[Amps] Capacitor and Alpha 374 question

[Ted]

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.
73,
Ted K8AQM


On 8/21/2016 12:00 PM, amps-request at contesting.com wrote:
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>     1. Re: Capacitor and Alpha 374 question (Roger)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sun, 21 Aug 2016 07:25:21 -0700
> From: Roger <ai7rogerroger at gmail.com>
> To: "amps at contesting.com" <amps at contesting.com>
> Subject: Re: [Amps] Capacitor and Alpha 374 question
> Message-ID:
> 	<CAEahN0JEP407XwVeENFY4uDR2W2-o1zMFM5GRYynULWUo7YZyQ at mail.gmail.com>
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> 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
> W7TZ
> CN83ia
>
> On Sun, Aug 21, 2016 at 3:36 AM, Rob Atkinson <ranchorobbo at gmail.com> 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
>>> 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.
>>
>> 73
>>
>> Rob
>> K5UJ
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