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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: Reason for grid resistors?, etc... posted by Sugardawg34 on December 25, 2005 at 19:47:01:
Grid “stopper†resistors are used to stop unwanted parasitic oscillations. These oscillations can occur, especially in high-gm tubes, due to the resonant interaction between the Miller capacitance (capacitance from plate to grid multiplied by the gain) and the inevitable stray inductance inherent in the wiring of the grid circuit. Adding a low-value resistor in series with, and very close to, the grid damps this resonant circuit. If it is damped enough, the resonant circuit will not oscillate. A value is usually picked by experimentation, since circuit wiring and layout is so variable. The goal is to find the lowest value which is still large enough to kill any oscillations under any combination of voltage levels and loads. It helps to have an oscilloscope to see the oscillations, which can occur at RF frequencies. Although they may be inaudible in themselves, these oscillations can drive the tube into non-linear regions and can cause audio-band distortions. Many times, depending on the tubes used and the circuit and layout environment of the tube, no stopper is needed at all. For the 300B, others may have recommendations for a range of grid stopper values to try. You might look at schematics of 300B amps on the web to get an idea.Running the heaters in tubes for very long without also passing current through the cathode can buildup an interface resistance layer on the cathode, sometimes called cathode poisoning. This will reduce cathode emission and increase noise. I would avoid using tube filaments for nightlights unless the tube is otherwise bad.
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Follow Ups
- Re: Reason for grid resistors?, etc... - BBeck 07:02:29 12/26/05 (1)
- Re: Reason for grid resistors?, etc... - BBeck 07:34:26 12/26/05 (0)
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