Flashover Protection for PAs

Arcing from the plate can quickly destroy the PA-tube. Fuses or resistors are not fast enough and should be seen as a very rough means for protection only. The high energy in the anode voltage condensator can damage the grids and cathode of the tube.

The circuit in Figure 1 opens the primary of the HV-transformer in case of overcurrent in the anode circuit, and simultaneously the high voltage condensator is shortenend by a thyristor. In the case of very high anode voltage (>1kV) it is replaced by a thyristor-cascade.

Figure 1: Flashover Protection Circuit


Normal operation: Pin 3 of CD4011 is high, transformer on via electronic relay, thyristor (SCR) off.

Overcurrent trip: High current in anode circuit caused by arcing or short circuit generates voltage drop across 10Ohm resistor. Flip-Flop toggles (Pin 3 low), switches off transformer and fires SCR to short HV capacitor.

For restart one has to switch off S1, the main power supply switch, and switch it on again after a few seconds. The series inductance for SCR protection is made from 10m of enamelled copper wire. Trip current is set by the 39k/8k2 voltage divider.

Figure 2: Thyristorcascade

Thyristors for voltages >1000 Volts are very expensive. For higher voltages you can use a cascade as shown above in Figure 2. I use four thyristors in series, switched by optocouplers, in my 3.5kV supply.

Before you test the circuit by producing a short, you should follow the step by step procedure, which is outlined as follows:
  1. Connect TP2 to ground, switch S1 on and short Pin 6 (CD4011) to ground. ===> Transformer must be off.
  2. S1 on, after plate voltage coming up, ground TP1 (transformer off), and then Pin 6 (CD4011). ===> SCR must fire now and ground high voltage.
  3. S1 on, ground Pin 6. ===> Transformer must be off and HV must be shorted.
  4. After successfully completing these tests, you should short the HV output with a thin wire to ground (0.12mm). This wire must not be melted when the HV-supply is shut down by the overcurrent protection.

Figure 3 shows a circuit for screen protection sreen in case of no anode voltage or screen overcurrent. Ig2>100mA will toggle the Flip-Flop via the optocoupler. The relay goes off and disconnects the screen. Voltage comes back after pushing the RESET key. Using the ordinary Z-diode stabilisation, overcurrent is limited to a harmless value during switching delay of the relay.



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Figure 3: Screen protection