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keep blowing the same capacitor

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  • #1

    keep blowing the same capacitor

    Hey, so does anyone have any ideas what might be causing me to repeatedly blow out the same filter capacitor in an old tube amp? It's an Ace Tone 601 - you can get some more details on it in my post from the vintage amps repair / rebuild section. The cap is rated at 250V and the highest voltage my amp sees from the transformer is 175V. I've never gotten any sound out of this amp except for some very low volume, scratchy stuff.

    First off, I know I need to rewire this so that the fuse is on the other side of the transformer, so that it will blow before the cap. BUT, that obviously won't solve my problem. I don't know if the power tubes are in working order, but if not, is it possible that non-working tubes could cause some sort of current feed back into the capacitor? That's probably totally ignorant, I'm very much a novice at this stuff. Anyway, ideas??
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  • #2
    after reading a bit more, any chance it's just because I need to slowly increase the voltage on that cap rather than taking it directly to 175V? seems like that would be unnecessary with brand new caps, but...

    also, the cap is definitely NOT wired in backwards.

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    • #3
      It's not explicit in your posts - are you putting a new cap in each time it blows?

      If so, and the new cap is rated for more than the working voltage, then it's a good bet that the rectifier is feeding it AC or a reverse voltage.
      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.

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      • #4
        Originally posted by R.G. View Post
        It's not explicit in your posts - are you putting a new cap in each time it blows?

        If so, and the new cap is rated for more than the working voltage, then it's a good bet that the rectifier is feeding it AC or a reverse voltage.
        yep. so far I've only gone through 2 capacitors, but it's the same one that blows every time.

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        • #5
          175vAC once rectified and filtered is only a couple volts shy of 250VDC. SO it shouldn;t pop the cap for overvoltage, but you got no headroom.

          Is the rectifier tube or solid state? A shorted one results in the AC RG referred to getting to your cap.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            If original, it has a tube rectifier. If that is still the case, the rectifier tube is suspect. If it has been converted to solid state rectifier, it could be a shorted rectifier diode.

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            • #7
              When I first glanced at your schematic (which was very blurry on my pc) I thought it was a doubler circuit. It isn't. Does the standby light come on and off? Have you measured the ripple on the B+ line? In something this old it's a good idea just to swap out the rectifier diodes. Have you run it up with the tubes out? Do you have a variac?

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              • #8
                Originally posted by olddawg View Post
                When I first glanced at your schematic (which was very blurry on my pc) I thought it was a doubler circuit. It isn't. Does the standby light come on and off? Have you measured the ripple on the B+ line? In something this old it's a good idea just to swap out the rectifier diodes. Have you run it up with the tubes out? Do you have a variac?
                Yeah, the standby light does come on and off! Someone told me that was just a manifestation of it being an old neon bulb, but is it more signficant than that?

                The rectifier diodes are original as far as I can tell, so swapping those is probably a good idea.

                I haven't measured the B+ variance. If it does turn out to be a large ripple, what might cause that?

                Finally, I did try running it up last night with the power tubes out. Same problem as before; I narrowly avoided blowing another capacitor.

                I think what i'll do tonight is trace out the circuit as I see it and compare it to the schematic, as well as post it on this board if any of y'all have good insights. Thanks everyone for the continued advice!!

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                • #9
                  Hmm, so I've traced the whole circuit and it seem to correspond to the schematic. Still can't figure out the cause of this capacitor issue. Strangely the other night I went ahead and replaced the cap in question and, while the amp didn't work, no particular problem... the cap didn't make it's typical "about to blow up" hum. the next day I plugged it in, without making any changes at all, and I'm back to the "primed to blow up" position. Any theories here? Very strange.

                  The only thing I can think of is that maybe there's something weird about having multiple power transformers with different primary voltages. I'm using one that has 220V primary and then two smaller transformers with 120V primaries. Is this a concern?

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