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Thanks for your reply.

I have double checked everything and my VR1 is connected on the positive bias side. With the input shorted I get -4.2V on the grid and it stays the same after I unshort it and adjust VR1 to 0v.  Yesterday voltage was drifting +-10mV with nothing connected at the input but as soon as I connect my preamp it drops to 0V and stays there. So should I be measuring DC with the preamp connected and just ignore the voltage drift when nothing is connected?

The big problem that I have is when I switch the amp off the voltage on the input creeps up to around 55v over 1 minute and then slowly starts dropping down as the last positive power supply cap - C8 starts to discharge. The last negative bias power supply cap C9 discharges quite quickly into R17 but C8 stays charged up. Something is not right. 

What are the R13 & R14 – these should not be connected to ground?

V1 on the schematics is the positive supply gas tube. I guess that the process for measuring the gas tube current draw will be the same. Please let me know if below is correct.

Thanks

Gintaras

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