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PJL · 20th Feb 2018, 11:24 am

The problem will be that the mystery valve is not biased correctly or is very low emissions (maybe wrong heater specification). The V3 cathode voltage is probably near zero and the incoming signal is rectified on the grid and cuts the valve off as the volume is increased.

Voltage measurements would give you the answer, heater (AC), Anode and Cathode.

20th Feb 2018, 11:24 am	#19
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: KB 381 5 valve AC/DC superhet The problem will be that the mystery valve is not biased correctly or is very low emissions (maybe wrong heater specification). The V3 cathode voltage is probably near zero and the incoming signal is rectified on the grid and cuts the valve off as the volume is increased. Voltage measurements would give you the answer, heater (AC), Anode and Cathode.