death cap explosion

[joshdfrazier]

So, when I first fired up my latest univox, I played on it for about 15 minutes, and I heard a loud bang followed by a TON of smoke billowing out of the rear of the amp. But there was still sound? I shut it off, looked inside and saw the guts of the death cap all over the inside of the amp. It was a .15uf electrolytic cap.

I've been using the amp without it, but it does hum badly at higher volumes. I've put a non polarized, very old .002uf Erie cap in its place with good results, but only for test purposes. Almost eliminates the hum. Any idea what might have cause it to explode as violently as it did?

[Paultergeist]

Wow......sounds pretty exciting!

I would think that you should be able to re-configure the amp power supply wiring without the death cap and still get the lower hum level you experienced with the death cap in place?

[eniam rognab]

three prong power conversion? eliminates the "fun" cap altogether, or in your case the surprise cap!

unless your one of those vintage types... i would bring it up to modern safety standards, but hey your amp!

old 'lytics just crap out sometimes, that one has probably seen some use!

[joshdfrazier]

The lead dress is fine, I don't know what causes the hum, really. Moving wires around does absolutely nothing. But with the death cap in place, the hum is gone! I taking my chances, and I'm putting the Erie cap back it.

With the cap in place, I get 80vac from one end of the cap to ground (110 vac with the two prong plug reversed) , which sounds scary, but the current is immeasurably low. I even *tried* to shock myself, and it wasn't happening. So, I'm going to say that this is safe, for now...

[Stevem]

Many of these Univox amps have the PT and OT mounted right next to each other and the 60 hz flux from the PT couples right in to the OT, atleast this the case in the U45B model, but anyway if the amp hums with 60hz as soon as you turn it on, then that's the what I just described above, if not it could be from the filaments be it from a tube or heater wiring or 120 hz ripple from the power supply.
my rule of thumb is to hook up a ac voltmeter across the speaker, plug in a guitar and turn its volume down and then crank the amp wide open, if what you read on the meter is less than 12mv then that is about the best you will get out of these amps.

[Mescalero]

If you're talking about an AC line filter cap, I strongly advise you to replace it with a less-likely-to-become-lethal modern safety cap. Here's an easy read as to why: http://www.justradios.com/safetytips.html

[teemuk]

But with the death cap in place, the hum is gone!

Well, that's the point of that cap, really....

You're lucky it failed by exploding to open circuit, instead of short circuit, which would have connected your amp's chassis to mains live.

And that's the point why it's called a "death cap" and why you want to get rid of it, really....

[Paultergeist]

But with the death cap in place, the hum is gone!

Well, that's the point of that cap, really....

You're lucky it failed by exploding to open circuit, instead of short circuit, which would have connected your amp's chassis to mains live.

And that's the point why it's called a "death cap" and why you want to get rid of it, really....

Perhaps this is obvious to others, but what are some of the methods by which the hum-suppressing function of the "death cap" can be engineered-anew into an older amplifer where the death cap it self was removed?

For example, most of the "death cap" circuitry I have seen involves a cap to chassis ground, on a switch which connects the other end of the cap to one (or the other) of the two in-coming AC supply legs. If we re-wire the chassis with a (modern) 3-wire AC power cord -- now with a dedicated chassis ground -- is that enough to make the amp as quiet as was under optimal "death cap" hum-suppression function?

It almost seems like -- with the death cap -- there is a bit of filtering of in-coming AC before the PT is even involved? Most modern amp designs don't seem to do this......?

[billyz]

If you replace the cap it needs to be a class x rated modern cap.
I would try the 3 wire grounded AC cable though. You should also test or replace the power supply caps, they are probably leaking .

[Leo_Gnardo]

Those shocker / death caps can let you know they're defective by exploding. One fine day just after I installed a grounded AC cable on a nice old Ampeg V2, upon firing up its death cap went rocketing right out of the chassis, with sparks & smoke, and leaving a spiral of foil in its wake, which luckily didn't land on the powered-up circuit. This is the kind of excitement we could do without. Ground chassis & clip out death caps: that's the solution.

[Structo]

Why didn't the fuse open?

[tubeswell]

Convert the amp to 3-wire mains with chassis earth and lose the death cap

[Zippy]

Why didn't the fuse open?

In many amps with "death" caps, the switch and cap are between the wall and the fuse.

Ex. http://elektrotanya.com/fender_bassman_ ... nload.html

[joshdfrazier]

The amp had a 30amp fuse installed.. I was so excited to play it I neglected giving it a once over. I popped in a 2 amp, maybe I should go lower? I don't know.

I am totally aware of the purpose of the death cap, but this hum is really bad. I will post an audio clip of the cap in and out of circuit so you can hear what I mean.

Also, PT and OT are on opposite sides of the chassis. There are some .01uf cap in the preamp that I should probably replace, though.

[Aurora]

Grounding cap AKA 'death cap' as electrolytic?????? If that is correct - it's a major design flaw!!
Suitable voltag ceramic or polyprop it should be......