[Amps] peak voltage rating for load cap

Jim W7RY jimw7ry at gmail.com
Sun Aug 20 13:23:55 EDT 2017

Take a few plates out of a lager value cap. That way it increases the 

Jim W7RY

-----Original Message----- 
From: Jim Thomson
Sent: Saturday, August 19, 2017 11:48 AM
To: amps at contesting.com
Subject: [Amps] peak voltage rating for load cap

Date: Sat, 19 Aug 2017 12:07:14 +0200
From: Conrad PA5Y <g0ruz at g0ruz.com>
To: amps at contesting.com
Subject: [Amps] peak voltage rating for load cap

<Hello what is the rule of thumb for conservatively estimating the
<required ratings for a load capacitor when used with a mono band Pi-L
<tank circuit?

Anode voltage 2750V

Tube design RL = 2600 ohms

Q = 12

I have searched on the internet an I cannot find any really convincing
info, I have something that will stand 1200V but that is marginal and
will not survive a fault I expect.

Alternatively does anyone have a small vac variable around 40-60pF that
is around 3 inches/76mm? That will obviously do the job.

Also where can I buy a couple of Ohmite Z-50 chokes in Europe? Anyone
got one or two that they can spare?


Conrad PA5Y

### Ok, what band is this for ????    You wanted a 40-60 pf cap, for the 
load cap of
a  PI-L amp.  The only thing I can think of is a 6M amp..and only if its a 
PI-L, as you desired.
A simple PI net on 6m would require a bigger cap than  40-60 pf.

##  The intermediate Z  across the load cap on any PI-L can be anything you 
want, like
from 60-1000 ohms.   Typ its aprx 250-350 ohms.    The peak V  across a 300 
ohm Z, is
obviously gonna be a lot more than a simple PI net load cap... which is at a 
50 ohm point.

##  If this is for a  mono band 6M amp, as I suspect, and with a 2600 ohm 
plate load Z, a better
option would be to use a L-PI, where some  uh is place before the main PI 
net. The added small uh
between the plate block cap..and the main C1 tune cap, forms a step down L 
End result is the main Pi can now be designed around a much lower plate load 
Z...and increased  harmonic
reduction.  Typ, with lower plate load Zs, harmonic reduction increases.

##  use GM3SEKs, free, excellent  PI  spreadsheet, he also has a PI-L...to 
get the correct values for both
coils in the L-PI..and also the correct values for the C1 + C2 caps.    Even 
in the L-PI  configuration,
the resulting ..transformed lower plate load Z  will result in a higher C2 
value..vs a normal plane PI.
Moot point, since the peak V across the load cap  will be across a 50 ohm 

## peak V  across a 50 ohm load cap, with 1.5 kw out  =  389 V.   Your 1200 
V rated load cap is more than
up to the task, even with some SWR.   I doubt you will be operating with a 
3:1  swr on 6m, or any other band.
Square root of 3 = 1.73     Even with a 3:1  swr, peak V across the load cap 
would then be  389 X 1.73 =  673 V peak.
Your  1200 V rated cap is still up to the task.

##  If this is for a monoband 6M, a simple way to make the main tank coil is 
to use copper strap, instead of copper tubing.
Copper strap, wound flat, has the advantage of conducting RF on BOTH sides 
of the strap.   Tubing only conducts
RF on the OUTSIDE of the tubing. IE:   .375 inch wide strap is the same as 
using .25 inch OD tubing.
Another advantage of strap coils, wound flat, is there is almost zero stray 
C  between
adjacent turns.   Hence the spacing between turns can be very close, like 
just a few mm.  Strap coils are very easy to wind,
in any diameter, and the diameter can easily be reduced..or increased. 
Strap coils are also easy to terminate,  just punch a hole
at each end to terminate to the plate block caps at one end.... and the load 
cap at the extreme other end.  The aprx middle
of the coil is also easy to punch, and terminate on the main C1 Tune cap. 
Strap can also be easily cut to any width you want.
10-15mm  wide strap would work superb on 6M.
Typ you want to use strap that is aprx .024 to .032 inches thick.  IE: 
.6mm  to .8mm  thick.   1mm thick is also fine.
Its a lot easier to wind a strap coil... vs a tubing coil, with the tubing 
being  6-10mm in diamter.    The diameter of the strap
coil can be easily tweaked, on the fly, so the extreme ends  end up right 
where you want them.

##  The L-PI  configuration also has the advantage of being able to use a 
lower loaded Q, vs a simple PI net....esp  on
15-12-10-6M bands.

##  whatever configuration you end up using, make sure to use 50 ohm coax 
between the output of the  PI,   L-PI,
PI-L, or  L-PI-L  and the TR relay.   The braid at each end has to be done 
right, so you dont end up with any Z bumps, otherwise
you will end up with the load cap at a High Z point.

Jim  VE7RF

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