Installed an electrolytic in reverse for 30 seconds, is it scrap ?

[Iplayloud]

I forgot for a brief instant that bias was negative and installed a fresh cap on the bias in my old Marshall head, the originals are bulging... so my bias voltage tanked from about -31v to -12v instantly probably because the cap started leaking being in reverse. Is the cap good for the garbage now ?

[R.G.]

Maybe, maybe not.

However, the bias filter cap is one of the most important parts in the amp in terms of not damaging anything else. How much would you trust it if it acted OK? What are the consequences of it failing at some random time? On stage? With no back up amp?

I'd pay enough money to buy a new one not to have to worry about it. But that's just me.

[Enzo]

Amen. COnsidering what happens to an amp if the bias supply fails, is it really the place to worry about saving a dollar part?

[Ronsonic]

I consider the part scrap.

Maybe it's fine, but not worth the risk - especially for a bias cap.

[Chuck H]

Another vote for scrap. I have a design that has V1 up front near the input jack. On the board the ground end for the V1 cathodes is on the "back" end instead of the front as they usually are. The resulting mistake is that I did install the cathode caps for V1 incorrect. When flipped back the right way they measured fine and performed fine so I left them in there. No way I'd take the same chance with a power tube bias supply though.

[cminor9]

Wait, why would it be catastrophic if the bias cap failed? You'd still get negative voltage only because of the diode; it'd just be " bumpy". Sure, you would be screwed on a gig, but what would be destroyed on the amp? Power tubes, sure. What else?

Not saying the cap shouldn't be thrown out, but I am just curious now.

[Chuck H]

Wait, why would it be catastrophic if the bias cap failed? You'd still get negative voltage only because of the diode; it'd just be " bumpy". Sure, you would be screwed on a gig, but what would be destroyed on the amp? Power tubes, sure. What else?

Not saying the cap shouldn't be thrown out, but I am just curious now.

Your scenario assumes the cap goes open. Let's suppose the cap shorted?!? Could burn the bias wind and would almost certainly burn the tubes.

[Iplayloud]

I searched Google to find out what happens when an e-cap is connected in reverse, apparently the oxide layer is held in place by the electric field, when hooked in reverse, the oxide layer dissolves into the electrolyte and allows a short circuit between the electrolyte and the aluminum. It didn't mention if the oxide layer fixes itself when connected properly...

[cminor9]

oh, right! I figured if it fails it would open. Didn't think about shorting. Thanks Chuck!

[Alex/Tubewonder]

That's when correctly dimensioned HT fuse should blow. Unfortunately they are hugely over rated in many amps.

[R.G.]

It didn't mention if the oxide layer fixes itself when connected properly...

That's because the current density can be so high at that point as to weld the two foils together. If they don't weld, it is possible, with enough care and luck, to heal the spot by careful manipulation of the voltages and currents through the cap in the right way to achieve some regrowth of the oxide layer if the spot isn't "poisoned" by whatever chemical combinations were formed in the heat from the short.

Some film caps are "self healing" because the current density at a short burns the metal back from the hole that let the two sides of the cap touch. Electros are not in general self healing in that sense. And I would not trust especially a bias cap that had been reversed even if it stood up and said "Really, coach, I'm OK. Put me in!" It's like putting your first-string quarterback back in the game after he's been knocked unconscious for 15 minutes. He may be fine...

[Ramsay]

In high voltages, like that used in tube amps, 30 secs is a eternity and sufficient to burn any capacitor.
In low voltages, like 10 or 12 V the tolerance is larger, depending of the current.