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Old 14th December 2007, 02:03 AM   #1
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Default 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?

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Old 14th December 2007, 02:09 AM   #2
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That circuit is using what's called grid leak biasing. Works great for small signal levels.
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Old 14th December 2007, 02:28 AM   #3
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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.
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Old 14th December 2007, 07:21 PM   #4
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Quote:
That circuit is using what's called grid leak biasing. Works great for small signal levels.
Yes exactly.

Shoog
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Old 18th December 2007, 05:00 PM   #5
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Lightbulb Possible applications

Maybe someone wants a simple circuit and high impedance input for small signals, then chooses that grid leak biasing, right?
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Old 28th December 2007, 02:18 PM   #6
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Post Grid leak bias in guitar amps

It seems that grid leak bias is a common way to bias a tube in a guitar amplifier input. Such as this one:

http://www.charlestonarea.com/Fender...s/champ5c1.gif
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