# Exploding bypass capacitor



## fishin' musician (Jun 19, 2008)

Working on a garage sale prize Gibson ga1-rvt. Replaced all of the electrolytics and when I repowered it, it played very quietly for about 2 minutes. During this time neither the reverb nor the tremolo would work so I was looking at the wiring, probing with a chopstick to see if I could find the short. Could find nothing, so I'm standing there, hands on hips, looking at the guts of the amp when POW!, the bypass cap across the rectifier explodes. Upon inspection it becomes clear that the capacitor WAS wired in correctly. Is there another reason why the cap might have blown?


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## fishin' musician (Jun 19, 2008)

After inspecting the 5y3 rectifier tube I found that the tube's base was somewhat loose. Could this be the cause of the problem?


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## Furtz (Nov 27, 2010)

I don't think a loose socket would cause this. Are you talking about the 20uf filter cap?
If you had the neg to ground it should have been good. Was the voltage rating high enough?
Tube Amp Schematic: Gibson GA1 RVT Reverb Tremolo


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## Cups (Jan 5, 2010)

If it was that 20uf cap then it's not a bypass cap but a filter cap. Could've been a bad cap? I'd put a small value resistor in series (before) the filter cap. Maybe 5 to 15 ohms, 2 watt and flameproof. Like Furtz alluded to, make sure the filter cap is properly rated. I think I read 200v out on the schem so the cap should be 300v at least.


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## fishin' musician (Jun 19, 2008)

It was the 10uf - 25v cap that is wired to pin #2 of the 6bm8 output tube. The lead from the output tube runs to the terminal for pin one for the rectifier tube, throught the 10uf-25v cap which is wired in parrellel with a 680K resistor, then ties in with the negative leads from the filter caps and goes to ground through a red and orange striped lead from the power transformer. I now realize that pin 1 on a 5y3gt is only there to help hold the tube in place and isn't actually wired into the tube. So, the cap isn't actually wired into the rectifier but that they were simply using the terminals like a turret board. 
Hmmm, too much voltage coming out of the power tube? The schematic says it should only be 22v, so the a 25v cap should have done the trick. Also, as I mentioned before, the bakelite base on the 5y3 is loose on the bottom of the tube (not the tube socket but the actual tube base) but this may be a red herring...none the less, I'm not going to power up the amp again until I get a new rectifier tube.


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## Fader (Mar 10, 2009)

Aren't you suppose to ramp up the voltage gradually with a Variac after a recap to allow the electrolytics to "form"?


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## dtsaudio (Apr 15, 2009)

That 6BM8 is probably no good. If it is drawing heavy current the voltage across the bypass cap will rise significantly, and as you found out - boom. 
When replacing bypass caps across power tubes I always oversize the voltage rating significantly. It prevents this kind of thing from happening.


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## fishin' musician (Jun 19, 2008)

dtsaudio said:


> That 6BM8 is probably no good. If it is drawing heavy current the voltage across the bypass cap will rise significantly, and as you found out - boom.
> When replacing bypass caps across power tubes I always oversize the voltage rating significantly. It prevents this kind of thing from happening.


I was wondering that myself. I think it has been a long time since this amp has been plugged in and judging from the rust on the chassis, it wasn't stored in the most favourable place either. Probably a good idea to replace all the tubes and resolder all of the leads. Then I'll try to figure out why the reverb and tremolo aren't working.


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## WCGill (Mar 27, 2009)

A 25 volt rating is certainly small in my books, especially when 22v is on the cathode. Taking into account the likely loose tolerances of the cap and our higher line voltage these days, you could be over spec quite easily and if the 6BM8 is drawing excess current, fireworks. Go with a 50v rated cap if possible. As well, electrolytics unused for a long period of time become unreliable.


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## dtsaudio (Apr 15, 2009)

You also want to check the capacitors that couple the previous stage (0.02uf) and the tremolo oscillator (0.1uf) to the 6BM8. If either of those caps is leaky, you will put a positive voltage on the output tube causing it to draw excessive current. This will put excessive voltage on the cathode bypass cap.


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