Robust VHF Parasitic Suppressor Design (Based on QST Oct 1990)

 

Robust VHF Parasitic Suppressor Design (Based on QST Oct 1990)

Based on Richard L. Measures, AG6K’s article "Parasitics Revisited" (QST, Oct 1990, pp.  32–35), the most robust method for suppressing high-VHF oscillations (common in tubes like 3-500Z, 8874, and 572B) is the Low-Q "U-Inductor" (Hairpin) Suppressor. 

Unlike traditional coiled suppressors that can magnetically couple to their resistors (raising Q and narrowing bandwidth), the U-design ensures magnetic decoupling, creating a broad, low-Q loss network that effectively swamps oscillations from 100 MHz to over 500 MHz. 

1. Design Theory & Mathematics

The goal is to lower the Q of the anode circuit's self-resonance without introducing significant loss at HF (3–30 MHz). 

  • Inductance ($L_s$): The U-loop acts as a low-Q inductor.

    • Target: 60 nH to 80 nH.

    • Formula approximation for a U-loop: $L \approx 0.02 \cdot l \cdot [\ln(2l/r) - 0.75]$ (where $l$ is length in inches, $r$ is wire radius).

    • Measures specifies a 3.5-inch span for 60 nH (3-500Z) and 4.0-inch span for 80 nH (lower voltage tubes like 572B). 

  • Resistance ($R_s$): Placed across the open end of the U.

    • Target: 100 Ω to 150 Ω total.

    • Configuration: Two or three 100 Ω, 2W metal-film resistors in parallel (yielding 50 Ω or 33 Ω) OR a single 100 Ω depending on the specific tube's VHF gain.  Measures suggests starting with two 100 Ω resistors in parallel (50 Ω) or just one 100 Ω if space is tight, but explicitly cites two or three 100-Ω, 2-W metal-film resistors soldered across the U. Correction based on text: The text says "two or three 100-Ω... resistors soldered across the U." If placed in parallel, this lowers resistance. If placed in series/parallel combo, it adjusts wattage. The standard implementation is often two 100Ω resistors in parallel to handle power and lower resistance to ~50Ω, or simply one 100Ω if the text implies a single equivalent. "consists of two or three 100-Ω, 2-W metal-film resistors soldered across the U." This implies a parallel arrangement to increase power handling and lower the net resistance to swamp the high gain. Let's assume a net 50 Ω to 33 Ω load across the inductor for high-gain tubes. 

  • Material: Nichrome 60 (NiCr) wire (No.  18 AWG).

    • Why? Nichrome has higher resistivity than copper, lowering the Q of the inductor itself. It also withstands high heat.

  • Geometry: The "U" shape minimizes magnetic coupling between the inductor legs and the resistor, preventing the formation of a high-Q transformer effect. 

2. Component List

  • Wire: No. 18 AWG Nichrome 60 wire (approx. 4.5 inches total length). 

  • Resistors: Two (2) 100 Ω, 2 Watt Metal-Film resistors (1% tolerance).

    • Note: Do not use carbon composition here; metal film is non-inductive and stable.

  • Solder: High-temperature silver solder (e.g., Stay-Brite) and appropriate flux, as nichrome is difficult to solder with standard rosin. 

3. ASCII Schematic

       TO TUBE PLATE (Anode)
            |
            |  (No. 18 Nichrome Wire)
            +-----------------------+
            |                       |
            |                       |  <-- Leg Length ~ 1.75" each
            |                       |      (Total Span A-B = 3.5")
            |                       |
            |                       |
            +-----------+-----------+
            |           |           |
          [ R1 ]      [ R2 ]      [ R3 ]  (Optional 3rd resistor)
        (100 Ω)     (100 Ω)     (100 Ω)
            |           |           |
            +-----------+-----------+
            |
            |  (No. 18 Nichrome Wire)
            |
       TO TANK CIRCUIT (Output Capacitor/Choke)

Dimensions:
- Span A to B (Resistor mounting points): 3.5 inches (for 3-500Z/8874)
- Leg Height: ~1.0 to 1.5 inches (determined by resistor length)
- Wire Gauge: #18 AWG Nichrome 60

4. Construction Steps

  1. Cut Wire: Cut a 4.5-inch piece of No. 18 Nichrome wire.

  2. Form U-Shape: Bend the wire into a "U" shape. The distance between the two parallel legs (Points A and B) should be 3.5 inches for high-voltage tubes (3-500Z, 8874) or 4.0 inches for lower voltage tubes (572B, 811A). 

  3. Mount Resistors: Solder the two 100 Ω metal-film resistors across the open ends of the U (Points A and B).

    • Tip: Keep the resistor leads short to minimize stray inductance.

    • Power: Using two resistors in parallel yields 50 Ω at 4 Watts total dissipation capability. 

  4. Install: Connect one leg of the U to the tube's plate pin (or anode cap) and the other leg to the tank circuit input.

  5. Orientation: In multi-tube amplifiers, orient adjacent U-suppressors at 90° angles to each other to prevent magnetic coupling and push-pull parasitics. 

5. Why This Works Better

  • Decoupling: The straight legs of the U and the perpendicular resistor placement ensure the magnetic field of the inductor does not couple into the resistor, maintaining a low Q.

  • Broadband: Unlike a coiled suppressor which might only dampen a narrow frequency, the U-inductor provides effective suppression over a wide VHF/UHF range (100 MHz – 1 GHz). 

  • Thermal Stability: Nichrome wire and metal-film resistors can withstand the heat generated during 10-meter operation or a parasitic event without drifting in value. 

Citations

  1. Measures, R. L., AG6K. "Parasitics Revisited." QST, October 1990, pp. 32–35. (Specifically Fig. 7 and associated text).

  2. Measures, R. L., AG6K. "Improved Anode Parasitic Suppression for Modern Amplifier Tubes." QST, October 1988, pp. 36–38, 66, 89.

  3. Measures, R. L., AG6K. "Calculating Power Dissipation in Parasitic-Suppressor Resistors." QST, March 1989, pp. 25–28. 

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