Reconditioning 572b Tubes and regulating the amperage

Based on the PA0FRI method, here is the specific How-To and wiring diagram for reconditioning (reactivating) a 572B tube using 44V DC. 

This procedure treats the tube as a diode by connecting the Grid (Pin 4) to the Plate (Top Cap), allowing it to draw significant current (250 mA) at very low voltage to clean the cathode surface without risking high-voltage flashover. 

⚠️ Critical Safety Warning

High Current: This test draws 250 mA at low voltage.  Ensure your power supply can handle this.

Hot Components: The tube envelope and anode will become very hot.  Do not touch.

Voltage: While 44V is "low," the current capability makes it dangerous if shorted. 

Step-by-Step How-To

Preparation: Insert the 572B into a UX4 socket mounted on a heat-resistant base. Ensure the socket is not connected to any amplifier circuitry.

Filament Warm-Up: Apply 6.3V AC to Pins 1 and 2.  Let the tube heat up for 30 minutes with NO high voltage applied.  This stabilizes the cathode temperature.

Connect HV Circuit: Connect your variable DC power supply as shown in the diagram below. Crucial: You must connect the Grid (Pin 4) directly to the Plate (Top Cap). 

Apply Voltage: Slowly increase the DC voltage from 0V.

Watch the milliammeter closely.

Stop increasing voltage when the current reaches 250 mA.

Target Voltage: A healthy tube will reach 250 mA at approximately 44V to 45V. 

Monitoring:

Hold this state for 30 to 60 minutes.

Watch the meter: As the tube cleans up, the current will try to rise rapidly. You must lower the voltage slightly to keep the current locked at 250 mA.

If the voltage required to maintain 250 mA drops significantly (e.g., below 35V) or the current spikes uncontrollably, the tube may have an internal short or excessive gas.

Completion: Turn off the HV supply first, then the filament. Let the tube cool completely before installing it in your amplifier. 

Wiring Diagram: 44V Reconditioning Setup

       VARIABLE DC POWER SUPPLY (0-50V, capable of 500mA+)

       +---------------------------------------+

       |                                       |

       |  (+) POSITIVE OUTPUT                  |

       |   |                                   |

       |   |                                   |

       |  [A]  <-- Milliammeter (0-500mA DC)   |

       |   |                                   |

       |   +-----------------------+           |

       |                           |           |

       |                        .--o--.        |

       |                        | TOP |        |

       |                        | CAP |        |

       |                        | PLATE|       |

       |                        '--|---'        |

       |                           | (Jumper)   |

       |                           |            |

       |                        .--o--.        |

       |                        | Pin |        |

       |                        |  4  |        |

       |                        |GRID |        |

       |                        '-----'        |

       |                           ^           |

       |                           |           |

       |  (-) NEGATIVE OUTPUT      |           |

       |   |                       |           |

       |   +-----------------------+           |

       |                                       |

       +---------------------------------------+

       FILAMENT SUPPLY (Separate 6.3V AC Transformer)

       +---------------------------------------+

       |                                       |

       |   ~6.3V AC                            |

       |   |                                   |

       |  [FUSE] (2A Slow Blow)                |

       |   |                                   |

       |   +------------+                      |

       |                |                      |

       |             .--o--.                  |

       |             | Pin |                  |

       |             |  1  | (Filament)       |

       |             |     |                  |

       |             | 572B|                  |

       |             |     |                  |

       |             | Pin |                  |

       |             |  2  | (Filament)       |

       |             '--o--'                  |

       |                |                      |

       |   +------------+                      |

       |   |                                   |

       |   ~6.3V AC                            |

       +---------------------------------------+

NOTES:

1. JUMPER WIRE: A heavy gauge wire MUST connect the Top Cap (Plate) to Pin 4 (Grid).

2. PIN 3: Left unconnected (No Connection).

3. MONITORING: Adjust the DC Supply voltage constantly to maintain exactly 250mA on the meter.

Interpretation of Results (PA0FRI Criteria)

Good Tube: Reaches 250 mA at 44V–45V.  The voltage remains stable over 1 hour.

Weak Tube: Requires significantly higher voltage (>50V) to reach 250 mA, indicating poor cathode emission. 

Gassy/Shorted Tube: Current spikes suddenly, or the required voltage drops below 35V to maintain 250 mA. Internal arcing (purple flash) indicates the tube is likely unrecoverable.