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

To: <>
Subject: [Amps] Replacing padder caps on an Alpha 77DX/SX
From: "Jim Thomson" <>
Date: Mon, 18 Mar 2019 12:07:55 -0700
List-post: <>
Date: Sun, 17 Mar 2019 16:31:27 -0400
From: "Gary Smith" <>
Subject: [Amps] Replacing padder caps on an Alpha 77DX/SX

<As there's no sign of overheating I can 
<find anywhere and I haven't been beating 
<on the amp, I'm assuming the drift is from 
<the 160M padder caps breaking down over 
<the years and they should be replaced.

<I understand the early models had one 500 
<pF padder and later models like this one 
<have two 1000 pF padders in parallel.

<The two paralleled 1000pF caps for 
<band-switch position B are easily 
<accessible from the side, the two 
<paralleled 100 padders for Band-switch A 
<are located deep where I can't get a 
<screwdriver in to undo them.

<Also, I'll probably get the HT58 padders 
<from RF Parts, any other worthy places I 
<might also check into?

<Thanks & 73,


##  The story I got was the original  77-DX  used a single  padder 1000 pf  
It would  drift on 160m,  on brand new amps,  so folks  replaced  it  with a 
pair of  500 pf  doorknobs. 

##  The  77SX, using  2 x 8877s  just compounds the problem..since  more 
padding is required, hence the
use of  2 x 1000 pf doorknobs. 

##  Replace the   2 x 1000  pf  doorknobs   with   4 x 500 pf  doorknobs, and 
you  wont have any more drifting issues
on 160m.   Use the  7.5 kv version of the  HT-50 /  58.   The  500 pf doorknobs 
 will handle more current than the  1000
pf version.   With 4 of em in there, they each only handle half as much current 
 as  the  1000 pf  units.   Problem solved.
I went through the same issue on a HB amp, using a 1000 pf  doorknob  for a 
160m padder.   Replaced it with  2 x 500 pf
doorknobs, zero issues after the swap.  The 1000 pf cap would run warm to the 
touch..and drift.  

##  The  Temp coeffient on the  1000 pf version is not as good as the  500 pf 
version, which compounds the problem, plus
the  1000 pf version wont handle a lot of current.   BEWARE there is 2 x 
versions of the  500 pf  doorknob !!!   Use the version
that has the lower temp coefficient.   Both are listed on HECs  master catalog 

##  If you opt for something else, like the 100 pf   3 kv  disc  ceramic cap, 
they will  also work, albeit, you will  require 20 of em
in parallel... which will get messy.     4 x 500 pf  doorknobs  simply  bolt to 
the chassis.... provided you have the room to fit  4 of em
in there. 

## ETO used a pair of  500 pf  air variables, for the loading cap, and both are 
gang tuned...and in parallel.   With only 1000 pf of loading available, you
can now see the need  for a lot of padding.   The padders  are handling most of 
the  current on 160m, not the air variables.   The saving grace is the use of 
PI-L  on 160m,  so lower values of  tune and load  C  are used... vs a PI net. 

Jim   VE7RF 

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