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 Post subject: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 1:49 am 
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Hello I have some amplifiers here that have capacitors in series in the power supply. The old capacitors that I replaced were insulated In cardboard tubes. What I replaced them with are blue mallory computer grade type capacitors. These mallory capacitors have a blue plastic insulation and I am wondering If the case of these capacitors should be further insulated.


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 2:29 am 
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j0shua wrote:
Hello I have some amplifiers here that have capacitors in series in the power supply. The old capacitors that I replaced were insulated In cardboard tubes. What I replaced them with are blue mallory computer grade type capacitors. These mallory capacitors have a blue plastic insulation and I am wondering If the case of these capacitors should be further insulated.
I am going to try to provide some helpful advice, but it will largely be based upon assumptions because there are a number of things that are unknown here.

Unknowns:
What is the reason the original capacitors are in series (usually a voltage rating issue, but is there some other reason?).
It there anything around the capacitors that could potentially penetrate the plastic insulation?
Is there a "physical support" reason for a cardboard sleeve around the capacitors?

My advice:
I figure the main reason the two capacitors are in series is that a single capacitor did not achieve the rated voltage for the circuit. Therefore the outside body of one of the capacitors would be sitting at 1/2 of the full supply voltage. If the original capacitors had no insulation on the outside case (it was bare metal), there was absolutely a need for the cardboard sleeve to avoid a potential short circuit to other objects in the amplifier.

If there was no physical support for the two capacitors, then the cardboard sleeve also provided stability of the joint between the two capacitors and also helped keep them from rattling around.

So it wouldn't hurt to add a cardboard sleeve to the new capacitors and might help prevent their insulation from being inadvertently penetrated by anything sharp that might be near the capacitor body.

The cardboard sleeve might be made from something as simple as the center of a role of paper towels.

Note: All advice may be completely wrong if the picture I got of the situation based upon your description is totally off base. That can only be addressed with additional information (pictures often help immensely along with specifications of the original and replacement capacitors).

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 4:02 am 
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It seems odd that an ordinary amplifier would have voltages so high that capacitors would have to be connected in series instead of just using a higher rated capacitor. Is this an ultra-high-power tube amplifier? Say 200 watts or more?

Are you sure the capacitors are in series? Capacitors in parallel is much more likely. If the wiring is a rat's nest it could be hard to tell. Take another look.

Or maybe it's a solid-state amplifier with dual power supplies. In that case each capacitor would be for a separate supply but the positive of one would be connected to the negative of the other but there would also be other connections (like ground) at the junction.

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 5:16 am 
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j0shua wrote:
Hello I have some amplifiers here that have capacitors in series in the power supply. The old capacitors that I replaced were insulated In cardboard tubes. What I replaced them with are blue mallory computer grade type capacitors. These mallory capacitors have a blue plastic insulation and I am wondering If the case of these capacitors should be further insulated.

I have some of that style Electrolytic cap, but they are old enough to vote...Twice over!

Have you tested the replacement caps to verify at least their capacitance?

And instead of making us guess what the electronic configuration of them is in the circuit, please advise the make and model numbers of the amps, and if possible their schematics. That way you can expect an educated answer, rather than a supposition.

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 2:29 pm 
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Many audio amplifiers used a voltage doubler power supply, which has two capacitors in series, for voltages of 300-450V. Are the caps full insulated (no metal showing on bottom of can)?

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 2:43 pm 
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Unknowns:
What is the reason the original capacitors are in series (usually a voltage rating issue, but is there some other reason?).
It there anything around the capacitors that could potentially penetrate the plastic insulation?
Is there a "physical support" reason for a cardboard sleeve around the capacitors?

The amplifiers are heathkit aa-81 B+ is around 560v so back then I don't think there was an electrolytic with such a voltage rating.
The new mallory's are being held by the usual metal capacitor clamp.
It is my understanding that the original astron caps were in cardboard tubes because the outside of the can on one of them was hot from being series connected.

So I am wondering are these newer caps the same, is the outside of one of them hot ?

I did test these caps for esr and mocked them up in series with bleeder resistors at their rated voltage in series 800v for an extended period of time.

The only schematic I have for these monoblocks is from the stereo version the aa-121.

Sorry I have limited experience for this stuff and know enough to be dangerous as you can tell.


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 3:08 pm 
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[url]https://photos.app.goo.gl/LK1otvcEG29QR2tt5

Tom the only metal I recall them having were the metal tabs used to attach to the slots in the wafers.
I went with the mallory's cus I thought they would look nice and since they were new in the box they could be formed up and used.


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 11:21 pm 
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j0shua wrote:
The amplifiers are heathkit aa-81 B+ is around 560v so back then I don't think there was an electrolytic with such a voltage rating.
Gee, what a difference it makes having a manufacturer and model number. Your description of capacitors in a cardboard tube had me picturing two capacitors stuffed into a single cardboard tube like two batteries in series. Now I know what you are talking about. Even though also had trouble finding a schematic I did find a picture (below).
Attachment:
aa-10_aa-81.png
aa-10_aa-81.png [ 1002.72 KiB | Viewed 517 times ]
Also indeed each capacitor on top of the chassis is inside of a cardboard insulator because yes, one of them is at 1/2 B+ voltage on the can.
j0shua wrote:
The new mallory's are being held by the usual metal capacitor clamp.
Make sure the clamp does not pinch through the insulating plastic on the capacitors and make sure that any exposed metal on the capacitors is "taped over" or protected from being touched. Only one of them is a shock hazard, but might as well treat both the same.

Note your aa-81 is electrically the same as the aa-10. If you go to https://audiokarma.org/forums/index.php ... ld.784525/ and scroll down you will see a file attachment aa-10 sch.pdf to download it you will have to become a member of Audiokarma (free)

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Last edited by Eickerman on Sep Sat 04, 2021 11:49 pm, edited 2 times in total.

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sat 04, 2021 11:44 pm 
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I'd be extremely careful, since the blue plastic insulation on the new caps may or may not be rated for the voltage on the outside of the cap. If that's the case, additional insulation, such as the aforementioned cardboard, might not be a bad idea. Or a few turns of electrical tape around any part that contacts a clamp or any part of the chassis. Etc ..............

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 12:15 am 
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Thanks guys I thought the terminals were the only parts of a capacitor that could present a shock hazard its a good thing I didn't end up with newer capacitors that are just painted.


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 12:35 am 
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j0shua wrote:
Thanks guys I thought the terminals were the only parts of a capacitor that could present a shock hazard its a good thing I didn't end up with newer capacitors that are just painted.
Well, it is possible that the outside IS isolated from the terminals, but that is not guaranteed (depends on the specific capacitor). You could do an ohm meter test between the outside metal and the terminal to see if they are directly connected (Zero ohms is directly connected, showing nothing different between open air and connected between the can and the terminal would be "isolated").

If you could point us to a spec. sheet for the capacitors in question it might be stated there somewhere.

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 12:40 am 
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I just did that on one of the spares I have here, just did a poke test and they look to be well insulated cg151t400r4c is the part number. ill see if I can dig up any info on them


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 12:42 am 
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In capacitors like that, frequently neither terminal is connected to the can. But there is usually significant leakage to the can so they need to be insulated even if an ohmmeter says that there is no connection.

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 12:49 am 
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Wait never mind I finnaly poked through the plastic and the reading is climbing as the capacitor is charging from the meter on both terminals. So I guess I need to insulate these better.


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 12:56 am 
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Tested a few more capacitors and it looks like they all could be dangerous in series operation...


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 2:58 am 
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I wanna thank everyone for possibly saving my life. I hate to loose the vintage look of the amp so I am going to source some clear acrylic tubing to further insulate these caps.


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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 2:06 pm 
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j0shua wrote:
I wanna thank everyone for possibly saving my life. I hate to loose the vintage look of the amp so I am going to source some clear acrylic tubing to further insulate these caps.

Or possibly Heat Shrink tubing? It's usually black and may look kinda original.

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 Post subject: Re: Capacitors in series should they be insulated.
PostPosted: Sep Sun 05, 2021 8:17 pm 
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Most manufacturers' detailed spec sheets will tell you that the blue, orange, yellow, or clear plastic shrink wrap on most capacitors is intended as an outer jacket and is not to be relied on for high voltage insulation. Indeed, I have never seen a spec sheet for a cardboard covered electrolytic that claimed it was rated for any particular voltage either. Mostly the plastic or cardboard was intended so there was not this big piece of exposed metal at some voltage to the chassis that could accidentally be shorted out or shock an unwary technician who reached in and touched it. But I'd be surprised if they ever intended that insulation (plastic or paper) to withstand more than a hundred volts or so. Usually when electrolytic cans were used at elevated voltages they take steps to insulate it more completely or put it under the chassis.

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