Date: Wed, 06 Sep 2017 21:51:52 -0500
From: donroden@hiwaay.net
To: amps@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
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