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Tuesday, January 17, 2012

Another JBOT Amplifier

Over the weekend I built another JBOT 5 watt linear amplifier (design by Farhan). I used a nice piece of copper-clad board that Dave, W8NF, sent me (thanks again Dave).

This time I chickened out regarding the possible conductivity of the anodized heat sinks. I didn't have any trouble with this on my first JBOT, but I worried that if the anodized layer gets flaked away, a heat sink might short one of those collectors to ground. To be on the safe side, I put small squares of Gorilla Tape on under the heat sinks. (PLEASE don't tell me that Gorilla Tape is conductive!)

For T1 and T2 I used FT50-43 toroids instead of the TV baluns used by Farhan. He had recommended FT37-43's as an alternative to the TV baluns, but I went with the slightly larger toroids. For T3 I rolled my own binocular core using four FT37-43 toroids stacked 2X2.

The amplifier has passed the smoke test. Next I have to put in the low pass filter (Steve Smith: Please note that I have left space on the board for the filter.) Then this version will face its real test when it goes into the 17 meter Azores SINGLE sideband rig.

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  1. Nice build OM. Here's hoping for amplifyin' and not oscillatin'!


  2. Sure, sure. I'll believe that when I see it (the filter). :-)

    Seriously, nice build there, OM. This one looks much neater than its predecessor.

    73.......Steve "Just call me 'Thomas'" Smith

    "Snort Rosin"

  3. Bill,

    I looked closely at your photographs after reading your description and there's just one thing I don't understand: What is that core to the right of your output transformer. It looks like a large bead.

    73.......Steve Smith WB6TNL
    "Snort Rosin"

  4. Good eye Steve! Yes, that is a large ferite bead. It it on the wire from the PA's RF choke to the center tap on the transformer. That line carries the 12 volts DC to the 4 final transistors. During the debugging of my first JBOT, putting a bead there seemed to help stabilize the amplifier. So I threw one in on this one too. I may also put a bit of shielding between the LP filter and the final transistors -- this too seemed to help last time around.
    LP filter will be finished tomorrow.


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