[Amps] Capacitor and Alpha 374 question

[Roger]

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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