The AM Forum

If I needed to replace a 12uF 3KV oil filled PS cap ..   Would I be best off to find another oil filled cap or use computer grade electrolytics (for example, 8 100uF@450V in series with appropriate equalizing R's)?  What are the tradeoffs?

Thanks ..Grant/NQ5T

I would stay with the oil cap.  Less money, a lot less hassle, takes up less space, more reliable.  I have had many electrolytics crap out over the years, but maybe one or two oil filled.  Electrolytics seem to have a finite life expectancy while oil filled last for ever.

I have a 4 mfd 3000 volt oil filled in one of my xmtrs that has the pre-1935 NRA logo stamped on it.  It still works fine.

I think you could go either way and things would work fine.

Personally, I don't like stings of electolytics with equalizing resistors. Seems like a poor design in that there are many parts that can fail, and if one does fail it likely takes the other parts with it.

Plus you are putting back in a part (electrolytic) that you know will only last a limited time before it has to be replaced again.

I doubt any commercial electonics designer, these days, would design anything with stings of low voltage capacitors in series.

So I prefer a single capacitor of sufficient capacity and voltage to do the job.

If cost is no object, then a single modern film capacitor would be good.

But then meny people use stings of electrolytics and it works.

I would rather get another oil cap.  I dont care for strings of electolytics.  One day you might be sitting there and KABOOM!  one of them wil just go. It really makes a mess and can damage other components. I got used Oil caps from W0vmc for my project.

Clark

I agree with the rest.....oil is the only way to go for HV caps.

Bill

always go oil if you're building or replacing an original oil.

if ya go lytic, don't buy into the "reforming" crap. You cant reform caps that have no electrolyte in them because it dried out 25 years ago.  Always buy new (NOT NOS) replacements, and increase the voltage rating per cap by about 15 to 20%. I always use 500VDC to replace 450 volt.

use the best 2% tolerance EQ resistors money can buy in a lytic cap string and hand match resistors with a vom for the closest values possible. To replace 2 watt carbons, use 2% 3 to 5 watt metal films. Overbuild by design and you'll never have any trouble.

one last thing: always use a blast shield around a lytic cap string. this will keep he rest of your rig clean and free of billions of tiny little cap fragments in case one does blow up.  :P ::)

This reminds of of a story one my Hamfriends told me..He said in highschool electronics class, He placed a 100UF cap in each table top power outlet socket before class.  The work tables had a socket at each workstation. There where about a dozen of them.  Then, When the teacher showed up the next morning and flipped the breaker on to power up the work stations he had one hell of a suprize!  hahahah. 

My friend said there where cap schrapnel all over the room.

Oil is best! If I could find enough oil to replace the electrolytics on the 3CX3000 amp, I'd do it. There was a supply of 100uF 4KV energy storage caps at one time.

i was looking at 3 of these (the ones at the bottom) to get 29 uF 3.6 KV

http://www.rfparts.com/caps_oil.html

That's quite a good amount of storage. Would that be for a choke input?

I ask because I see a very common figure for voltage doublers and bridge rectifiers with C inputs starting about 50-66uF total string. If more volts or uF is needed they can be stacked in rows and columns. I'm wondering what the insulation voltage is on those. looks high.

fairradio seems to have the same, 'dented' for a little less  -$22. They claim to be checked.