Explosive bias cap

[Smokebreak]

I fired up a new build last night, a 6V6 JTM kinda thing. Bias supply is pretty standard fare and coming off the HT. SS rectified.
I powered up with no tubes, and got voltage everywhere it needed to be. Plenty of neg voltage off the bias supply, for starters, and a good sweep.
So far so good.
Put in tubes, fired it up, and within 5 seconds got burning, sizzling, POP....and a big ol mess.
My 150k/1W between HT and reverse diode was smoked, and the first bias cap literally looked like a discharged party popper. Fuse did not blow.
I stared at everything for a while, made sure my orientations and circuit were correct, and popped back in another 150K and 10u/160v.
Everything was fine, amp works, etc. I shut it down for the night at that point.

I'm trying to figure out what could have happened.

This morning I took some readings with everything as I left it last night. Tubes were biased very cold(purposefully), plates at 420V. I got it biased up just fine, with plates now at 400V. Amp seems fine.

I kinda chalked it up to a "bad cap", but the fact that the bias resistor burned up has me scratching my head.

This is the same circuit I put in all amps w/o a bias tap, basically the Express circuit, and I've never had a problem. The fact that I replaced the busted components with the same values, and it actually worked is also puzzling .

Any thoughts?

[gui_tarzan]

Is it possible the first resistor wasn't the right value? Did you measure it before you put it in just to make sure?

[xtian]

Just guessing, because I like to play: If you indeed had a bad (shorted) cap, it was providing an easy path to ground for current, which is what smoked the 150K resistor.

[Smokebreak]

That's possible, and I didn't read it beforehand. So say it was 15k, that would have caused voltage on the other side of it to exceed the cap's rating, but technically shouldn't fry the R.
If it was 1.5M, that would have caused a massive V drop, possibly frying the R, but shouldn't have fried the cap. Do I have all that correct?

[Smokebreak]

Just guessing, because I like to play: If you indeed had a bad (shorted) cap, it was providing an easy path to ground for current, which is what smoked the 150K resistor.

This could very well have been the case, too

[KellyBass]

Did you notice (in that violent 5 seconds) the tubes redplating?

This almost exact incident happened to me before...a couple of years back...and I'm still sctratching my head over it. Explosions have that effect!

[Smokebreak]

Did you notice (in that violent 5 seconds) the tubes redplating?

This almost exact incident happened to me before...a couple of years back...and I'm still sctratching my head over it. Explosions have that effect!

I didn't notice. As soon as I heard the sizzling I thought "I should turn that off". I didn't quite make it;)

Strangely enough, and I know that exploding caps is no laughing matter, I just started laughing when it happened...albeit a nervous laughter. It scared the piss out of me!

[Blackburn]

Did you make sure the cap's polarity was wired correctly, with its positive side to ground? Easy to forget about the proper orientation.

[Smokebreak]

Did you make sure the cap's polarity was wired correctly, with its positive side to ground? Easy to forget about the proper orientation.

Yep I spent a good 5 minutes tracing out the bias circuit after it happened. That was my first thought when it happened though

[Leo_Gnardo]

Could be your bias cap was mis marked. You THOUGHT you had it in right...

It can happen!

About 3 months ago I had a 47 uF 500V, didn't explode but the power surprise wouldn't go past 65V while bringing it up on Variac. It did heat up noticeably. According to the wrapper, it was wired correctly, but it behaved as if reversed. This brand has black plastic/rubber seals on each end. Many caps have - end metal and + end plastic.

None like that before nor since, thank goodness. But I always bring up freshly replaced HV caps on variac, one of my few good habits.

[xtian]

Wouldn't it be cool to have an infra-red camera pointed at your circuit as you power it up? Super fast way to see if any caps are going supernova.

FLIR for amp builders(TM)!

[Structo]

Sorry if I didn't read every post but I blame the beer.

Did you check the bias cap polarity in regards to the ground.

The + side should go to ground as to give you a negative voltage to the grid.

[JazzGuitarGimp]

Sorry if I didn't read every post but I blame the beer.

Did you check the bias cap polarity in regards to the ground.

The + side should go to ground as to give you a negative voltage to the grid.

Yup, it's the beer….

[Structo]

damn

[Leo_Gnardo]

Wouldn't it be cool to have an infra-red camera pointed at your circuit as you power it up? Super fast way to see if any caps are going supernova.

FLIR for amp builders(TM)!

No FLIR by a wide margin, but I did get a laser thermometer last year, Parts Express had 'em for @ $25. Very handy for checking tube temperatures. Let your amp warm up & how can you tell which output tubes are running hot and which cool. Laser thermometer tells you. I've scanned suspect resistors, caps, transformers etc. with it. I suppose it could be used when first firing up new filter caps, but heck, you can touch 'em IF YOU DARE and sense any heat the ol' fashion low tech way.