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????

[sluckey]

It's not uncommon for filter caps to fail all on their own. When a filter cap fails shorted it usually damages some other components. Change the filter caps and then check for any low resistance reading from each positive terminal to ground before turning the power back on.

[rafe]

Thanks Steve, I found one resistor that I put in looks a bit cooked I test it one way and there is -0- resistance, if I switch the leads it reads 1.4 m and kept climbing I stopped as it approached 2m I did not expect that. any way that one goes. What am I expecting to see on the lead to ground if it's healthy? nothing.

[sluckey]

What am I expecting to see on the lead to ground if it's healthy?

You're looking for a high resistance reading. It may vary from one amp to another. But low is bad, high is good. You really have to look at the schematic to see exactly what to expect. Looking at your schematic, I would expect very high resistance (1M or more) on all four filter caps for a healthy amp.

[Rontone]

I test it one way and there is -0- resistance, if I switch the leads it reads 1.4 m and kept climbing I stopped as it approached 2m I did not expect that. any way that one goes.

Don't want to hijack the thread but I've experienced this a few times when testing in circuit, also last night testing what I thought was a blown bridge rectifier or transistor in a solid state PSU,

Is this dialectric absorption? The rising resistance is the cap still holding charge and pulling some back in?

You're looking for a high resistance reading. It may vary from one amp to another. But low is bad, high is good.

That reassures me as I have experienced the same as rafe, a reading that climbs up to 2Meg then higher until the meters limit....

[AlNewman]

When measuring resistance with a multimeter, your meter applies a small amount of voltage across the 2 leads, so the increasing resistance is your caps charging.

[rafe]

When measuring resistance with a multimeter, your meter applies a small amount of voltage across the 2 leads, so the increasing resistance is your caps charging.

                                                                                                                                                                                                                                                                   So the resistor has zero resistance when hooked up one way and when I reverse the leads it's charging a cap upstream, I take it??? I have seen resistors short to open .....but it's the first time I've had one lose resistance   

[sluckey]

So the resistor has zero resistance when hooked up one way and when I reverse the leads it's charging a cap upstream, I take it??? I have seen resistors short to open .....but it's the first time I've had one lose resistance 

Sounds like you are checking the resistor while it's still connected to the circuit. If that's the case, then examine the schematic to see what else is connected to that resistor. Better yet, tell us which resistor you are checking, and we'll help you figure it out.

[rafe]

I'll post that up tomorrow, Thanks

[rafe]

It's R-28  The tremolo switch was removed at some point and I have not replaced it , so I don't think that resistor is even used as R-30 goes to the cathodes of V-5,6....I don't think it's an issue just odd that it has -0- resistance one way.Still waiting on the caps zzzzzzzzz

[sluckey]

It's R-28  The tremolo switch was removed at some point and I have not replaced it , so I don't think that resistor is even used as R-30 goes to the cathodes of V-5,6....I don't think it's an issue just odd that it has -0- resistance one way.Still waiting on the caps zzzzzzzzz

R28 (150Ω/10W) is absolutely necessary. It's the cathode resistor for V5 and V6. Without it the amp is dead and will produce no sound.

You should be able to check R28 in circuit. A good resistor will read 150Ω regardless of how the meter leads are connected, unless C15 is bad.

[rafe]

My Brainfart, It's R-30  the 1k ....The 10w 150 is OK ....I checked it first thing and the coffee hadn't kicked in yet....

[sluckey]

R30 allows the tremolo tube to conduct a little bit, but probably not enough to create the tremolo effect. Think of it as a "keep alive" circuit that will cause the tremolo effect to quickly start when the switch is turned on. When the switch is turned on R30 is bypassed and allows the trem tube to conduct fully, applying the trem signal to modulate the output tubes' cathodes. Without R30 the tremolo effect would be slow to turn on when the switch is turned on.

I don't see anything that would cause the weird meter readings you got, other than maybe just a poor connection with the meter probes. Can you duplicate those readings?

[rafe]

Different today. I hooked up the leads and it started @ 6megs and dropped pretty quickly, I stopped @ 2m and it was still dropping ....I reversed the leads and 350k and started climbing got close to a meg and I stopped ....Some kind of chicanery Caps are here I hope to get on it in the next few days. I'll put in the new caps and see what happens . It was quiet with no issues until the caps popped.....Quiet as a mouse with great overdrive if pushed ....again I have not hooked a switch up I may jump it to hear the tremolo, but I really don't use tremolo much