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Re: [Amps] Oil-caps delating 2/3 ratings?

To: <>, <>, <>
Subject: Re: [Amps] Oil-caps delating 2/3 ratings?
From: "k7rdx" <>
Date: Thu, 6 Mar 2008 10:36:57 -0800
List-post: <">>
Gerald,how did your capacitors fail? Did they short internally? Did they 
rupture?In my experiance over the years with oil filled caps, I have found 
the pressed case types are generally designed for uses such as Heart 
defibulators, and some I have had were from airport pulse strobe light 
supplys,hardly rated for constant use.If the cap is small with a higher than 
realistic capacity and voltage rating, then it`s probably for the described 
uses. There are so many hams out there that buy this kind of junk for big 
prices and really have no idea that when used in a typical ham HV supply, 
they overheat internally very quickly and more times than not self-destruct. 
Sure makes a mess in the power supply when one of these things goes "Poof" 
and spews dielectric all over everything,not to mention the fire danger.
There are good oil filled capacitors out there but your`e going to pay the 
big bucks..I have GE welded case capacitors in my HV supply here...I have 
run them many times for several hours and they seldom get even mildly warm 
to the touch..Yes,they are very expensive.I had a 35mfd/7500vdc rated cap 
mfg by Maxwell I used for several years with my old 4-1000x2 
amplifier...That one had an idling plate voltage of 6700vdc...Used it many 
times and put lots of hours on it...Never a problem..Have you priced Maxwell 
caps?.Moral of my story: If you want to play you have to pay...And know what 
your`e paying for!..Jim,K7RDX..
----- Original Message ----- 
From: <>
To: <>; <>
Sent: Thursday, March 06, 2008 8:48 AM
Subject: Re: [Amps] Oil-caps delating 2/3 ratings?

> Han, I have lost two similar capacitors running at about 3500vdc over the
> years. Both of my capacitors were purchased from Peter Dahl Co and were 
> rated
> for 5000vdc working voltage. I came to the conclusion that oil type 
> capacitor
> ratings are not as robust as electrolytic type ratings.
> I think the failure mode has to do with ripple current and associated
> heating. Both failures happened while calling cq for long periods in an 
> EME  contest.
> 73,
> Gerald K5GW
> In a message dated 3/6/2008 4:03:05 A.M. Central Standard Time,
> writes:
> Recently  an old man told me that all of the caps are energy storage jobs
> and they  are usually derated for continuous duty to about 2/3 rating.
> I thank him  very much.
> I am using cheap 32uF 4500V GE non-pcb caps.
> Should I use  it at 3000V maximum?
> Fortunately I am using this potential level  now.
> If someone use 16uF 7500V Arioso caps like Henry,
> maximum working  voltage should be 5000V?
> Would you please tell me the reason for the  "2/3 derating"
> practically when using them for our linear amps?
> For  a long time I had a question why GE had not indicate working
> voltage and  test voltage but only "4500V".
> Additionally anyone did a  destructive/non-destructive test on it?
> Seemingly they are sustainable at  5kV, but if the number 2/3 work
> their life should be very short.
> de  Han JE1BMJ
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