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Simple ALC Monitor for the Heathkit SB-200

  Simple ALC Monitor for the Heathkit SB-200 The stock Heathkit SB-200 ALC circuit samples RF from one grid, causing imbalance, and can generate excessive negative voltage (up to -35V) that damages modern transceivers (which often require only -8V to -10V).  Here is a simple, safe ALC Monitor/Clamp design you can build externally or internally. 1. The Problem Voltage Mismatch: The SB-200 can output -30V to -35V on the ALC line when overdriven.  Radio Limit: Most solid-state radios (Icom, Kenwood, Yaesu) are damaged or shut down by anything below -10V .  Imbalance: The stock sampling capacitor (C29) connects to only one tube grid.  2. Simple External ALC Clamp Circuit This circuit connects between the SB-200's rear ALC jack and your transceiver. It clamps the voltage to a safe level while allowing you to monitor the ALC activity with an LED or meter.  Components: D1: 1N4735A Zener Diode (6.2V) or 1N4737A (7.5V). Note: Connect Cathode to Ground, Anode ...
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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). ...
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