Re: The "Sussex" Homebrew Valve Tester.
Have you fitted R6 on the PCB - it's not shown on the layout on P26 of the manual - assuming you are using the original board layout. The holes are there between AJ & AK - top right of the drawing.
Have you checked the audio osc output level that feeds to the grid?
The extra diode added in series with D16 was fitted for a reason, but I can't remember why now, but it's described somewhere - found it on page 23.
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