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[Amps] Capacitor and Alpha 374 question

Subject: [Amps] Capacitor and Alpha 374 question
From: Ted <>
Date: Sun, 21 Aug 2016 13:35:25 -0400
List-post: <">>
Thanks to all who saw the pf vs uf issue. As Paul Harvey use say, "Here is the rest of the story...."

      You're right the pf was caught by five others....BUT....not my
fault!  Note in the picture of the power sulpy it says the oil filled is C24 
and in the parts
list for C24 it says "40pf," not uf.  However after getting
toasted on the pf thing I did check the schematic and it says 30uf there
for c24.  What's a dummy like me to believe!

Thanks to all who replied.  Actually I never noticed but since my replacement 
caps are all uf what I really wanted to know was whether I had to use resistors 
and caps...which everyone said to use so thank you all for catching the pf/uf 
thing but mostly, thanks for the resistor/cap question I had.  I just didn't 
want to do more work than necessary but wanted to make sure I was safe.

On 8/21/2016 12:00 PM, wrote:
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Today's Topics:

    1. Re: Capacitor and Alpha 374 question (Roger)


Message: 1
Date: Sun, 21 Aug 2016 07:25:21 -0700
From: Roger <>
To: "" <>
Subject: Re: [Amps] Capacitor and Alpha 374 question
Content-Type: text/plain; charset=UTF-8

I find it strange that the op specified the suspect capacitor as "40pf" /
5kv. Either this is a slip of the fingers on the keyboard , most likely, or
something is sorely out of place.

73, Roger

On Sun, Aug 21, 2016 at 3:36 AM, Rob Atkinson <> wrote:

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
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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