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swordholder · 8th Feb 2010, 10:27 pm

Hi Dan,
First let me explain the BC237 circuit. The emitter is at -ve voltage, and the base is tied to -ve. The transistor is cut off and the LED is off.
If either the anode or G2 has a leak to ground, (+ve with respect to emitter & base) a positive voltage will appear at the base and the transistor will conduct lighting the LED. The higher the +ve voltage (the lower the leakage resistance) the brighter the LED.
The leak to ground will be either through the grid or suppressor grid.
VR1 sets the reliability of the audio oscillator and is adjusted for consistant starting with a good waveform
VR2 sets the amplitude applied to the grid (about 105mV)
VR3 sets the grid voltage (read on the grid DVM).
Hope this answers your question

Mike

8th Feb 2010, 10:27 pm	#139
swordholder Rest in Peace Join Date: Aug 2009 Location: East Preston, Sussex Posts: 162	Re: The "Sussex" Homebrew Valve Tester. Hi Dan, First let me explain the BC237 circuit. The emitter is at -ve voltage, and the base is tied to -ve. The transistor is cut off and the LED is off. If either the anode or G2 has a leak to ground, (+ve with respect to emitter & base) a positive voltage will appear at the base and the transistor will conduct lighting the LED. The higher the +ve voltage (the lower the leakage resistance) the brighter the LED. The leak to ground will be either through the grid or suppressor grid. VR1 sets the reliability of the audio oscillator and is adjusted for consistant starting with a good waveform VR2 sets the amplitude applied to the grid (about 105mV) VR3 sets the grid voltage (read on the grid DVM). Hope this answers your question Mike