Sunn Concert bass thermal shutdown

Discussion in 'Amps and Cabs [BG]' started by john m, Aug 18, 2017.

  1. john m

    john m Supporting Member

    Jan 15, 2006
    I just got a Sunn Conceret bass head-- red knob-- about circa 1976.

    Cleaned up some scratchy pots and played it for about 1/2 hour. It had more audible 60 hz hum than I remember my old (silver face) one had in the early 70's. The amp ran pretty hot with an 8 ohm load, and went into thermal shutdown after 1/2 hour of light jamming with Youtube videos. It did reset after it cooled down.

    I suspect it needs power supply caps. My question: could the constant 60 hz hum from the leaky caps be the cause (I think yes).

    Anyone else experience this with old ss lead sleds?
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Unlikely that power supply caps caused this. What needs to be done is a thorough analysis of the operating points of the amp. This will determine what is actually going on with the amp.

    An older tech familiar with single ended amp topologies might be a good idea (if I'm remembering correctly) because there are some unique failure modes that are common to this type of amp. Output symmetry and DC leakage can greatly change the effective bias of any stage within the amp. This is different than just setting the bias.
    BadExample and Passinwind like this.
  3. john m

    john m Supporting Member

    Jan 15, 2006
    Thank you aged horse.
    Stumbo and agedhorse like this.
  4. CrashAlpha


    May 16, 2012
    Funny, I stumbled on this thread. I came to find info on this issue. My silver face model had a plate on the back that explicitly stated thst if you don't run 4 ohms the amp will overheat and shutdown. Just plugged it into a Hartke 4x10 cab tonight, first time I've used it in years. What a monster this amp is! But running it at 8 ohms it does suffer after an hour, losing its crisp cold tone for something warmer, compressed and bland. I have run it on hot days like that and it will go past that point, start to distort then shut down... I thought it was just old because I had never noticed the warning plate. I also had to replace my leaky caps.
    Stumbo likes this.