From: Jim Thomson
Sent: Tuesday, April 23, 2019 7:15 AM
Subject: disc ceramic capacitors
> What parameters or characteristics should one seek out when selecting
> disc ceramic caps for power rf applications such as plate coupling and
> padder caps used on the pi network.
## I have yet to see a disc ceramic that even list a RF current rating.
Typ, larger diameter disc ceramic caps will also handle more RF current.
I just bought 2 dozen .01 uf at 1.3 kv rated disc ceramics. These are
25mm diameter types. They are also thicker bodied....vs the smaller 15mm
diameter .01 uf at 1 kv types. How much current a disc ceramic will
handle is largely determined by experiment. Alpha uses several paralleled 6
kv rated disc ceramics
for their plate block caps..which will handle the current on 10M.
## For coupling and bypass caps, temperature coefficient and actual value
is a non issue, provided the value chosen meets specs. IE: sufficient C used
lowest freq involved. For bypass caps, typ a higher value is used for the
lower bands, to ensure
## For plate block caps, they have to be able to handle both the DC
bias.... IE B+
AND the AC voltage drop across the plate block cap. Plate block caps have
far more current
through them on the highest bands, like 10m + 6m.... vs 160m. All the
current that flows through the
anode to grounded grid in the GG triode, also has to flow through the plate
block cap. You also have to
factor in all stray C, from anode to chassis. RF voltage across the....
anode to grounded grid is assumed
to be aprx 60% of key down loaded B+ value. Once you measure the total C
from anode to grid..with tube
in socket, then its an easy calculation to determine the XC...on band
extremes...like 160m..and also say 10m.
## On 10M, the tube C will make up most of the total tune C. On 10m, the
C1 tune cap is basically a tiny,
variable padder, in parallel with the tube C. Problem is, the plate block cap
is in between the tube..and the C1
main tune cap. ALL the calculated RF current that flows through the tubes
C, also has to now flow through the
plate block cap. Any plate block cap has to be sized to handle calculated rf
current on 10m...for modes like FM, and RTTY.
## actual pf value for the plate block cap is a moot point. 100-170-500 pf
will work just fine on 160m, provided that it will also handle
the calculated current on 160m. You cant go by KVAR ratings either. You
can see this on the HEC ( high energy corp) site.
Use their charts and graphs to determine the biggest bang for the buck.
For example, a 200 pf HT-50 or HT-57 cap will handle a lot
of current. Henry radio used 4 x 200 pf caps in parallel on its 4 k ultra,
which is a 80-10m amp. A single 200 pf HT-57 cap will handle
15A on the upper bands. On the low bands, the saving grace is there is very
little current flowing through a plate block cap on 160m.
Even with lower values of C, on 160m, the RF AC V drop across the plate
block cap is very little. Calculated XC X current = V drop
across the cap. That calculated V drop across the cap + .... B+ voltage,
can not exceed the V rating of the plate block cap.
## Padding caps for padding say a tune cap..or a load cap, different ball
game. For padders, they are now part of a tuned circuit. In this case
we dont want the padder to start drifting in value. Here temp coefficient
has to be taken into account..and also the current rating at the freq in use..
Padders are typ used for padding a load cap on 160m. For a simple PI net,
1st calculated the total required load C. Use the largest practical
value variable load cap you can get your hands on..that will also fit into the
amp, for say a hb amp. . That will minimize the required load C for the
worse case, typ 1800 khz..and some swr. Peak V across both the variable load
cap and also the padder, will be the same as the peak V across
the dummy load. Calculate the XC across the padder cap. RMS voltage across
the dummy load...divided by the calculated XC of the padder
= the current flowing through the padder. Knowing all of that, then its a
simple matter to figure out how many paralleled padders required. A single
padder might barely suffice in some cases. In other cases, 2 or more
padders..even though their TC is not as good, will easily work well. With
are one step worse than a NPO cap, their current rating has to be examined.
IF each of the the paralleled padders is handling minimal rated current, their
is not an issue, they wont drift, cuz they are not heating up in the 1st place.
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