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Sunn Spectrum 1 with a rising B+

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  • Sunn Spectrum 1 with a rising B+

    I need to start off this posting with this statement: "THIS AMP CURRENTLY WORKS".

    I'm refurbushing a Sunn Spectrum 1,( recap, new cord, I/O wiring and like that, nothing really over the top.) This amp is basically a Dynaco MKIV. I'm shooting a YouTube series on this and I'm in the final episode where I set the bias and then noodle out some riffs on my guitar as proof it works, tie things up and roll credits. 1 problem however when I first biased the amp the B+ measured 470VDC. The second time I fired it up the B+ was 575VDC. The third time I fired it up the B+ was 660VDC. All these startups and shutdowns are just a result of shooting video and staging shots.

    It has a 5AR4 rectifier and I thought I would look for some advise here as to what might be up with this. I'm a little gun shy about firing it up again and seeing an even higher B+.

    Oh yeah, It's on a variac, dummy load, no signal.
    ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

  • #2
    OK, it works, right? Get it off the variac, eliminate the potential problem. Any change?

    WHen you measure your B+, measure the HV AC from the transformer too. And if you don't have a mains monitor, check the mains voltage every time too.

    And check the battery in your meter.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      OK, the amp directly on the mains, the rectifier pulled and I'm measuring the power transformer output... 815VAC. Did my power transformer just go bye bye? The mains is 120VAC60Hz

      I also swapped out meters just in case, same results from all.
      Last edited by Sowhat; 06-12-2020, 12:38 AM.
      ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

      Comment


      • #4
        I never checked up till now but the replacement transformer is 400-0-400 output so 815VAC would not be too far from nominal, so, maybe my transformer is OK and maybe my rectifier is whacky?

        Damn! I don't have a spare rectifier around here.
        ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

        Comment


        • #5
          I assume that is center tapped? In which case roughly 400v on either side. With a rectifier tube, that ought to get you 500v and some.. Is it still changing by a hundred volts each time you measure?


          The hotter teh tube is biased, the more it drags down the B+, but not by hundreds of volts.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            This is why I retired. Now I'm getting a steady 500 volts! Tried and measured it a couple time, no change. What the heck was all that about? It's just that I have never seen over 600 volts on these things and I got a little frightened. I was worried that the new 525V multi filter can was going to pop. I still can't see how that voltage could read that high and marched up incrementally and now everything is peachy. The only thing I did was take it off the variac, pull the 5AR4 and then put it back in... I hate that!
            ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

            Comment


            • #7
              We ran into someone with like a 350vAC transformer and was reading almost 1000vDC and was all concerned the voltage "was that high". My reaction was that I CAN"T get 1000v from 350vAC and the problem is with my measuring equipment or procedure. Same for yours, I see no way for it to actually step up the voltage to something silly. And a 400v transformer does not generally have a failure mode of raising the voltage.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                I'm not sure if this is OK on this forum but here's a link to my video playlist for this amp, I trying to document some of my work video style these days, just like the young-uns do.

                https://www.youtube.com/playlist?lis...FQMxuKPzPm-yBP
                ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

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