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

[Jim Thomson]

Date: Thu, 28 Mar 2019 15:42:54 -0500
From: Rob Atkinson <ranchorobbo at gmail.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] Replacing padder caps on an Alpha 77DX/SX


>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.

You imagine incorrectly.  Even brand new HEC 15 KV is about half that price.

>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.

<No, they don't use them because hams are cheap customers, most don't
know any better, the duty cycles are usually low, and manufacturers
buy in lots of 1000 or more.  But a single ham with one amp can afford
quality.

<73
Rob
K5UJ

##  Typ  HT-50  +  HT-58  door  knobs in the  5  /  7.5 kv variety   are in the  $12.00   to  $14.00   range..in single lot  quantities. 
Buy them in huge quantities   and  prices drop like a rock.   A  single  HT-58 in 700 pf, would easily handle the required current
as a padder on  160m... and still not drift.   Screwing down the  HEC  doorknob to the chassis  heat sinks the doorknob,   increasing its
current rating.     Although caps  with better temp coefficients  are preferred  as padders, this is normally not required.  IF the padder
in question has more than ample current carrying capacity, the cap  wont heat up in the 1st place.    Once that is factored into the equation,
drift becomes a non issue. 

##  The 15 kv  HEC  doorknobs  Rob is referring to  are called  HT-57s...and typ  cost  $40.00   Again, in single lot quantities.   Not required
for a padder  application.      20 kw into  50 ohms  is only  1414 Volt peak.    

## Forget ham amps, look at how commercial amps are built, they use the real deal. 

Jim   VE7RF