pentode with grid AND cathode tied to gnd?

xdissent · 14th December 2007, 01:03 AM

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

Gary P · 14th December 2007, 01:09 AM

That circuit is using what's called grid leak biasing. Works great for small signal levels.

tubelab.com · 14th December 2007, 01:28 AM

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.

Shoog · 14th December 2007, 06:21 PM

Quote:

That circuit is using what's called grid leak biasing. Works great for small signal levels.

Yes exactly.

Shoog

Nikolas Ojala · 18th December 2007, 04:00 PM

Maybe someone wants a simple circuit and high impedance input for small signals, then chooses that grid leak biasing, right?

Nikolas Ojala · 28th December 2007, 01:18 PM

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

14th December 2007, 01:03 AM	#1
xdissent diyAudio Member Join Date: May 2003 Location: nashville, TN	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

14th December 2007, 01:28 AM	#3
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	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. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

18th December 2007, 04:00 PM	#5
Nikolas Ojala diyAudio Member Join Date: Aug 2004 Location: Finland	Possible applications Maybe someone wants a simple circuit and high impedance input for small signals, then chooses that grid leak biasing, right? __________________ All distortions are not equal. By the way: I hate overuse of acronyms.

28th December 2007, 01:18 PM	#6
Nikolas Ojala diyAudio Member Join Date: Aug 2004 Location: Finland	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 __________________ All distortions are not equal. By the way: I hate overuse of acronyms.

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14th December 2007, 01:09 AM	#2
Gary P diyAudio Member Join Date: Apr 2002 Location: Portland OR	That circuit is using what's called grid leak biasing. Works great for small signal levels.

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