Posted by rude52 on April 25, 2007 at 23:42:27

Hello all,

After 5+ excellent years, the blue smoke got out of my SET 300B amp (non-bottlehead but I do have a FPII). I had heard some popping in one channel and had been trying to see if one of the tubes was bad by swapping back and forth. Last night I was letting it warm up and heard a strange noise. As I turned around, smoke was pouring out the top. Based on the little I know, I think it may have been a cap.

On quick inspection today, nothing stood out. No popped cases or scorched parts. However, one of the electrolytics was very convex on the base (i.e. popped out). Does this mean it failed?? They seem to be built to do that with creases in the bottom. It would be the correct channel that I heard the popping earlier.

Unfortunately, I don't read schematics, so I don't know exactly what this cap does (may be coupling cap?). Therefore, I am not sure if it's failure would cause a lot of problems downstream.

Should I just replace and see what happens?

Thanks a lot in advance. I could post elsewhere but this is always the best forum.

Rich

• Re: electrolytic cap help (longish) - mrclassicman 00:31:51 04/26/07 (7)
  In Reply to: electrolytic cap help (longish) posted by rude52 on April 25, 2007 at 23:42:27:

  Just a couple of questions. Do you want to fix this yourself? Is the wiring point to point or on a circuit board? Can you read the value of the capacitor without removing it from the circuit? The esiest scenario would be ,Yes I want to fix it myself, it is point to point, and I can read the value without removing the cap. If this is the case there are three things to watch out for with a cap, 1) uF value 2) voltage rating 3) polarity symbols. Rule of thumb would be stay close to the uF, prefer higher voltage rating and type of cap either non polar or electrolytic. In knowing where in the circuit the offending cap is helps in determining the best type to use. Some capacitors can have a noticable effect on the output. So if an exact match cannot be made, they may need to be replaced in both amps to maintain equal sound. This can be more criticle in some areas than others. I hope this helps! Many here would be happy to help, so post your findings, and questions.

  Terry
  
  

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  • clarification - rude52 10:03:08 04/26/07 (6)
    In Reply to: Re: electrolytic cap help (longish) posted by mrclassicman on April 26, 2007 at 00:31:51:

    Hi Terry,

    Sorry that I wasn't very clear. To answer your questions, I built this amp myself from a kit and it is PCB. I know the part value exactly and how the polarity goes for that section. Also, I would replace both the left and right channel.

    I was just more curious whether what appears to be a bulging cap would likely be the one to start with and whether there is likely collateral damage around it.

    I guess that there is only one way to find out! On the upside, it does give me an excuse to upgrade a bunch of parts in my amp now that I have to partially disassemble it.

    Thanks for the input.

    Rich
    
    

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    • Re: clarification - mrclassicman 18:45:35 04/26/07 (0)
      In Reply to: clarification posted by rude52 on April 26, 2007 at 10:03:08:

      Sound as if you have a good handle on the situation. Thanks for the clerification, initially I thought there was more ambiguity. This is certainly not the case. Have fun, and let us know how the upgrades work out!

      Terry
      
      

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    • If you tell us how many uF and what voltage - Russ57 11:06:08 04/26/07 (4)
      In Reply to: clarification posted by rude52 on April 26, 2007 at 10:03:08:

      We can make an educated guess as to what job it does and possible reasons for failure. Sometimes parts are speced on the edge and it helps to go up a step in voltage and/or temperature rating. I'm inclined to think it might be a bypass cap for the 300B bias resistor. If so I'd look at voltage and temperature rating.

      
      Russ
      
      

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      • IIRC 100uF and 100V (nt) - rude52 14:30:14 04/26/07 (3)
        In Reply to: If you tell us how many uF and what voltage posted by Russ57 on April 26, 2007 at 11:06:08:

        nt
        
        

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        • Just as I thought - Russ57 04:59:45 04/27/07 (2)
          In Reply to: IIRC 100uF and 100V (nt) posted by rude52 on April 26, 2007 at 14:30:14:

          I'd certainly up the voltage rating to at least 160VDC. Also go for a 105C temp rating. There should be a resistor near by, a large high wattage one. Check that and see if it is still the proper value. I'd use a 12 watt mills. Black gate would be nice for the cap. Neither are cheap but those parts are quite important to the sound of the amp.

          Oh, don't forget to have your tubes tested. We don't know what failed first. And yes a bulging cap is a bad one.

          Russ
          
          

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          • Re: Just as I thought - Paul Joppa 11:38:14 04/27/07 (0)
            In Reply to: Just as I thought posted by Russ57 on April 27, 2007 at 04:59:45:

            Also check for leakage of the coupling capacitor, and a good grid resistor. If the grid bias drifts up from the above or a gassy tube or whatever, it will raise the cathdoe voltage, possibly driving the cap to over-voltage. Heat and voltage are the cap-killers.
            
            

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          • thanks all - rude52 09:57:13 04/27/07 (0)
            In Reply to: Just as I thought posted by Russ57 on April 27, 2007 at 04:59:45:

            Hi Russ and everyone,

            Thanks for the advice. I will check out the specs and the tubes.

            As for the blown caps, a quick web search showed me more than I would ever want to know about elctrolytic cap failure in motherboards! I should have realized that the design on the bottom of lytics is a vent.

            Cheers

            Rich
            
            

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