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1971 Sonaro overvoltage?

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  • 1971 Sonaro overvoltage?

    First post here! I love old tube amps ! I have a basic understanding but am by no means a "tech". I have a Sunn Sonaro Head and 1x16 cab, that was left at my house about 20yrs. ago. They are very pristine in condition, and couldn't have been out very much.
    I am having "rock might" at my house and am supplying backline.
    The head is a 71' and it had two missmatched 6550's in it. I replaced all the electrolytic caps, and put in new power tubes ( from the old Svetlana co.) On powering it up I noticed that the power tube plates were beginning to glow red, in order to stop the glow I had to turn up the bias voltage all the way( the pot pegged)
    The readings I got are: pin 3 ( plate) 540vdc, pin 4 grid 541vdc, pin 5 bias -71 and via transformer shunt ( I think! ) 41.3ma current. This high voltage appears right away at the first filter junction.
    I don't have a schematic for this amp, and the only one I could find that seemed close was for a Solaris, but this amp has a solid state rectifier. I checked all the components I could and they all seem ok ( not so good at checking caps) I am guessing that this voltage is too high, and am wondering if the power transformer is bad, and or If I could drop some voltage with a dropping diode and still use this amp, or maybe just use it "as is" and keep my fingers crossed.
    Sorry for the long post, but I wanted to to introduce myself, so no one would think I am more qualified as a tech.
    Thanks so much, and I look forward to any advise you may be able to share with me!

  • #2
    6550 will take 100 volts over what you are showing them right now, so 540vdc is not a problem for them. What is your heater voltage? Should be 6.3 to 6.9VAC from pin 2 of a 6550 to pin 7, big current here so make connections with the amp unplugged from the wall, use meter clips & double check that there's no chance of shorts before powering up).

    Your plate voltage will drop a little if you bias the power tubes hotter, say 50mA per tube.


    • #3
      Those Svetlana 6550's run pretty hot in those old Sunns, but the plates shouldn't be glowing at 40mA. Can the caps you used stand the 540V? Most of the amps I've seen with solid state rectifiers had 20uF @ 600V caps on either side of the choke. These amps have Pi filtering for the B+ and the screens are connected to taps on the output transformer.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !


      • #4
        Thanks so much for jumping in guys!
        As suggested, I checked the heaters and they read 6.38vac~
        So that seems good.
        As far as biasing the tubes hotter, that is the problem.
        I get a plate glow, and the stripe just disappears when I have the bias full up ( reading -71vdc/ 41.3 ma: no glow) ) If begin to decrease the bias voltage ( is that correct, as it is minus?) ,increasing the current, the red stripe begins to show up on the tubes.
        So I seem to pretty much be stuck with what I have for current and voltages.. This is the only Sunn amp I have ever had and this is also the first amp with 6550's ( I have the old Svetlana not the present day version, 6L6's in my Super Reverb and I run them pretty hot and they sound great and have lasted, so years ago I bought the 6550's but am only just now using them)

        So I get no plate glow at around 40ma, but I can't go much hotter without causing glow.
        I replaced the two caps you mentioned loudthud, with Sprague 600v caps
        The amp right now is quiet ( much quieter than my Fenders!) unless I turn the bass way up, then a "bass hum" appears.
        What I would really like to know is if this high voltage is typical for a Sunn amp? if so ok, but if not would I still be ok to use the amp as long as i keep it as is ( no tube glow but perhaps not ideal bias) ??
        I was concerned that the power transformer might be "over-voltage"or otherwise pooched
        I can't thank you enough for the help


        • #5
          With today's line voltage, 540V is about right. Be sure to use the standby switch to protect the preamp caps from seeing that 540V before the tubes warm up. Many people use KT88's although it's debatable if there is any real difference in construction these days. Those amps don't have any screen resistors. Installing a 1K 5W resistor in series with each screen grid would reduce power slightly but increase tube life and protect the output transformer.
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !


          • #6
            Thanks SO much,
            That's just what I needed to know!!!!!!
            I have a small fleet of old Fenders and one funky little Gretsch single ended amp, but I am not used to seeing this kind of voltage!
            "Rock night" at my house tonight with the Old Phahts, so it will get it's first workout!
            I will keep an eye on the tubes, and I will install those screen grid resistors as you suggest.
            Again THANKS SO MUCH!!!
            Hopefully the Sunn will shine tonight!


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