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Short-term Cap Overvoltage?

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  • Short-term Cap Overvoltage?

    I built a tweed Deluxe style amp but using SS rectifier+ bulk resistance instead of a glass rectifier. The problem is that until the 6V6s warm up and start drawing current, the B+ goes up to 480-485 volts for several seconds across 450v rated filter caps. The voltages are good once the tubes heat up.

    Now, I know this is a Bad Thing, but is this dangerous, or will it just decrease the cap lifetime? If earnestly not recommended, is there a simple solution? Or should I just order 500v parts and swap them out? I wouldn't leave things this way for a customer, but this is just my personal 'basement' amp.

  • #2
    A standby switch before the reservoir might be cheaper than new caps?
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


    • #3
      It's probably non-fatal. Caps don't break over at some well-defined voltage like a zener does.

      Electro caps come with a "surge" factor that means that the leakage goes up as the voltage goes up. It's almost certainly non-linear, but the current that flows forms more oxide as long as the heat generated at any point doesn't start DE-forming oxide at the same time. It's the same trade off that's made when making electros anyway.

      I would not sweat it as long as it's for short times.

      You might, if you want to experiment, take the tubes out, and stick a 100K resistor between the diodes and first cap. Measure the voltage across the resistor. When it goes down to less than 20-30V, the cap has been formed up to the voltage. Removing the tubes lets the voltage go up above the oxide leakage point so forming can happen.

      The life will probably be shorter even with forming, because there will be long periods of no voltage so the oxide can UN-form, and one day the voltage surge will win. But it'll be a long time in this case.

      ... I think.
      Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

      Oh, wait! That sounds familiar, somehow.


      • #4
        What you describe is not unusual.
        Electrolytics have a Working Voltage Rating which is what is written on the side.

        They also have a surge voltage rating which is usually specified (in the datasheet fine print) as:
        - For short periods not exceeding 30 seconds
        - At intervals not more often than 5 minutes

        Most 450V Working Voltage Caps will have a Surge Voltage Rating of 550V.
        I would not be too fussed, particularly for your own amp.



        • #5
          I work on Leslie amps, and all 122s and 147s (SS rectified) used a 475V rated Mallory FP can. The B+ supply always hits ~510V for a few seconds before the 6550s start to conduct. The original filter cans tolerate this for decades.

          On the other hand, the Dynaco MkIII can hit 580V before the output tubes heat up, and they are notorious for blowing their filter caps even though they were built with 525V rated multisection cans.

          Without looking at the surge ratings, my experience mirrors what Gingertube wrote above. My sense is that 450-475V aluminum electrolytic caps will tolerate surges into the low 500s, but once you get closer to 600V, they will blow up.


          • #6
            Simple solution means to add parallel resistor in circuit.This resistor will affect the voltage over entire supply rail before it,no matter if preamp tubes draw current or not. Spot on the right supply point to insert this resistor in respect by you circuit. With a expence of some extra mAmps draw you can drop some voltage over series network maybe enough to protect you caps when no load condition. You have to recalculate the values for series supply resistors to achieve the right voltage in supply points then, taking in consideration the extra current draw. Usually we talk about preamp supply circuit so no big problems with lot of mA draw or big power disipation there. RC supply networks will be affected,still
            Series zenners works better, think,but never used
            Last edited by catalin gramada; 02-22-2017, 12:03 AM.
            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."


            • #7
              It's the main and screen supply caps I was concerned with. It would need to be a hefty load to keep those down, then I'd be heating up the secondary once the outputs started conducting. I just got a bunch of 500v caps, I might just replace the first two.