doubt I replaced 3 of the four filter caps. Problem persisted.
    I replaced silver mica caps in the preamp just in case one had gone bad.
    Measured the voltages and they all are within normal ranges:

    AC where the PT enters the bias circuit: 106VAC
    AC at the diodes that connect to the Standby switch: 553VAC
    DC at the diodes that connect to the first filter cap: 497V

    Valve voltages:

    V1P1: 123V ECC83
    V1P3: 0.88V
    V1P6: 162V
    V1P8: 1.64V

    V2P1: 134V ECC83
    V2P3: 0.65V
    V2P6: 205V
    V2P7: 3.65V <-- seems low but it's the way it's always been with this amp.
    V2P8: 58.2

    V3P1: 265V ECC82
    V3P3: 12.41V
    V3P6: 242V
    V3P8: 12.41V

    I checked the sockets for signs of arcing and found none. Didn't use contact cleaner on the pots and jacks because the one I have is also a lubricant, got to get a non-oily one.

    What should be my next move short of rebuilding the preamp section? :(
    Can a problem in the OT create the boomy sound in the sample that I posted previously?

    As a reminder, the problem appeared from one moment to the next while I was playing. So the amp was working fine when I just turned it on and got bad after 15 minutes or so of playing.
    Thanks so much for your help.
     
  20. neikeel

    neikeel Well-Known Member

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    Worrying thing is that your problem resulted in a melted DC feed wire between rectifier and first filter cap.
    Presume that has not happened again?
    Where is your HT fuse in relation to these?
    Maybe time to add a bundle of pics as Mickey said we are all flying a little blind.
     
    Last edited: Mar 28, 2019

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