henry alc zener

Description: The Automatic Loading Control (ALC) 100 volt 5 watt zener diode in Henry RF amplifiers is known to fail. Initially, it shorts, causing a high voltage (HV) surge, which typically results in the zener blowing open and tripping the main circuit breaker. After the circuit breaker is reset, the amplifier may appear to function normally; however, the ALC circuit will no longer operate. Occasionally, after resetting, the circuit breaker may trip again if the zener did not open following the HV surge. To test the ALC zener, one end should be removed from the circuit and tested with a basic volt/ohm meter. One polarity should show infinity ohms, while reversing the meter leads should yield approximately 100-200 ohms in the opposite polarity. In the Henry 3K-A model, the ALC zener is identified as diode D206. This zener is connected in parallel across the high voltage bleeder resistor nearest to the B- side of the HV power supply. The anode of the zener connects to the B- side, while the cathode connects to the plus side of the last bleeder resistor in the series. For the Henry 3K-A, this connection occurs at the junction of D206 and R116, with the junction of D206 and R117 leading to the RF deck ALC voltage threshold adjustment potentiometer. The Henry 3K-A schematic provides complete details. The circuit operates by mixing the negative ALC voltage from the transceiver with the -100V ALC reference voltage, which subsequently feeds back to the tube bias circuit. If the voltage exceeds the threshold set by the ALC adjustment pot, the gain of the tube is clamped, preventing overdriving the input grid and potential tube damage. Additionally, this mechanism helps avoid non-linear conduction of the tube under overdrive conditions, which can lead to distortion and splattering. This circuit is not essential; due to the zener's frequent failure, some users opt to remove D206, R117, and R118 entirely from their 3K-A amplifiers, instead manually monitoring the grid drive meter to prevent overdriving the amplifier grid input.

The ALC circuit in the Henry 3K-A RF amplifier plays a critical role in managing the operational integrity of the amplifier by regulating the gain of the RF power tube. The ALC zener diode (D206) serves as a reference voltage point, ensuring that the amplifier does not exceed safe operating limits. It is important to note that the zener diode's failure can lead to significant operational issues, including the risk of damaging the RF tube due to excessive grid drive.

In the event of a zener failure, the recommended testing procedure involves isolating the diode from the circuit and using a multimeter to assess its functionality. This ensures that the amplifier can be restored to operational status without the risk of further damage. The connection points of the zener diode are strategically placed to allow for effective voltage regulation, with the B- side of the HV power supply providing a stable reference point.

The design of the ALC circuit aims to prevent distortion by clamping the tube gain when the input signal exceeds a predefined threshold. This is particularly crucial in high-power RF applications, where even minor distortions can lead to significant signal quality issues. Although some users report removing the ALC components to simplify the circuit and avoid zener-related failures, this practice may compromise the amplifier's ability to handle dynamic input signals effectively.

In summary, the ALC circuit, while prone to failure, is an essential component of the Henry 3K-A RF amplifier, designed to protect the amplifier and maintain signal integrity. Proper maintenance and testing of the ALC zener diode are crucial for ensuring the longevity and performance of the amplifier.The ALC (Automatic Loading Control) 100 volt 5 watt zener in Henry RF amplifiers is a known problem for going bad. It will first short causing a HV surge, then the HV surge will normally blow open the zener and the main circuit breaker will trip.

After resetting the circuit breaker, the amplifier will appear to function OK, however the ALC circuit will no longer be operational. Sometimes after resetting the circuit breaker it will trip again because the zener did not open after the HV surge. To test the ALC zener, remove one end from the circuit and test with a basic volt/ohm meter. You should have infinity ohms in one polarity direction, reversing the meter leads, you should have approximately 100-200 ohms in the opposite polarity direction.

In the Henry 3K-A, the ALC zener is diode D206. The zener connects parallel across the HV bleeder resistor which is closes to the B- side of the HV power supply. The anode of the zener goes to B- side of the HV power supply and the cathode side of the zener goes to plus side of the last bleeder resistor in the bleeder resistor series.

For the Henry 3K-A this is the junction of D206 and R116. The junction of D206 and R117 then goes to the RF deck ALC voltage threshold adjust pot. See the Henry 3K-A schematic for complete details. The way the circuit works, the negative ALC voltage from your transceiver will mix with this -100V ALC reference voltage, which then feeds back to the tube BIAS circuit. If the voltage goes higher then the ALC adjustment pot threshold is set to, the gain of the tube will be clamped off.

This prevents you from over driving the input gird and damaging the tube. Its also prevents the tube from conducing non-linear when being over driven, which will cause distortion and splattering. This circuit is not really required. Since this zener is a common failure point, I personally removed D206/R117/R118 totally from my 3K-A amplifier.

I found D206 to be a real headache, shorts all the time, poor design. I just manually watch the grid drive meter and don`t over drive the amplifier grid input.