[Amps] More on Toroidal Tank Circuit Issues

Sat Mar 22 16:25:21 EDT 2014

So I used the measurement technique from Tom W8JI that he has on his site and found that the 160 meter toroid was certainly resonate on 10 meters! No matter what I did. Sure, I could move my hand around the toroid and get the the lamp to dim and sometimes go out. But there didn’t seem to be any place moving the windings that would help. 
http://www.w8ji.com/rf_plate_choke.htm

So..... I made a center tap on the 160 meter toroid and simply shorted it to one end with a normally closed vacuum relay. I open the relay only on 160 meters.  

See here:
http://s900.photobucket.com/user/w7ry/media/8877%20Amplifier/IMG_0250_zps17bca129.jpg.html?sort=3&o=0

Seems to work just great. No arcing or sparking. As you can see, the leads are pretty long and the relay is just taped to the Plexiglas. I used the bench power supply to energize the relay on 160. There was no interaction with anything. Even had the lid off when testing 160 meters. 

The toroid seems to run perfectly cool (about a 10 degree temperature rise on 10 meters measuring with my Fluke IR thermometer) when operating on all bands. Tuning seemed better and more power output with less drive and lower plate current for the same output power (1600 watts) on 10 meters just like removing the toroid all together. 

I'm going to move the 160 meter toroid back to where it was, and move it further from the load vacuum cap. If I had it to do over again, I would have mounted the caps closer to the outside walls to give me more room to work between them.

I’ll also have to wire up the relay to energize on 160 meters. Rather than tie into the tuned input switching, I’ll just put another switch contact between the front panel and the sub panel.

73
Jim W7RY

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

  I was simply going to move it away from the loading cap and see what happens. I was also thinking of install a wire coil. I have a ceramic coil form that would work. 30 turns, 2”dia 3.5” long which is 20uH. I have room to mount this.

  Thanks all!
  73
  Jim W7RY

  From: Dan Hearn 
  Sent: Friday, March 21, 2014 3:02 PM
  To: Jim W7RY 
  Cc: AMPS 
  Subject: Re: [Amps] More on Toroidal Tank Circuit Issues

  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