How To Short Pins 4 And 5 On 12ax7 Tubes

How to Short Pins 4 and 5 on 12AX7 Tubes: A Comprehensive Guide

Modifying 12AX7 tubes by shorting pins 4 and 5 is a popular technique among audio enthusiasts seeking specific tonal alterations. This modification, often referred to as a "shorting mod" or "cathode follower mod," alters the tube's operation, typically resulting in a warmer, thicker, and potentially more compressed tone. However, it's crucial to proceed with caution as improper modification can damage your tube or equipment. This guide will walk you through the process safely and effectively.

Understanding the Modification

Before diving into the practical steps, let's understand why we short pins 4 and 5. These pins correspond to the tube's plate and screen grid. By connecting them, we essentially create a cathode follower configuration. This circuit modification changes the tube's gain characteristics and impedance, influencing the overall signal path. The result is often described as a decrease in high-end frequencies and an increase in low-end warmth.

Potential Benefits of Shorting Pins 4 and 5:

• Increased Low-End Warmth: Many users report a noticeable thickening of the low-end frequencies.
• Reduced High-Frequency Harshness: The modification can tame overly bright or harsh tones.
• Increased Compression: The altered gain stage can lead to a more compressed, saturated sound.

Potential Drawbacks of Shorting Pins 4 and 5:

• Reduced Gain: Expect a decrease in overall gain.
• Potential for Instability: Incorrect implementation could lead to instability or oscillation.
• Irreversible Modification: Once shorted, the modification is generally not easily reversed.

Materials You'll Need:

• 12AX7 Tube: The tube you intend to modify.
• Small Jumper Wire: A short length of thin, insulated wire is ideal. Make sure the insulation is suitable for the voltage in your circuit.
• Soldering Iron: A low-wattage soldering iron is recommended for delicate work.
• Solder: Use rosin-core solder designed for electronics.
• Helping Hands (optional): This will free up your hands during the soldering process.
• Multimeter (optional): For testing the connection after soldering.

Step-by-Step Guide:

1. Preparation: Ensure your equipment is completely unplugged and disconnected from the power supply. Safety is paramount.

2. Identify Pins 4 and 5: Carefully examine the tube's base to identify pins 4 and 5. Reference a tube pinout diagram if necessary. Double-check to be certain before proceeding.

3. Cut the Jumper Wire: Cut a small piece of jumper wire, long enough to comfortably connect pins 4 and 5.

4. Strip the Wire Ends: Carefully strip a small amount of insulation from both ends of the jumper wire.

5. Soldering: Holding the wire in place, carefully solder one end of the jumper wire to pin 4, then solder the other end to pin 5. Use a minimal amount of solder to avoid creating unwanted bridges.

6. Inspection: Once cooled, carefully inspect the solder joints to ensure they are clean and secure. Use a magnifying glass if necessary.

7. Testing (Optional): Using a multimeter, check the continuity between pins 4 and 5. You should read zero ohms indicating a good connection.

8. Reinstallation: Carefully reinstall the modified tube into its socket.

9. Testing: Power on your equipment and listen for any unusual sounds or behavior. If you hear any unusual noises or distortion, immediately power down.

Important Considerations:

• Tube Selection: The effect of this modification can vary depending on the specific 12AX7 tube. Experimentation may be necessary.
• Circuit Compatibility: This modification is not suitable for all circuits. Research your specific amplifier's circuit before attempting.
• Risk of Damage: There is a risk of damaging your tube or equipment if this modification is done incorrectly. Proceed with caution and at your own risk.

This guide provides a basic overview. Always prioritize safety and consult relevant resources before attempting any modifications to your electronic equipment. If you're uncomfortable performing this modification, seek the assistance of an experienced electronics technician.