Jim,
Thanks for sharing the 160M toroid details and the pics. I have a
PA-70V and I am about to add 160M to my amp. RF concepts was not able to
provide any guideance here but I was able to find a copy of the original ETO
instructions with the step by step detail on how to add 160M to the amp.
The only caveat I would add is that the coil turns count seem to vary
between recommended 27 turns, your 28 turns, and I heard of one reference
using 29 turns to optimize 160M. I plan to start out with 29 turns and
adjust accordingly.
If anyone wants a copy of the ETO conversion to 160M instructions for a
PA-70V please contact me off list.
73 Mike K9MK/5
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Jim Barber
Sent: Monday, June 18, 2012 11:47 PM
To: Rob Atkinson
Cc: Amps@contesting.com
Subject: Re: [Amps] Alpha PA77 vs 77Dx plate choke?
And just so it's in the archives:
The components for adding 160m to the original Alpha 77, PA-77, "D" and "S"
models are are below. (apparently ETO built "D" and "S" models that did not
have 160m, because according to the docs the models that did are "Dx" and
"Sx")
According to RF concepts:
The toroid is (3) T200-2 iron-powder cores glued together, wrapped with
glass tape and wound with 28 turns #12 temp-coated wire. (like Thermaleze
from Amidon)
** Please note the "Dx" *also* has (2) 100pF doorknob caps that are switched
in parallel with the loading caps. The old ETO notes for modifying the
original 77 for 160 make reference to a "2nd load cap", but don't specify
type or quantity. To confuse things further, a factory-modified PA-77
carcass in my garage has the toroid, but does not have any additional
doorknob caps beyond the 500pF unit normally found in unmodified amps. For
the amp I'm working on right now, I'm going to add the two 100pF caps just
to be safe.
The toroid stack should be spaced off of the chassis to avoid cutting the
field or shorting out turns. ETO originally used rectangular strips of
unplated PC board across the top and bottom of the stack, with a bolt-hole
drilled in the middle of each. I like a little more spacing than that, but
bear in mind that it can get hot down there; common plastics like acrylic or
polystyrene might melt. PTFE or Delrin will work, fiberglass may be easier
to get.
If you dig for it a bit, the current RF Concepts site has a hand-drawn
schematic of the PA-77 bandswitch connections for the modification. I won't
re-post it for fear of copyright issues, but if it ever disappears and I'm
still alive you'll be able to email me for it. I will, though post a picture
of my factory-modified PA-77 tank circuit. You can't see the inductor really
well without taking the side cover off the amp, but this should give enough
of an idea to know where to mount it:
http://www.n7cxi.com/alpha77/160toroid.jpg
And finally, please note the picture is from a parts amp, I try not to let
cobwebs accumulate in my working ones. :-)
73,
Jim N7CXI
On 6/18/2012 8:04 PM, Rob Atkinson wrote:
> work up some kind of plastic fork wand thing a few feet long for
> holding the IR thermometer close to the choke or just tape it to a
> length of PVC.
>
> thanks for the cap information.
>
> 73
>
> Rob
> K5UJ
>
>
>
> On Mon, Jun 18, 2012 at 1:21 PM, Jim Barber <audioguy@q.com> wrote:
>> That might be the right question.
>>
>> Starting at the supply end on the 77Dx:
>> From the 15uH molded choke, there's .002uF to ground.
>> At the junction of the 15uH and 90uH chokes, there's .005 total to
ground.
>>
>> Again the two chokes are in series, with the 15uH going to the supply
>> and the 90uH being the "main" choke.
>>
>> WRT input from Carl & Paul, I think what I'll do in the short term is
>> add the additional bypass caps to make it fit the 77Dx schematic,
>> fire it up and see what happens. The choke is far enough from the
>> tube to measure temp with a non-contact IR gadget, or if I can't get
>> good readings I can borrow a thermal imaging camera for a couple of
>> days. ( I secretly lust for one anyway, but can't justify ~$2K to buy
>> one) If the chokes or bypass caps get hot enough to worry about after
>> a few 300-watt AM transmissions then I can consider what to do next.
>>
>> Thanks to all,
>> Jim N7CXI
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