This amp certainly did not pass UL or CE requirements. I agree, the surrounding can of an electrolytic capacitor should not be electrically connected to either terminal. Put a cover on those puppies before someone gets hurt. I'd also alert the vendor of your discovery. Perhaps they'll offer to swap out those caps for a better design.

sounds like the caps are in series. I have that on my MFA 75 monoblocks.

It has 6 750uf 330Vdc caps,and the rails are +550VDC. What they do, is stack the caps in series, two per voltage rail. So..550/2 eagles...275VDC. If the little plastic caps are not on the tops of the 'snap-on' style caps, the voltage I'd get 'hit' at, would be...275VDC. Rest assured, I've been hit by that voltage..and far worse.

The MFA 75 utilizes 6550 output tubes.

Sound familiar?

It's not a flaw in the amp's circuitry, the capacitors simply need to have their tops covered.

Internally, the capacitors are NOT connected efficiently to that measured voltage, so the 'zap' you receive is not all that fun to endure, but is far less intense than if it was fully coupled to that 'half way' 275VDC voltage.

The metal cans on a snap mount or any other common capacitor, for that matter..are not designed so the metal can is the 'negative' terminal, but there is also the thing called 'leakage'..so the can is actually 'floating' high, with it's apparent 'ground potential'. That +250VDC that you measured.

You can't get capacitors above 500VDC rating very easily, and they are notably more expensive. Basically, I've barely found 500VDC rated caps, nevermind ones with higher Voltage ratings (this is for electrolytic metal can types). As well, the surge voltage at turn on, and with 120VAC line fluctuations, will cause the voltage to peak higher than the 500VDC cap's ratings. So it HAS to be a series-split capacitor bank in the vast majority of 6550 (and similar) amplifier power supplies, due to the high voltages they run at. The major exceptions, are amplifiers with enough money spent on the parts..that the main capacitors are all film type in nature, which would make the amplifier come in at a retail price of about $1000 more.

I'd imagine that the unit was originally spec'ed or designed with capacitors that have proper and full 'skins' or 'covers', but..someone put something more efficient (financially, I'd guess) in their place.

For the moment, power the unit down, and then cover the tops with a small piece of plastic, and tape that on top. It's a tight space, so it's going to be tricky.

Another trick would be to cover them with a smattering of silicone. That would be the easiest, and for the moment..the most permanent and safe.

Let it sit for a day to let the silicone dry. Then fire it up again. Should be fine for a decade or so, after that.

Thanks for the accurate and detailed analysis KBK. I had neglected to consider that the rail voltages required would be substantially higher that what's required in a solid-state amplifier thus necessitating series-connected electrolytics. Although the danger presented by the leakage voltage appearing on the capacitor cases is much less severe that of a direct connection, covering them is still a good idea.

Thanks, KBK!

Great info and very interesting.

I'm sure the manufacturer (and their legal dept) would like to know that you found this. Since there's vanishingly little likelihood that other buyers will be measuring voltages from chassis to capacitor tops, probably every other owner is at risk of a nasty shock. Manufacturer should be able to offer a fix of some sort. Probably would rather send out covers than get sued for injury.

A small tube of bathtub silicone from the local corner 'half-way' hardware shop. You know, the big city type corner stores with lotsa stuff in them.

1)turn amp off.

2)Go read a book. It's good for you.

3) After reading that article in Hustler, (20 minutes, getting old) get a small wooden stick (popsicle, etc) and smear a THIN layer of the silicone on the top, or exposed metal of the capacitors. Make sure it is even, with no gaps. Clean up the excess.

4)Let it dry overnight. The amp will be playable immediately after applying the silicone, but you can let it dry overnight if you wish.

This is by far, the quickest way to get this fixed. The proper 'skin' covers are built ONTO the capacitor, when it is made. Yes, all that stands between you and 250VDC, is that thin little skin. Which is why many an amplifier of the tube type has the capacitors on the inside.

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I'd also like to know if that unit has an Orange CSA sticker on it, or a blue 'Entela' sticker. Either one is 100% necessary for sale of such in North America. I very much doubt it has either, with that total blunder of a PS supply implementation.

Entela does CSA and UL approval on individual items. Their sticker is blue, with a file reference #. Same for the orange 'special inspection' CSA stickers. (Canadian Standards Association)

I don't in any way think that the importer is aware of this situation, so I would appreciate it, and so would he..if you informed him of this situation. Tell him to go for the Entela approvals, they are much cheaper to obtain and will cover his ass with respects to proper analysis, and legal considerations, the kind filed under 'due diligence'.

Customers getting amplifiers that can kill them is not all that common these days, but hey, as you can see..it still happens.

As stated, I expect that someone on the production line who had little understanding of the power supply design..tried to save a buck. And that might only be for that given production run! All the rest might be fine.

Researching this a bit further, I found this page documenting the structure of aluminum electrolytic capacitors. Turns out the case is in direct contact with the electrolyte and, therefore, is at the same potential as the cathode (negative terminal). The isolation resistance of this connection can range from just a few ohms to a few thousand ohms. The shock danger from this amp is not trivial. Your vendor should be notified.

As suspected, someone made a parts swap during production. And ended up with this situation of raw can cases. If you do a search (for images, etc) on the net, you will find some with the caps as you have, and other photos with properly (for this use and placement) made/used caps.

as one in the legal field, i would amplify the comment about informing your dealer. in a defective product case, everyone gets sued. but the only likely person that will have to pay a judgment is the dealer. the chances are slim to none that the asian manufacturer is going to make an appearance and offer to pay a judgment, so the top of the food chain, so to speak will be the dealer or his insurance if he has any. many of these new "distributors" are very small time folks who may have a great deal at stake.
in all of my posts, i have expressed support for the low-cost asian imported tube amps because i think it will help to bring new people into the hobby. but, in regard to the unexpected parts swapping - that has been the story with asian made goods of all stripes, expediency and profit drive these manufacturers as they do all however, there are no regulatory agencies in most of those countries. one person mentioned the caps on the outside - a perfect example, if made in the US to UL standards those caps would be at least covered if not on the inside and a warning would be placed in a conspicuous place.
caviat emptor.

tom collins

BTW, if one does get zapped by a high voltage blast, be aware that it temporarily drops your white blood cell count to darn near zero, and it takes a few days to get back up.

So..getting zapped at this 250VDC in the morning (Hello! Good morning!), then taking the train, subway, and then walking into a big office building..could get you the latest cold or flu that is floating around. This..as you won't have much internal bodily protection for a few days.

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BTW, if one does get zapped by a high voltage blast, be aware that it temporarily drops your white blood cell count to darn near zero, and it takes a few days to get back up.

Dude, I think you just found a treatment for leukemia.

Quote:

Quote:
BTW, if one does get zapped by a high voltage blast, be aware that it temporarily drops your white blood cell count to darn near zero, and it takes a few days to get back up.

Dude, I think you just found a treatment for leukemia.

That's bizarre. But...stranger things have happened. I've no idea of the true nature/behaviour/issues of leukemia.

But, note that trickles of electricity have been used as of recent to turn ordinary cells back into Stem cells.

Please look up "Dr. Royal Raymond Rife" to see how nefarious the chasing of money can really be. Dr Rife freed us from cancer issues about 50 years ago, and for his efforts - his laboratories and equipment were destroyed and burned down. He died in Mexico as a despondent alcoholic. One of the most intelligent men that ever lived. He built two optical microscopes that were capable of looking at live virii. he used monochromatic spectrum to achieve this wonder, in the late 40's, and it has not even been close to equaled to this day. He designed every bit of the 7000 parts that went into each microscope. For the crime of saving humanity from massive medical industries --- he was destroyed. Look it up. As true as it gets. And what is the record of the past-still goes on this day. Just like it was current - then.