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

[Jim]

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.


Thanks
73
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.
>
> 73
>
> Rob
> K5UJ
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