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
On 6/18/2012 3:57 AM, Rob Atkinson wrote:
> What's the total value of the bypass caps in the 77Dx?
>
> tnx
>
> rob
> K5UJ
>
> On Mon, Jun 18, 2012 at 1:01 AM, Jim Barber <audioguy@q.com> wrote:
>> I'm getting around to servicing the blower (again) in my very last
>> complete Alpha PA77, and decided that since I'll have the side off I
>> might as well add the toroid and caps to put it on 160m. Note it's been
>> 20 years or so since I've done this, so memories are a bit foggy... :-)
>>
>> Many things confuse me, but what's relevant to this discussion is that
>> the plate chokes for *both* the PA77 (10-80m) and the 77Dx (10-160m)
>> appear to be 90uh on a ceramic form plus a 15uH molded choke in series.
>> The 77Dx also has a smattering of additional bypass caps, (that part
>> makes sense) but isn't a total of 115uH a bit light for 160m? There's
>> room to put in something like an RFC-3, but I'm wondering what the
>> original strategy was, assuming anyone remembers.
>>
>> I also notice the the 77Dx has no tuned input in the 160m position. I
>> can probably ring out the approximate cathode impedance "offline" by
>> putting 50 ohms (or so?) between the cathode and ground, pulling in the
>> input relay then putting the VNA on the input connector. What I find
>> puzzling is why there would be no tuned input for 160m on the 77Dx,
>> since it was shipped as being capable on 160m.
>
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