K&T filter caps in 52 LP GA-40 'Who let the smoke out"

[rafe]

Sooooo, I was set to play through the LP the other night as I do pretty often.... I flipped the switch and sat down strummed and it was faint, I thought it was the jack but wiggling it said it wasn't.....so I checked the amp end and smoke was billowing out from the back cover ....I unplugged it and opened it up and there was a good-sized (BIG) bubble on one of the caps 16/16 500 the other has a smaller bubble 16/16 500. I was working real well for a few years. It calls for 3-10's and one *20* all450v ....I think that (Hope) was the culprit. I had those caps and thought it would be ok.....The question being are F&T's low quality these days or did I skimp where I should not have, knowing that you aren't supposed to go lower capacitance....That big bubble was on the one that should have been 20.....anywho, I think Iagain I have answered my own question, but feel free to chime in......

[passaloutre]

F&Ts are as quality as anything. And the difference between 10,16, and 20 uf isn’t going to let the smoke out. I’d look elsewhere. In my experience caps don’t pop for no reason, usually something near them is malfunctioning. If it’s the main reservoir cap it could be a lot of things.

Got a schematic? Any voltage readings before things went south? Got a light bulb limiter?

[rafe]

Yes , a few different schematics ....LBL, Variac amongst other such stuff, I don't have the voltages, I had them long ago when I went through it completely, It was working just fine for a long time.I ordered new caps and will go through it again.I'm wondering if I had a power surge.

[pdf64]

The most likely cause of the reservoir cap blowing is a shorted rectifier https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga40.pdf

It's a really good idea to fit back up silicon diodes in series with each valve rectifier anode to prevent that happening.
https://www.premierguitar.com/the-immortal-amplifier-mod

[rafe]

Good stuff. That makes sense to me because that cap with the huge bubble is the first thing that rectifier would hit. I pulled it and saw no signs of failure. It's a coke bottle RCA and it's only 75 years old ....so on second thought the silver coating on the inside around the base is 65% gone and the 2 center posts (out of 6) small silver circles above them, maybe a touch of black soot.....if those silver coatings are gradually used up. They are used up. I am going to put in the diodes for sure.....5V4G Made 13 week of 51....That schematic is the next iteration ...mine is the early one. It does not have a serial number so It's EARLY ......I'm waiting on the capacitor bus.   Gibson_ga_40_54_early.pdf      caps are C10,11,16,17...

[rafe]

Wellllll, I tested the 5V4G and no shorts and it tests good both sides are 60 and well into the green.....TV-11 ....I'm leaning towards the K&T's I think a surge hit them, We were (are)having storms in the vicinity almost daily.....I think I have some diodes handy......and I ordered 3 different sets of caps ....and i'll go over things ...it could be the choke between the first two filter++++++++++ caps????