Snap - Geek posted as I was about to.
AC Coupled Cathode Follower info:
http://www.valvewizard.co.uk/accf.htmlTube Cathode Followers, MOSFET Source Followers, Transistor Emitter Followers all need grid/gate/base stopper resistors.
For 6N3P try a 4K7.
If the circuit is on a Printed Circuit Board then cut the track to the grid right at the tube grid pin pad and solder a surface mount resistor across the track cut.
With 150 odd volts at the cathode (seems high) you will need to elevate the Heater Supply to say +80 or +90 Volts so as to not exceed the heater cathode maximum voltage spec.
Tube Datasheet for reference:
http://www.tubes.ru/techinfo/HiFiAudio/6n3p.htmlYour measurement of 150V suggests 1.5mA tube current. 6N3P like more than that. Are you sure this circuit was'nt intended for 6N2P.
Cheers,
Ian