Date: Sun, 17 Mar 2019 16:31:27 -0400
From: "Gary Smith" <Gary@ka1j.com>
To: amps@contesting.com
Subject: [Amps] Replacing padder caps on an Alpha 77DX/SX
<As there's no sign of overheating I can
<find anywhere and I haven't been beating
<on the amp, I'm assuming the drift is from
<the 160M padder caps breaking down over
<the years and they should be replaced.
<I understand the early models had one 500
<pF padder and later models like this one
<have two 1000 pF padders in parallel.
<The two paralleled 1000pF caps for
<band-switch position B are easily
<accessible from the side, the two
<paralleled 100 padders for Band-switch A
<are located deep where I can't get a
<screwdriver in to undo them.
<Also, I'll probably get the HT58 padders
<from RF Parts, any other worthy places I
<might also check into?
<Thanks & 73,
<Gary
<KA1J
## The story I got was the original 77-DX used a single padder 1000 pf
doorknob.
It would drift on 160m, on brand new amps, so folks replaced it with a
pair of 500 pf doorknobs.
## The 77SX, using 2 x 8877s just compounds the problem..since more
padding is required, hence the
use of 2 x 1000 pf doorknobs.
## Replace the 2 x 1000 pf doorknobs with 4 x 500 pf doorknobs, and
you wont have any more drifting issues
on 160m. Use the 7.5 kv version of the HT-50 / 58. The 500 pf doorknobs
will handle more current than the 1000
pf version. With 4 of em in there, they each only handle half as much current
as the 1000 pf units. Problem solved.
I went through the same issue on a HB amp, using a 1000 pf doorknob for a
160m padder. Replaced it with 2 x 500 pf
doorknobs, zero issues after the swap. The 1000 pf cap would run warm to the
touch..and drift.
## The Temp coeffient on the 1000 pf version is not as good as the 500 pf
version, which compounds the problem, plus
the 1000 pf version wont handle a lot of current. BEWARE there is 2 x
versions of the 500 pf doorknob !!! Use the version
that has the lower temp coefficient. Both are listed on HECs master catalog
!!
## If you opt for something else, like the 100 pf 3 kv disc ceramic cap,
they will also work, albeit, you will require 20 of em
in parallel... which will get messy. 4 x 500 pf doorknobs simply bolt to
the chassis.... provided you have the room to fit 4 of em
in there.
## ETO used a pair of 500 pf air variables, for the loading cap, and both are
gang tuned...and in parallel. With only 1000 pf of loading available, you
can now see the need for a lot of padding. The padders are handling most of
the current on 160m, not the air variables. The saving grace is the use of
the
PI-L on 160m, so lower values of tune and load C are used... vs a PI net.
Jim VE7RF
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