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 to ALC Line.

  • D2: 1N4148 Signal Diode (Protection).

  • R1: 1 kΩ, ½ W Resistor (Current Limiter).

  • LED (Optional): Red LED + 4.7 kΩ Resistor (Visual ALC indicator).

  • Connector: RCA or 3.5mm plug (matching your radio).

ASCII Schematic:

       FROM SB-200 ALC JACK
            |
            |
          [ R1 ]  1 kΩ
            |
            +----------> TO TRANSCEIVER ALC INPUT
            |
          [ D1 ]  Zener (6.2V - 7.5V)
            | (Cathode to Ground, Anode to Line)
            |
          [ D2 ]  1N4148 (Anode to Ground, Cathode to Line)
            |
          GROUND

  • How it works: When the SB-200 generates negative ALC voltage, D1 (Zener) conducts once the voltage exceeds its rating (e.g., -6.2V), clamping the line to a safe level. D2 protects against any positive voltage spikes. 

  • Visual Monitor: To add an LED, connect the LED (with a 4.7 kΩ series resistor) from the ALC line (after R1) to a small positive voltage source (like a 9V battery) or use a dual-LED setup to show activity. Simpler: Just use a high-impedance voltmeter across the ALC line to monitor.

For best performance, modify the internal sampling point to balance the tubes and limit voltage at the source. 

  1. Balance the Sample: Move the stock sampling capacitor C29 (5 pF) from the grid of one tube to the junction of the two 33 Ω grid resistors (R21/R22).  This samples both tubes equally.

  2. Add Internal Zener: Solder a 1N4737A (7.5V) Zener diode directly across the internal ALC output terminal and ground (Cathode to Ground, Anode to ALC line). This prevents the voltage from ever exceeding -7.5V internally.

  3. Adjust Threshold: The stock ALC potentiometer can then be adjusted to set the "turn-on" point without fear of over-voltage. 

4. Usage Tips

  • Setting ALC: Tune the amplifier to full power (600W). Increase drive until grid current is ~80-100mA. Adjust the ALC pot on the SB-200 until the ALC voltage just begins to appear (or your radio's ALC meter shows slight deflection). 

  • Monitoring: Use a multimeter on the ALC line. If it exceeds -10V, reduce drive or adjust the ALC pot. 

  • Safety: If your radio does not have an ALC input, do not connect the ALC line.  Instead, carefully monitor grid current manually to avoid overdrive.

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