'66 Super Reverb Clipping Issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Troy T. Blues, Feb 1, 2008.

  1. Troy T. Blues

    Troy T. Blues Comstock Station

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    My '66 Super makes a hard clipping sound when hitting the bass strings hard..pick attack. It happens at pretty normal levels.

    I brought it to my tech and there are no obvious things wrong in the circuit other than going through each individual component by process of elimination. I've tried the obvious things as well like changing tubes, etc, etc. The speakers were also a possibility, so I took the amp home. I also have a Super RI, so tested it by going through the '66 into the RI's speakers and visa-versa. It's been confirmed that it is not the speakers.

    Any ideas about what this could be? Your help would be greatly appreciated. Thank You! :AOK
     
  2. SouthernShred

    SouthernShred Member

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    Does it sound like this at all?
    http://www.youtube.com/v/FZ4ztdQ6RTQ

    That's what MY '66 is doing...dunno if you're having a similar issue. Todd Shock (Wakarusa) is my tech and he suggested a plate load resistor(s) could be bad
     
  3. Troy T. Blues

    Troy T. Blues Comstock Station

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    LOL...I'm at work so I had to listen to it on my Phone...but nope, that's not it...The clipping sound only happens when I strike the strings. On this You Tube example, you hear it when the note(s) are ringing/decaying. The only way I can try to explain is....Like shuffling a deck of cards with your two thumbs as it strikes the the surface..In this case, when the strings are hit.

     
  4. SouthernShred

    SouthernShred Member

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    Have you unplugged the reverb tank? Checked your tubes?

    Thanks for listening to the video...that's my Super going haywire. :)
     
  5. Pete Cage

    Pete Cage Member

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    Try something: Pull the 12AT7 reverb driver tube and see if it still does it.

    Problems in the reverb driver circuit (including a bad reverb transformer, poor connections to the tank, etc.) will make produce those symptoms.

    My $0.02,
    PC
     
  6. Troy T. Blues

    Troy T. Blues Comstock Station

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    Oh that's you? Thanks for that. I have checked the tubes, but I have not unplugged the reverb tank. I think I tried it through the normal channel and it did the same thing, but I will have to reconfirm that tonight.
     
  7. mark norwine

    mark norwine Member

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    See, I just can't square these statements.

    Hard clipping stands out on a scope like a pig at a ballet. Judicious probing will show where the issue is [or at least which stage it's in] in about 4 minutes. From there, centering in on the issue / component isn't really very hard.

    Does your tech have a scope? Did he use it?
     
  8. Troy T. Blues

    Troy T. Blues Comstock Station

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    Yes he did and could not find anything abnormal.
     
  9. phsyconoodler

    phsyconoodler Member

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    If the amp has carbon resistors on the power tube sockets,those need to be changed right away.And if the plate resistors on the preamp side are original,those need changing too.That will eliminate those as a possible source right off the bat,and should be done anyway.
    It may be a parasitic oscillation and that should show up on the scope when playing the amp.It may not show up with a sine wave.
    The 1.5k grid stopper resistors on the power tubes may be suspect.Again,if they are carbon,get them out of there and pop in a some metal film 1/2 watters.
     
  10. Troy T. Blues

    Troy T. Blues Comstock Station

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    Thank you....I pass that along.

    (edit) Response from the tech: all of those components mentioned had been replaced, but it doesn’t mean that they're still good.
     
  11. Blue Strat

    Blue Strat Member

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    Without actually hearing what it's doing, I don't know what you mean by "clipping". People who've listened to an amp while observing clipping on a scope know what "clipping" sounds like. Others are guessing. I suspect that you're guessing.

    With what little I know about this amp so far, I'd start looking for intermittent connections on the board and in the wiring. With the amp on the bench and powered up I would start banging around, gently, to see if anything is intermittent.

    I'd also check all the ground connections to the buss bar by pulling on them.
     
  12. GuitarsFromMars

    GuitarsFromMars Member

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    you have experts in Mark and Blue Strat here,I will stay out of this.
     
  13. Troy T. Blues

    Troy T. Blues Comstock Station

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    Hi Mike!

    Like I said, the best way I could describe this type of "clipping" is - it sounds like when your shuffling a deck of cards with your two thumbs and the clipping sound would sound exactly like when the cards strike the surface of a table..In this case, when the strings are hit....pick attack.

    I can't describe any better than that.
     
  14. Blue Strat

    Blue Strat Member

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    Still not sure I'm hearing it correctly;)

    Does it do this at any volume? Only above a certain volume? Both channels? Do you hear anything unusual when applying "percussive maintenance" (banging on the top of the amp with the amp on)?

    Remove all preamp tubes except V1 and V6 and play through the normal channel. Still do it?
     
  15. Troy T. Blues

    Troy T. Blues Comstock Station

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    Thanks again Mike.

    I will try and re-confirm all that tonight when I get home.
     
  16. Troy T. Blues

    Troy T. Blues Comstock Station

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    Pete, you got it my friend. I pulled the 12AT7 (reverb driver tube - V3) and no clipping. Also no clipping when I disconnected the reverb tank, even with the tube in. Have I actually isolated the problem?

    Thanks so much!
     
  17. Pete Cage

    Pete Cage Member

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    Ah, good news. You've narrowed it down.

    Make sure the cable and connections on the 'send' side of the reverb driver are all good. Be sure to get inside the jacks on the amp and the tank and snug up the contacts for the center conductor as well as slightly crimping in the shield connections on the cable. Check the cable for good continuity. Re-tension the tube socket contacts.

    If everything is tight and solid, try a different 12AT7. If it's not that, try hanging a 500pF cap between the plate and the cathode like the SF amps.

    If all that still doesn't cure it, it's probably an arcing reverb transformer. They can die from being run into an open load just like a regular OT. Fortunately, they're cheap.

    Let your tech do the work if you're not 100% confident around tube amps and high voltages.

    PC
     
  18. Troy T. Blues

    Troy T. Blues Comstock Station

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    Awesome!!! :BEER

    I'll let you know how it goes....Thanks a million for your help Pete!! :AOK
     
  19. Blue Strat

    Blue Strat Member

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    I had one like this one time. Turned out NOTHING worked (the problem would occur with or without cables, with a new reverb transformer, cap was stock) except finding the "right" 12AT7.
     
  20. Troy T. Blues

    Troy T. Blues Comstock Station

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    Hi Mike,

    I don't wanna jump the gun, but I think you have something there. I only had a brief time to double check that this morning and it looks like that was the case. Out of eight 12AT7's, only three did the trick. However, I have to reconfirm that later. Out of eight, my two Telefunkens worked as well as an old Sylvania. The others that didn't work were (surprisingly) the Mullard CV4024's & Jan Phillips. I will report back later, so far, so good! I wonder why only some (possibly) fiixed the problem? Either way, I'm thrilled if the problem is actually fixed!!!!! Thanks so much for your help!
     

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