On tuesday July we lower the cathode heater to swap the anode-cathode bias. While lowering the cathode heater a short on the anode developed. 

The polarity swap was not possible because the target/tube assembly has also a short to common. 

We return the configuration has it was and we added two 100 Ohms: 5 W resistor between the common and the heaters to prevent current to flow back to the common. 

The anode short disappear during the cathode and target heating.