[Amps] More on Toroidal Tank Circuit Issues

Sat Mar 22 08:52:49 EDT 2014

Jim,

Self resonance in a coil is generally from the middle of the coil to 
surroundings, or from a middle area to the ends.

It is NOT across the whole winding. That would be a harmless parallel 
resonance. You will not shift the resonance much that way.

The mechanism is just like an RF choke series resonance:

http://www.w8ji.com/rf_plate_choke.htm

Two things you can do is minimize excitation of the toroid by shorting it 
with a very low impedance short and moving it out of the intense fields 
around the tank components, or add a center tap that you also short when not 
using the toroid (so you have an end-to-end-to-center short.

You also can spread the turns at the midway point to make an air gap, which 
will move resonance up.

You are having a common problem with large inductors. This is why large 
rollers cause problems and burn up near ten meters, and why GOOD band 
switches should be progressively shorting pick up and  hold contacts.

Test your toroid like I show a choke is tested in that link, and you will 
see the issue.

73 Tom

----- Original Message ----- 
From: "Jim W7RY" <w7ry at centurytel.net>
To: "Dan Hearn" <n5ardxcc at gmail.com>
Cc: "AMPS" <amps at contesting.com>
Sent: Saturday, March 22, 2014 12:08 AM
Subject: Re: [Amps] More on Toroidal Tank Circuit Issues

Thanks for the idea Dan...

I moved the toroid away from the loading cap. Made a huge difference. It’s 
now hooked to the back of the loading cap and the bandswitch. Now it runs 
pretty cool. No more smoking or turning the fiberglass tape brown. The core 
temp gets to about 125-140 degrees when transmitting on 10 meters.

10 is the only band where the 160 meter toroid heats up. I may try to wind 2 
toroids instead of just using one? The toroid is 3ea T200-2 cores wound with 
fiberglass tape.

After I moved it, I put a 100 pF silver mica across the toroid. It 
immediately went up in flames!  Then I tried a 1800 pF cap. It got pretty 
hot but it didn’t really help anything. Amazing how much RF is across that 
toroid even though it’s shorted out electrically. 10 meters must really 
create quite a bit of stray current around the chassis and on the output 
side of the tank. Not sure if it’s the Pi/L circuitry or the fact that it’s 
10 meters.

Not sure if a wire coil would be any better, but it’s worth a try. I have 
one that would work perfectly for both 80 and 160 (42 uH ceramic coil-form) 
but it’s only wound with #14 wire. I think that may be a bit lite for 80 
meters contesting RTTY. But it may be worth a try. Perhaps tomorrow.

From: Dan Hearn
Sent: Friday, March 21, 2014 5:47 PM
To: Jim W7RY
Cc: AMPS
Subject: Re: [Amps] More on Toroidal Tank Circuit Issues

Jim, I would try a small air wound coil similar to what you would use in a 
parasitic suppressor in series with the toroid, maybe in the lead to the 
switch.

Dan, N5AR

On Fri, Mar 21, 2014 at 4:46 PM, Jim W7RY <w7ry at centurytel.net> wrote:

  Both leads are shored by the band switch when on 10 meters guys. 
Interesting that 3 of you came up with the same possibility...

  Not sure where to put the capacitor if both leads are shorted together. 
Perhaps I could center tap the toroid (bare the Teflon wire) and solder a 
cap from the center tap to one of the output leads.

  I removed the toroid and the major heat in the 3 turns of wire is where 
the winding is within 1/4 inch of the vacuum loading cap. Which is probably 
to close.
  See: 
http://s900.photobucket.com/user/w7ry/media/8877%20Amplifier/P1010217.jpg.html?sort=3&o=16

  Jim, I think your 160 toroid when shorted is self resonant with its 
distributed capacitance on 10m.  You might be able to solve this by putting 
a small cap across the 160 m toroid to move its self resonance.

  73, Dan, N5AR

  On Fri, Mar 21, 2014 at 1:08 PM, Jim W7RY <w7ry at centurytel.net> wrote:

    I may have to eat some crow here about using dedicated toroids on 160 
and 80 meters...

    I built an 8877 amplifier based on the K8RA (SK) 3CX1500D7 amplifier. 
http://www.k8ra.com/index_049.htm

    Mine is shown here: 
http://s900.photobucket.com/user/w7ry/media/8877%20Amplifier/DSC_0037.jpg.html?sort=3&o=47

    And here: 
http://s900.photobucket.com/user/w7ry/media/8877%20Amplifier/P1010225.jpg.html?sort=3&o=13

    The lower toroid is 160 only and the upper one is 80 meters only (with 
10,15,20 and 40 coils added in of course).

    I’m using a shorting band switch 6 position.

    On 10 meters, the 160 meter toroid (lower one) gets so hot, the 
fiberglass tape has turned brown, and the Teflon wire is melting the 
Plexiglas that the toroid's are mounted on.

    I know all about trying to use a tapped toroid for multiple bands is 
wrong (except on the L coil of the Pi-L circuit, and see my postings about 
the Commander HF-2500 and K1TTT’s pictures of same) but WHY is the 160 meter 
toroid getting so hot when using 10 meters?  It's mounted very close to the 
loading vacuum capacitor is the only thing that I can think of. But the 
weird part, is that this just started. During all of the testing, there was 
no problem, but now, its really getting HOT!

    When testing, I had the top cover off of the amplifier, now that I’m 
using it, the top cover is on. I’m going to try the test again with the top 
off and see what happens.

    On 80 and 160 meters, the toroids are normal temperature (around 100 
degrees) and everything works just fine since I’m not shorting turns on the 
toroids.

    Ideas?
    Thanks and 73
    Jim W7RY
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  -- 
  Dan Hearn
  N5AR

-- 
Dan Hearn
N5AR

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