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Sunn Concert Bass - Oscillation

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  • Sunn Concert Bass - Oscillation

    Hey fellow techs,
    The symptom is not just a constant oscillation but when you hit a note and them mute it, that's when you get a short spurt of the oscillation. Just sounds like a very short buzz. Easy to see on the scope. Almost doesn't occur when you drive the amp with an oscillator or consistent source.
    I looked through all the Sunn Concert Bass or Lead entries for this symptom & did not find it, however I did find the schematic that a tech posted on this forum that included the parts list as well as a output stability mod that Sunn put out in '73. I was about to just bridge some disc caps across the B-C of the driver xistors when I stumbled on these mods. Thanx to whomever posted that. Can't find it now.
    I guess even in '73 there must have been some higher frequency response T03 power transistors that had a higher freq response then the orig 2N3055's which is usually the cause of these type of issues.

    I guess the point of this post is to enlighten some of the existence of the mod as well as the fact that it actually works. The mod had you also removing the insulating washers from the speaker jacks but on this version that actually shorts the output to ground, so just be aware there are different versions of this amp out there. I didn't swap the + and - of the speakers in order to be able to ground the jacks as that would have also changed the input to output phase relation which I didn't think would be wise in the long run.
    They also suggested removing C305 in a 3 state pi filter in the power supply which I did not do.
    Attached Files

  • #2
    Does Q1 drive Q2 -5 through T2 ?


    • #3
      Yes. All those individual T2 notations are just sections of the same transformer.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        This reminds me of problems I had with a Vox amp (can't recall which model) that needed its 2N3055s replacing. I was certain I'd repaired it - the transistors came from RS components - but I still had problems. I'd read (and then dismissed the idea in my own mind) that the original spec devices needed to be used. After getting nowhere, I replaced the transistors with some old but good 'pulls' as a last resort and these worked perfectly - no overheating or oscillation.