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Something you did reduced the readings to a normal level, what did you do?

Hello zendick,
I suspect three things:
- A too long connection between the PCB and the tube. Make a direct, as short as possible connection, and see if it improves the behavior.
- A poor tube, that's happening sometime with NOS SBM20. If this is the case, nothing to do, put a new one... BUT, before attempting this exchange, see below...
- Wrong voltage: that's possible as the radmon plus kit doesn't really measure the voltage and lock on it, but only assumes it for a given duty cycle and associated components. Only one thing to do, really measure the voltage with a high impedance probe (> 100Mohms or better 1Gohms) - Remove the tube before and put the probe at the junction of R6 and R7, you should have around 400V for a SBM20. This should match the setting of the kit.
Another thing to take in account: check R7 for the correct value (4.7 megaohms), if you are not using an anode resistor, jumper "RL_JMP" must be open. Anyway, a resistor is mandatory in the anode line (R7 or the external one), if for any reason this resistor is missing or shorten, this could quickly destroy the tube...