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Re: [Amps] Replacing padder caps on an Alpha 77DX/SX

Subject: Re: [Amps] Replacing padder caps on an Alpha 77DX/SX
From: Jim <>
Date: Tue, 26 Mar 2019 18:21:07 -0500
List-post: <>
I would imagine that a 500 or 750 pF doorknob that could handle the current on 160 meters is at least $150.00 or more.

That is why modern amplifier manufactures don't use them. They use what I used, and more likely surface mount types on a small circuit board.

Jim W7RY

On 3/20/2019 3:21 PM, Rob Atkinson wrote:
There's nothing wrong with doorknobs, but you have to get the right
kind of doorknob.  A class 2 RF doorknob specified for a few amps at
1000 KC is in my experience far better in a 160 m. output network than
disc ceramic caps, but if you have true class 2 disc caps and you can
parallel several of them for the needed value and voltage, have at it.
The temp. coeff. isn't extremely critical; some hams have an idea that
they have to have NP0...not true, but the 1000 KC current spec. IS
important.  You can't just throw in any old doorknob from a
hamfest--it's a dice roll as to whether it blows or not.

It is also incorrect that micas are no good--most h.v. fixed value RF
caps in phasors and matching networks in broadcast service are micas.
Gates used micas and variable inductors in the output networks in some
of their medium wave broadcast rigs.  But, old F case micas from
hamfests leak half the time, so in a critical application, it's better
to stick with new RF HEC or Russian doorknobs, Sangamo or CDE black
cast micas.


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