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Sunn Concert Lead Thoughts

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  • Sunn Concert Lead Thoughts

    I have now worked on (rebuilt) 3 silver face Concert Leads and am on my second Red Knob Concert Lead all of which have suffered significant heat events on the Power Amp boards. I have worked and built amps over years (mainly tubes) and it is obvious to me that Sunn transistor amps were under designed especially now that modern AC Volts are 120+ with 123VAC common. (I worked on one 3 years ago and rebuilt power section and it has been gigged 3 years.... I have worked on the rest this year and I forgot about the foot switch and fortunately I found my photos and notes from first one I worked on- I take picts and make drawings of all amps I work on - it helps me....)

    When I hook my Variac to the power and dial in 110VAC I get the voltage readings the schematics claim; at 120 VAC the voltages are at least 5VAC higher in circuit making some CAPS rated at 50V handling 40+. For 20% components this will cause failure in relatively short time. The early silver face amps used under rated resistors that were later upgraded to higher wattage in the red knob versions.

    The 2N3055's are 60v rated which is low and I now use 140v Output transistors. I also now go thru and rebuild the power amp boards with higher power rated resistors and caps and I have had great success with one amp gigged every day for 3 years now. The process isn't expensive but is time consuming but the payoff is reliability and sound. Yes I replace carbon comps with metal film and I suggest you try it and don't drink the cool-aid.

    I have always done this for old Fenders, changing bias caps and diodes on solid state because of the voltage change but old transistor amps seem more affected.....

    Anyone else have thoughts on this?

  • #2
    by the way-- it looks like two of the Concert Leads I've worked on had switch problems and were miss-wired by someone. The last one worked but had low volume and when I removed transistor covers I found that one of the transistors was missing the screw inside the cover that grounds the case/collector... so I guess they didn't realize Sunn only used a single outer screw for the black cover and the inner screw on the other side was the case ground!
    Last edited by elektron; 08-21-2015, 07:22 PM.