[Amps] Plate coupling capacitor value

[Jim Thomson]

Date: Fri, 8 Sep 2017 08:50:09 +0100
From: Chris Wilson <chris at chriswilson.tv>
To: amps at contesting.com
Subject: Re: [Amps] Plate coupling capacitor value



<Hello John,

<What  value  blocking  cap  would  you  suggest  for a 136kHz amp I am
<building using a single GS35B tube? Thanks.

##  1800 khz / 136 khz = 13.24  
XC is proportional to freq.   At the max end, assuming 2000 pf on 1800 khz, 
I would use   2000 pf x 13.24 =  26,470 pf. 

##  However, JI sez that  170 pf on 160m is just fine.    So  170 pf x 13.24 =  2250 pf. 
One of JIs  friends used 100 pf on 1800 khz,  zero heating.   So  100 x 13.24 =  1324 pf. 

 https://www.w8ji.com/blocking%20capacitor.htm  read through this.   Its only 1.275 amps,
and that’s  peak, not rms.     Caps are rated in rms current. Peak rating of a given cap will be a lot more
than the stamped  rms rating.  

##  My best guess is... 2000 pf  - 10,000 pf  would be ample.     Since it’s a monoband
  136 khz amp,  you have some wiggle room..... you don’t have to worry about higher freqs. 

##  Ceramic HT-50s and HT-58s handle less current at lower freqs.. vs higher freqs.... like 160 vs 10m.
Mica caps are the opposite, they handle more current on lower freqs.  Typ used in lower freq
2-5 mhz marine and AM broadcast...and also below 500 khz.   Those mica  types are readily avaialble, all over the
place, typ with flange plates at each end, or simple screw terminals at each end, and usually stamped 
with a CCS current rating at a specific, typ low freq.   

Dont get too concerned, just leave enough room, in case you have to parallel more caps, use longer caps, etc.
Who the heck is down on 136 khz ?  Any ant on that band will be a nightmare at best.   PI net values..if a PI net
is used,  will be bizzare.   How much harmonic suppression is required ?? 

Jim   VE7RF