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

[Jim Thomson]

Date: Tue, 26 Mar 2019 18:21:07 -0500
From: Jim <jimw7ry at gmail.com>
To: 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.

<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


###  A single  500-700 pf in a HT-50  will easily handle  8 A  CCS  on 160m. 
If you really want to go crazy,  3 x 170s  in parallel will handle   stupid amounts of current on 160m.
And  all  of the above will easily hi pot test to well  over 8 kv.    These HEC  doorknobs are easy to
mount, just bolt em to the  chassis...which will also  heatsink them..and increase their  CCS current ratings even more !    

##  Before  everybody goes off the  deep end,   you have to 1st  do all the required math calcs  on your 160m amp,  or
the  160m portion of the  160-10m amp  etc.   Is the  circuit a PI  net.... or a PI-L network ?   Then decide what tube or
tubes are being used, what  the loaded B+  will be....and what the loaded  plate current will be.   Then stuff  everything into 
Gm3SEKs  pi  /  pi-l  spreadsheet.    Then also factor in stuff like the uh of the plate choke,  stray C  from anode to chsssis, and 
any stray  uh  between anode   and   C1  tune cap.    The results will spit out everything you need to know, like  plate load  Z,
correct values of the  tune and load caps, and the main tank coil....and also the  2nd coil   if a   PI-L is used.    Also, to get a 
good  Q  on the upper bands, a small of amount of uh is  inserted between plate block caps  and   main C1  cap...forming a 
step down L network on the upper HF bands. 

##  Once we know the correct tank values, and loaded  B+, then its easy to calculate the  current flowing through everything in there.  
Load cap RF  current is easy to calculate.   If the variable load cap is not big enough for  160m  use, a padder is used, and the current 
division between the air variable load cap and the padder is also easily calculated.   Once we know exactly how much current  flows through
the padder  assy,  the rest is simple.  Use a single padder, or paralleled  padders  that will handle the current involved.  No rocket science here
folks.   On any PI net, the peak V across the variable load cap..and any padder is the same as the peak V  across the output coax connector...
which is like    388  volts peak.... into a 50 ohm dummy load.   Peak V  in a PI-L is higher, since the load cap is  at  typ  a 200-400 ohm point. 
Typ  PI-L  network uses  Way less  load C  vs a simple PI.    Less  load C means the XC of the load cap  increases a bunch.   So even with the higher
voltage across the load cap on a PI-L, the  resulting current   through the load cap remains aprx the same as the PI. 

##  As it happens, a local buddy is building a  hb  160m   YC-156  monoband amp next month, to supplement his existing  80-10m  YC-156 amp.  
Im supplying most of the parts, including the fixed value  edge wound  coil, and ceramic  vac  tune and ceramic vac load caps.   For the load cap, we
will be using a small jennings  50-4000 pf  unit, rated at 5 kv.  The  160m monoband amp will use a simple  PI net.   The idea here is...use the biggest
variable load cap  you can get your hands on.... then if any padding is required, it will be a lot less........  vs a smaller value variable load cap.

Jim   VE7RF