• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

pentode with grid AND cathode tied to gnd?

Is this possible? I just got an old bell pa head that has a 5879 pre that appears to be doing just that - cathode is grounded and the grid tied to it with a 2M resistor. I can't seem to figure out how it's actually functioning without SERIOUS distortion. Sounds clear as a bell. Just a plate resistor going to B+ and a 2 resistor voltage divider driving the screen (half B+). Suppressor is grounded as well. Is there something magical going on here biasing the grid to somewhere near the recommended whopping -3V? I'm so confused. I thought I FINALLY understood all the basic tube circuits and then I run across THIS crazy thing. Ideas?

handsome greg
 
This is "grid leak bias". It was common in the early days of vacuum tubes. When the tubes cathode is hot, electrons are "boiled off" resulting in a "space charge" cloud of electrons surrounding the cathode. Some will collect on the grid resulting in a negative charge (voltage) on the grid. The "grid leak" resistor bleeds a controlled quantity of these electrons off of the grid resulting in a negative voltage on the grid. This requires a tube with a good vacuum, a coupling capacitor that doesn't leak, and a very high value resistor with reasonable tolerance. Since these were (and still aren't) always achievable the practice dissapeared a long time ago.

Why can't I measure the negative voltage on the grid? The impedance on the grid is set by the 2 meg resistor. Your meter probably has a lower input impedance. We had "vacuum tube voltmeters" back then with an 11 megohm input impedance.