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Roller Coaster Inductor For Homebrew Amplifier?

Discussion in 'Amateur Radio Amplifiers' started by MM0IMC, Oct 9, 2019.

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  1. MM0IMC

    MM0IMC Ham Member QRZ Page

    I'm gathering parts together to make my first HF valve (tube) linear amplifier. Looking at the tank circuit, what's the pros and cons regarding a roller coaster inductor? I'm sure I could wind my own tank coil, if required...
  2. WR2E

    WR2E Ham Member QRZ Page

    I've never heard them called "Roller Coaster" inductors.

    I think you mean "Roller Inductor", no 'coaster'.
    AE8W likes this.
  3. MM0IMC

    MM0IMC Ham Member QRZ Page

  4. W1BR

    W1BR Premium Subscriber QRZ Page

    Complicated drive system--usually requires an expensive turns counter.

    Roller inductors are not cheap... but then again a good band switch is also expensive.

    Contact resistance issues possible when the roller contact rides on the coil.

    Will still need a band switch to change the input matching for the cathode or grid.

    Very slow tuning if changing bands--have to adjust the coil and tune and load caps.
    KU3X likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Most of the Henry amps used a roller inductor and it became the "PLATE TUNING" control, so the only variable cap is the LOAD control. In lieu of a variable capacitor for plate tuning, they used switched-in fixed value capacitors and used the roller for tuning. Instead of using a rotary wafer switch for this, they used a camshaft to open or close contactors (heavy coin-silver contacts), and a sprocket on the same shaft with a chain drive to another sprocket below the chassis to turn a regular rotary switch for the input tuning networks.

    Mechanically complex, but electrically simple. Someone who likes mechanical work and has a good tool set can do this.

    They also used (mostly) an edge-wound, heavy silver-plated roller inductor they fabricated in house. The whole system was pretty elegant and rarely failed.

    My old Johnson Thunderbolt (c. late 1950s) used a more conventional roller inductor; they've been common in amps and tuners since the WW2 era, so when I was younger there were thousands on the surplus market pretty cheap.
  6. WR2E

    WR2E Ham Member QRZ Page

  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, we just call them roller inductors.

    If you can't sit in it and ride around, it's not a coaster.:p
  8. K6BSU

    K6BSU Ham Member QRZ Page

    Large RF currents on roller inductors will cause burning of the coil wires if adjusted at high power. Antenna tuners with roller inductors have this problem. Using a roller inductor for amplifier main tuning would require tuning at reduced power to limit arcing on the inductor.

    Silver plating oxidizes quickly. Tin plating is subject to burning wherever roller inductor contacts have local resistance.
    KU3X likes this.
  9. K9STH

    K9STH Ham Member QRZ Page


    Amplifiers are best tuned at full power.

    I have tuned my Henry / Tempo amplifier, at up to 1400-watts output, for over 45-years and the roller inductor is still in excellent condition. Also, my Hallicrafters HT-20 uses a roller inductor but it only runs 100-watts output.

    Glen, K9STH
  10. AI3V

    AI3V Ham Member QRZ Page

    Dit dit :)

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