[Amps] Plate coupling capacitor value

[Jim Thomson]

Date: Wed, 06 Sep 2017 21:51:52 -0500
From: donroden at hiwaay.net
To: amps at contesting.com
Subject: [Amps] Plate coupling capacitor value

Is there any hard or fast rule about how much capacitance to use  
between the tubes and the pi-net ?   if 1000pf is good, is 2000pf ....  
3000pf better ?
Better to parallel several 500pf Centralabs or use one big Russian ?

Don W4DNR

DonR

##  check out w8jis site, where he swaps out a 2000 pf plate block cap for
a 170 pf unit.   No difference in po..on any band 160-10m.   Only difference
was a tiny bit with the tune + load settings. https://www.w8ji.com/blocking%20capacitor.htm

##  However,  the tubes plate to grounded grid C  is directly in parallel with the C1 tune cap. 
The two are in parallel.... but the plate block cap is what parallels them together.   On 10-12-6m
bands, the tube C is 80-90%  of the total tune C required,  with very little C1  tune C required. 
All that  anode to chassis current now has to flow through the plate block cap to even get to the 
C1  tune cap. 

##  The plate block cap handles very little RF current , esp on the lower bands, like  160-20m.
Its on  15-12-10-6m where it has to handle more current.    Plate block cap value is a non issue, 
you can use whatever you want.  Most of the older and also newer engineering  formulae tend to use
way too much pf for the plate block C.   So base your plate block requirements on higher current handling
capability  on the upper HF bands. 

##  JIs notes show actual current flow... but that is only for 160-20m.  Its a heck of a lot more on 12-10m. 
The more the tube C, like with multiple tubes, or tubes with a lot of anode to grid C,  the current requirements
on 12-6m  increase a lot.   You not only have tube C to deal with, but also stray C from the anode of any metal
tube... to the chassis below, and also side, top, and rear walls.   Case in point,  my 3CX-3000A7 tube has  24 pf
from anode to grid..when measured on a wooden bench  with the lcr meter.   Which then rises to 33 pf, when
installed in the socket.   Its not the socket either, as I removed the socket, and left just the grid ring in place..and
its still 33 pf.   The extra 9 pf came from the proximity of the lower anode fins...to the chassis  directly below the tube. 

##  The only time you require those giant  3 inch diam russian  plate block caps is if running 25 kw CXR out on 11M..which is
where they are used. 

##  Typ, a pair of 500 pf  HEC  or centralab TX type doorknobs is ample  for a 1.5 kw ham amp... up to 6M.   Like the small
5 kv types. 

Jim   VE7RF 

Jim   VE7RF