Here the plate block cap assy is installed.
5 x 200pf @ 15 kv rated doorknob caps are used to make up the assy. The
200 pf cap's were chosen since they are rated at 15 amps CCS of RF current
on any freq > 5.11 mhz. Total max current rating is now 75 amps. Total
capacitance is 1000 pf. (The 200 pf HT-57 cap is the....'go to'.... cap
for the HT-57 series since their 200 pf cap has the highest current rating
of the entire HT-57 series). Similarly for the bigger HT-59 series, the
...'go to cap'.....in a HT-59 is their 250 pf cap, rated at 35.5 amps CCS
on any freq > 16.0 mhz).
1000 pf = 88 ohms of XC @ 1.8 mhz The general rule for plate block cap
total value is.... The XC of the plate block cap assy shouldn't be any more
than 5% of the plate load Z...at the lowest freq used (1.8 mhz in this
case). In this case, with 7 kv loaded B+ @ 2.5 amps, the plate load Z =
1555 ohms. 5% of 1555 ohms = 78 ohms. 1200 pf would have been 73 ohms of
XC.... but in this case, there is severe space restraints, and only 5 x
caps can be used. Close enough for this application.
2 x copper straps used, one above, and one below..... with the input at the
tube end on top.... and the output at the opposite end...on the bottom.
Done this way so the total physical, combined distance in + out, stays the
same for each cap. Done this way, the RF current is equally divided
between the 5 x caps. (Another similar method used, when 6 or 8 caps are
used, is 2 rows of 3 caps.... or 2 rows of 4 x caps. In those cases, the
input is dead center between 2 x caps at left side of the top plate...and
the output is dead center between the last 2 x caps, at the right side of
the bottom plate. Same deal, the current will divide equally between the
caps).
HEC (high energy Corp) sez the PEAK RF voltage ACROSS the cap (or
paralleled cap assy).....PLUS any ...'DC bias' can not exceed the stamped
cap's voltage rating. What HEC calls ...'DC bias', we call B+. The
saving grace at the low freq end (160m) is.... there is hardly any RF
current flowing through the plate block cap asy.....and the peak RF voltage
is very little.
At the upper end of the HF spectrum, 10m, there is a LOT of RF current
flowing through the block cap assy. Reason is..... the anode to grid C on
the tube makes up 90% of the total required C1 tune C. Think of it as the
tube C is 90% of the required C....and the actual C1 tune cap is a tiny,
variable padder. Issue is.... the plate block cap resides between the
anode and the C1 tune cap. All that current that flows through the tube C
now also has to pass through the block cap assy.
The XC of a 1000 pf cap on 29 mhz = 5.5 ohms. So even with high current
flowing through the block cap, the peak RF voltage is still miniscule.
Just to recap, enough C is required for the lowest freq....and the cap has
to be able to handle a lot of current on upper HF.
https://www.youtube.com/watch?v=bMmCXeB_cew
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