Query: Conversion of Ganged Johnson Variable Capacitor to Differential Type

Discussion in 'Ham Radio Discussions' started by K5HU, Nov 18, 2021.

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  1. K5HU

    K5HU Ham Member QRZ Page

    I've noticed that differential type variable capacitors are scarce for building T-Network antenna tuners. Has anyone tried to slip the plates of a Johnson dual-ganged variable capacitor to be 1/2 turn out of step? If anyone has been successful in doing this, please provide some guidance about how to accomplish it. If it is not possible, please say this too.
    Thanks and 73, Bill, K5HU
    Last edited: Nov 18, 2021
  2. K7MEM

    K7MEM Ham Member QRZ Page

    IMHO, you would be better off using two single variable capacitors, connected with a coupler. The variable capacitors would need to have the shaft extending out the rear, so it can be coupled to the other variable capacitor. One of the capacitors can then be offset 1/2 turn.
    W1PEP, W0FS and N2EY like this.
  3. WA9SVD

    WA9SVD Ham Member QRZ Page

    Either option ( yours or that proposed by K7JEM; there are probably others as well) will depend entirely on the exact construction of the variable capacitor you intend to use. If the plates are welded (as some I've seen) then such a modification would be nigh onto impossible. IF they are simply "compression fit," then it MIGHT be possible, (again, depending on the cap's construction) but realize you may not be able to reconstruct the capacitor they way YOU want, and you may (instead) destroy an otherwise "good" capacitor for what it IS (not for what it COULD be.) Personally, I'd keep searching to find the appropriate capacitor rather than taking a chance of ruining a good one, but YMMV.
  4. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    You might want to look at the Ultimate Transmatch as opposed to a T network. It has the same imprdance matching range, and it uses two standard variable caps, and a roller inductor. I built one years ago, even included a 5 KW 4:1 Balun in it, it still works great, and will handle 3 KW all day.

    The design is in the ARRL antenna handbook. Just use the biggest capacitors you can find, with plenty of plate spacing, and you can't go wrong.
  5. N2EY

    N2EY XML Subscriber QRZ Page

    KA4DPO likes this.
  6. W4HAY

    W4HAY XML Subscriber QRZ Page

    How is the capacitor made? Are the plates soldered to the shaft or held in place by a couple of nuts and spacers between the plates?
    If the plates are held by the nuts and spacers, and the shaft is round, I.E. no flat spots or grooves, the nuts can be loosened slightly and the plates rotated.. If soldered to the shaft, it's a no-go.
    MFJ has a 40-300 pF differential capacitor P/N 282-2015 used in their Differential-T tuner . It's gonna be pricey, though.
  7. W1VT

    W1VT Ham Member QRZ Page

    A machine shop can be used to make another shaft. A Sherline miniature lathe and milling machine will easily handle the brass and aluminum stock typically used to make electronic parts.
  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The larger EF Johnson variable caps, which are assembled with nuts, bolts, washers and screws can be easily modified. I have even made my own by re-assembling parts salvaged from several of these caps to assemble one to my own specs. Just take the capacitor apart, and remove the rotor section. Carefully loosen the bolts that hold the bearings and rotor plates together. Rotate the plates of one section 180 degrees. Making sure the plates are precisely lined up and exactly 180 degrees from the opposite section, re-tighten the nuts that sandwich the plates and spacers together. Re-assemble the cap.

    Be very careful not no lose the small fibre washers under the screws and nuts that hold the end plates to the long connector rods that hold the capacitor together; replace them exactly as they were originally. They serve an important purpose of insulating the connecting rods from the end plates, to avoid a closed loop from shaft-end plates- connector rods, that otherwise might carry unwanted circulating currents.

    Although less important with a differential capacitor than with a balanced split stator, one should make sure all the plates on the rotor are correctly aligned, i.e. every plate is at maximum or minimum mesh at the same setting, and not have some a degree or two off. If this is not carefully observed, a split-stator unit will not be perfectly balanced at all settings, and will cause problems in circuits where balance is critical. I use a flat steel plate with a slot in it, removed from a defunct table saw, as a tool to align the plates, but this might be tricky to do with a differential capacitor, but as stated previously, exact balance is less critical since the sections are 180° out of balance with each other anyway, so a slight unbalance of a plate or two might not hurt anything.
    W1PEP likes this.
  9. W0FS

    W0FS Ham Member QRZ Page

    If you could obtain a capacitor in kit form, like those used in the Heathkit SA-2060 tuner, I believe you could build one that way.
  10. K8BZ

    K8BZ Ham Member QRZ Page

    There are some excellent videos showing the construction of very nice high voltage variable caps. Looks like an excellent home brew project. This is just one example, not necessarily the best example but it shows the technique. Making a differential cap with this technique wouldn't be hard.

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