Tube DIY Asylum

RE: Buffer Circuit for Passive Preamp - Questions

98.206.47.57

In Reply to: RE: Buffer Circuit for Passive Preamp - Questions posted by Mike C on May 10, 2008 at 02:24:33

Circuit A will have a very-well-determined tube current, since the not-quite-known grid-cathode voltage is only a small fraction of the well-determined grid-ground voltage from the voltage divider. (The cathode current equals the cathode-ground voltage divided by the cathode resistor.) However, the voltage divider can couple hum from the B+ line into the input. With the signal source removed, roughly half of any hum on the B+ line will appear at the cathode, coupled through the voltage divider to the grid. A reasonable source resistance will reduce the hum if the two resistors RA and RB are much larger than the source resistance. Replacing the upper resistor RA with two equal resistors in series and adding a bypass capacitor to ground from their junction will reduce the hum.
Circuit B, as Eli says, is a normal bias circuit re-connected. The DC operating point is not so well determined as in A, (the grid-cathode voltage is now compared with the voltage across the first cathode resistor RC) but the hum route has been removed.

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