Date: Tue, 26 Mar 2019 18:21:07 -0500
From: Jim <jimw7ry@gmail.com>
To: amps@contesting.com
Subject: Re: [Amps] Replacing padder caps on an Alpha 77DX/SX
<I would imagine that a 500 or 750 pF doorknob that could handle the
<current on 160 meters is at least $150.00 or more.
<That is why modern amplifier manufactures don't use them. They use what
<I used, and more likely surface mount types on a small circuit board.
<Thanks
<73
<Jim W7RY
### A single 500-700 pf in a HT-50 will easily handle 8 A CCS on 160m.
If you really want to go crazy, 3 x 170s in parallel will handle stupid
amounts of current on 160m.
And all of the above will easily hi pot test to well over 8 kv. These HEC
doorknobs are easy to
mount, just bolt em to the chassis...which will also heatsink them..and
increase their CCS current ratings even more !
## Before everybody goes off the deep end, you have to 1st do all the
required math calcs on your 160m amp, or
the 160m portion of the 160-10m amp etc. Is the circuit a PI net.... or
a PI-L network ? Then decide what tube or
tubes are being used, what the loaded B+ will be....and what the loaded
plate current will be. Then stuff everything into
Gm3SEKs pi / pi-l spreadsheet. Then also factor in stuff like the uh of
the plate choke, stray C from anode to chsssis, and
any stray uh between anode and C1 tune cap. The results will spit out
everything you need to know, like plate load Z,
correct values of the tune and load caps, and the main tank coil....and also
the 2nd coil if a PI-L is used. Also, to get a
good Q on the upper bands, a small of amount of uh is inserted between plate
block caps and main C1 cap...forming a
step down L network on the upper HF bands.
## Once we know the correct tank values, and loaded B+, then its easy to
calculate the current flowing through everything in there.
Load cap RF current is easy to calculate. If the variable load cap is not
big enough for 160m use, a padder is used, and the current
division between the air variable load cap and the padder is also easily
calculated. Once we know exactly how much current flows through
the padder assy, the rest is simple. Use a single padder, or paralleled
padders that will handle the current involved. No rocket science here
folks. On any PI net, the peak V across the variable load cap..and any padder
is the same as the peak V across the output coax connector...
which is like 388 volts peak.... into a 50 ohm dummy load. Peak V in a
PI-L is higher, since the load cap is at typ a 200-400 ohm point.
Typ PI-L network uses Way less load C vs a simple PI. Less load C
means the XC of the load cap increases a bunch. So even with the higher
voltage across the load cap on a PI-L, the resulting current through the
load cap remains aprx the same as the PI.
## As it happens, a local buddy is building a hb 160m YC-156 monoband amp
next month, to supplement his existing 80-10m YC-156 amp.
Im supplying most of the parts, including the fixed value edge wound coil,
and ceramic vac tune and ceramic vac load caps. For the load cap, we
will be using a small jennings 50-4000 pf unit, rated at 5 kv. The 160m
monoband amp will use a simple PI net. The idea here is...use the biggest
variable load cap you can get your hands on.... then if any padding is
required, it will be a lot less........ vs a smaller value variable load cap.
Jim VE7RF
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