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[Amps] Capacitor and Alpha 374 question

To: "" <>
Subject: [Amps] Capacitor and Alpha 374 question
From: Rob Atkinson <>
Date: Sun, 21 Aug 2016 05:36:52 -0500
List-post: <">>
>I have a three tube Alpha 374, it has an oil-filled capacitor rated at 
>40pf/5KV. After running awhile in cw an AC hum occurs on my signal. I am 
>thinking that after heating up this >old oil-filled capacitor may be going bad 
>and breaking down.

Run the amplifier until you hear the hum then shoot a temperature of
the cap with an IR thermometer.  Take the cap temp. when at room
temperature first.  The temperature shouldn't rise much if any.  If it
does, then it has resistance and is leaking.  It is unusual for this
to happen but if it is mounted so the lugs are on top, then the lug
seals can age and leak in air which can cause some oxidation or
contamination of the dielectric, which in this case is probably oil
impregnated paper.

Don't just assume the cap is bad and blow a pile of money.  Make sure
first.   There are other ways hum can be getting in.

>It seems impossible to find another such oil-filled capacitor so I bought a 
>capacitor replacement kit from "Tube Meister." The add says it is for 
>Alpha/76/78/374. I do have two >other Alpha 374A amplifiers and the 
>replacement kits are great.

You can find oil caps that are 30 mfd or more at your voltage rating
at hamfests and on eBay.   Don't pay hundreds of dollars for a new one
unless you have more money than you know what to do with.   If a
replacement doesn't fit, so what--leave it outside the cabinet and run
h.v. wire to it.

>My question is since there are just two leads from the oil-filled capacitor 
>can I just simply connect the 6 210pf/450 volt capacitors in series and run 
>the leads to where the oil->filled capacitor connected? Am I missing some 
>important step here? I'd rather not blow the 374 up! Perhaps even someone 
>might suggest where one finds an oil-filled 40pf/5kv >capacitor?

Rather than answer the question so you can just plug and play, I
suggest you take advantage of an educational opportunity and study the
amp schematic and look at other amp schematics that use electrolytic
filter caps and figure it out.  Then ask yourself, Why do they go that
way? and read Wikipedia on capacitors and circuit books to find out
why.  I'm not trying to be a !@#$% but everyone had questions like
that at one time including me, and I studied and figured it out.  You
miss out on well over half the fun of radio if you don't learn this
sort of thing.


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