Electrolytic Cap on Fender Champ 6v6 keeps exploding/HELP!

[bbrownsbrick1]

Hello,
I havnt been tinkering much with amps as I've been very busy recording/touring, but I thought id give it a shot.

I picked up a 69' Fender champ for a good deal, but a few days later i noticed the mains switch was extremely finicky and wasn't working often enough.

I then opened it up to notice the mallory 25uf cap on the 6v6 was bulging, so I went ahead & replaced it with a fresh 25uf 25v sprague.

Turn it on, hum and a huge explosion. Checked the polarity and it was correct.
Did it again. Big explosion.
Fuming mad, I decided my batch of spragues were faulty, replaced it with the original mallory. Mallory went up in smoke.

Could this be due to the heater current flowing to the 6v6 from the on/off switch? Whats going on here? Extremely confused. Thanks much.

[Diablo]

This is the cathode bias cap on the 6v6 socket...right? Did you connect it to the cathode pin of the socket - Pin 8? If so, you may have an internally shorted tube. I'm guessing the plate is shorted to the cathode. Put in a new cap, pull the tube and turn it on. I bet the cap doesn't explode. Turn the amp off and stick in a new/good 6V6.

[bbrownsbrick1]

yes, cathode bypass cap is on pin 8, I pulled the tube, fired it up, and the cap seems like its doing an ok job thus far. I subbed the 6v6 from the faulty champ into my rebuilt single ended silvertone and it sounded great. could it STILL be the tube? perhaps tube socket arcing?

Also, the 470 resistor on pin 8 looks burnt in the middle but reads normal on the multimeter. i think this might just be from the cap exploding right next to it. Im afraid to sub another 6v6, but will report results.

Happy Holidays to all,

[Jazz P Bass]

How about measuring the voltage on the cathode resistor.
Remove the capacitor, install a known good 6V6, turn on the amp & measure the cathode voltage.
I have attached a view of the cap showing the polarity.
This is important.
Plus to the tube, minus to ground.
It would not hurt to have a 50Volt cap in there.

[Diablo]

yes, cathode bypass cap is on pin 8, I pulled the tube, fired it up, and the cap seems like its doing an ok job thus far. I subbed the 6v6 from the faulty champ into my rebuilt single ended silvertone and it sounded great. could it STILL be the tube? perhaps tube socket arcing?

Also, the 470 resistor on pin 8 looks burnt in the middle but reads normal on the multimeter. i think this might just be from the cap exploding right next to it. Im afraid to sub another 6v6, but will report results.

Happy Holidays to all,

Yes it could be the tube socket arcing. JP Bass has some good ideas, especially going to a higher voltage cap (doesn't effect tone). I'd still sub in a known good tube. It can only be higher voltage that's killing the cap. You could also test it without installing the cap - the voltage on the bias resistor should be the same. Measure that voltage.

[stokes]

With no tube in the amp that cap is not subject to any current,so checking it with no tube is pointless.With a tube,depending on where the idle current is,the 25v rating may be marginal.Use a 50v or better just to be safe.I would also change the resistor,it may read okay,but when things get cooking the value may drift eneough to cause that cap to explode or the tube to go south.

[Diablo]

With no tube in the amp that cap is not subject to any current,so checking it with no tube is pointless.

It shouldn't have any voltage on it with no tube in place, but if the socket is wired wrong or shorted somehow, then it might have plate voltage on the cap. I know it's unlikely, but strange things happen.

[R.G.]

Yes it could be the tube socket arcing. JP Bass has some good ideas, especially going to a higher voltage cap (doesn't effect tone). I'd still sub in a known good tube. It can only be higher voltage that's killing the cap. You could also test it without installing the cap - the voltage on the bias resistor should be the same. Measure that voltage.

With no tube in the amp that cap is not subject to any current,so checking it with no tube is pointless.With a tube,depending on where the idle current is,the 25v rating may be marginal.Use a 50v or better just to be safe.I would also change the resistor,it may read okay,but when things get cooking the value may drift eneough to cause that cap to explode or the tube to go south.

It shouldn't have any voltage on it with no tube in place, but if the socket is wired wrong or shorted somehow, then it might have plate voltage on the cap. I know it's unlikely, but strange things happen.

Back at Three Initial Corporation we used to describe this state of affairs as you guys being in violent agreement.

The most likely reason for a new cap in that position failing repeatedly is over voltage. The question of how it gets there is open to a few interpretations. All of the ones noted are possible:
- bad charring on the socket letting high voltage leak directly
- shorted wiring on the socket, or a random wire sneakily making connection somehow
- an open, opening, or badly drifted cathode resistor could put more voltage across the cathode cap than it could take.

All of these need checked.