[Amps] Alpha PA77 vs 77Dx plate choke?

[Jim Barber]

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 at 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