Whats the purpose of Screen resistors?
Bill Kahle: In simple terms, this resistor is used for current limiting to prevent over dissipation of the g2 screen grid. Ken (ride5000): bill’s right. they also serve the secondary purpose of lowering the Q of the screen grid. in plain english, they make the screen much less likely to oscillate. this can easily happen if you have multiple screen grids fed from the same supply node without the stoppers. • Do they help protect the Output Transformer if a power tube shorts? Bill Kahle: Not really…. It depends on the condition of the tube failure. Sometimes a failure will occur that involves the screen. If a short occurs that involves the screen and another [tube] element that is near or below ground potential, the excessive screen current will burn the screen resistor. This will shut the tube off. Maybe others can elaborate on failure modes that involve the screen? Ken (ride5000): well, the simple fact is most (i’d say a good 90%) of tube failures are failures of the screen grid.. it’
