Date: Thu, 7 Oct 2010 17:54:01 -0700
From: "Jim W7RY" <w7ry@inbox.com>
Subject: Re: [Amps] Autotuner for Amp Input
You should be able to pick up a 23rd edition on Amazon.com.
73
Jim W7RY
## I found 38 x mistakes in that book, and am only 1/2 way through it.
Most of em are typo's and stuff, so be careful. Some of it is silly stuff,
like calling it C1 and C1, when they meant C1 and C2. In his red covered
book, they have C1 and C2 reversed on the tuned input, with C1 being
closest to the cathode and C2 facing the xcvr. That caused Jon Ogden
no end of grief with his 4-1000 tuned input.
> Playing with the models shows that designs between 25 and 70 ohms
> cathode impedance meet the 13pf/meter criteria using a pi-net input with
> a Q of 4. Above 70 ohms not so much.
## 13 pf per meter is not even close. I found the better way to do it is
to use GM3SEK's PI spreadsheet. That works out dead on for tuned
inputs every time. Beware, it will not accept 2 x equal Z's... like
50 ohms in and 50 out, or 70 in and 70 out. We have to 'fool' the
software a bit..since it was made for high power pi nets.. and factors
in stray tube C, and plate choke Z, etc. If the tube input Z is say 40 ohms,
you enter 50 ohms for the 'plate load Z'.. and 40 ohms for the output Z.
IE; ur trying to match 50 ohms to 40 ohms. Enter 999999 for the value of the
plate choke, and enter 0 for all stray C and stray L. Enter a Q =3.3 for
the
operating Q. It will spit out the correct values. The input Q will be
something like
2...and the output Q will be 1.3 The sum of the input and output Q = total
network Q. The C1 cap is based on the input Q of 2 in this example.
[same deal with conventional hi-power pi nets in kw amps, these days we call it
a loaded Q of 12... in the old days they called it a loaded Q of 10. The
network
Q is actually 12..... 10 on the input, and 2 on the output. ]
## don't get the Q too high..esp on the higher bands. On my 3CX3000A7, input
Z is aprx 50 ohms. I tested the tunable pi net by putting bird wattmeters on
both
sides of it..plus heath dummy load on the output. With 200w applied to the
input,
I was only getting 160w out on 20-17-15m. [and 198w on lower bands] This was
after
tweaking the C1 +C2 caps for flat swr. I had to increase the coil taps by
just 1/4 turn
on each of the higher bands [ and decrease the value of C1 +C2] . Ok, now I
was getting
195 watts out on the upper bands.
## I even tried a Q of 5+...and the solid 6 ga cu wire that the 4 uh tapped
coil was made from,
would actually start getting warm with 200w cxr applied. 200w=2A into 50
ohms. With a Q=5,
you end up with 10A of circulating RF current. I verified that by inserting
an RF ammeter on input
end [ low bands]. Point here is, get the Q too high, and you lose power in
the tuned input coil..esp
with typ 16-22 ga material. Here I thought I was putting 200w into the
cathode.. and was burning up
40 w ! After the fix... then it all goes to the cathode. The result is more
grid current/plate current..
and way more power out.
## I didn't want to mess with 9 x tuned inputs, and design them to each
handle 200-300w... so opted
for a tapped 4 uh coil made from 6-8 ga wire [ or .125" cu tubing]. Coil
consisted of 17 turns on a
1.5" ID. 2 x 2160 pf broadcast variables used. Each was 4 x sections... and
each section is 17-540pf.
All 4 x sections simply strapped in parallel. Each broadcast cap is padded
on 160m....with 4 x 500 pf
doorknobs [HT-50/58 type]. 4" ball drives, [with 6:1 redux vernier] made by
jackson bros was used on each
cap. They are marked 0-100, in fine increments, across 180 deg of the skirt.
4" diam = 12" circumference,so
the 0-100 scale is marked over 6" . Tune it once for each band, then write
down the numbers.
## Then you are guaranteed a flat input swr, across the entire band..on every
band. For a laugh, we tested
the tuned input, with the bird wattmeters on both sides, into the dummy
load..with a 800 w cxr... and it
runs just fine, zero heating, all bands. But the Q can't be too high. Run it
up till the PO starts to drop
off, then back the Q down a hair. Ok, now it's dialed in correctly on the high
bands. On the low bands
you can get away with higher Q, [4-6] but it doesn't buy you much. It's fine
for a manual tuned input, since
you can have 2 x settings.. CW and phone band. For fixed PI nets, it would
be a pita...as swr will rise
on band edges.
Jim VE7RF
>
> If I ever find a copy of Orr's 23rd edition at a reasonable price I'll
> pick it up. I see quotes from it zip past here every now and then.
>
> 73,
> Jim N7CXI
>
>
> On 10/7/2010 11:58 AM, Gary Schafer wrote:
>> Bill Orr recommends around 13pf per meter for the capacitance at the
>> cathode
>> of a GG amplifier.
>>
>> 73
>> Gary K4FMX
>>
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