When to use a grid leak resistor?

bequerel · 24th April 2009, 10:19 AM

I have studied a lot of schematics lately to try to find a rule regarding when to use a grid leak resistor. There are a lot of eg. 12b4a linestage scematics floating around here at diyaudio.com. Some use a grid leak resistor, and some not. What is the deal?

SY · 24th April 2009, 10:50 AM

There has to be some sort of DC path to ground or to to some reference voltage that has a DC path to ground. So, there's always some sort of grid leak if the grid isn't returned through a transformer or set at some reference voltage.

bequerel · 24th April 2009, 11:04 AM

Quote:

Originally posted by SY
There has to be some sort of DC path to ground or to to some reference voltage that has a DC path to ground. So, there's always some sort of grid leak if the grid isn't returned through a transformer or set at some reference voltage.

Thanks for chiming in, SY

Hmm..

In a linestage where there is usually a pot or attenuator on the input, a grid leak resistor could be skipped then since there is a dc path to ground via the pot?

SY · 24th April 2009, 11:27 AM

In that case, the pot IS the grid leak.

As a matter of course, I will always use a resistor from the grid to ground anyway, just in case there's an intermittent between the wiper of the pot and its resistive track. It's not needed in normal operation, but could potentially save all of the downstream equipment in case of a fault.

bequerel · 24th April 2009, 11:46 AM

Quote:

Originally posted by SY
In that case, the pot IS the grid leak.

As a matter of course, I will always use a resistor from the grid to ground anyway, just in case there's an intermittent between the wiper of the pot and its resistive track. It's not needed in normal operation, but could potentially save all of the downstream equipment in case of a fault.

Thanks gain SY.
And now it is time for a "leak".

tomchr · 24th April 2009, 06:38 PM

I believe the deal is that a floating grid will collect charge. This collected charge causes the grid voltage to rise, thus, eventually resulting in positive grid-cathode voltage. This will cause a large current to flow in the tube, which will likely destroy it. The grid leak resistor is there to bleed off the charge. An input volume pot is likely to serve the same function, but a 1Mohm resistor is not that expensive so why not just put it on there and be done.

There are some cases where the grid leak isn't strictly needed. But you will always need a DC path from the grid to some sort of ground.

~Tom

Wavebourn · 24th April 2009, 07:14 PM

Quote:

Originally posted by bequerel

Thanks for chiming in, SY
Hmm..
In a linestage where there is usually a pot or attenuator on the input, a grid leak resistor could be skipped then since there is a dc path to ground via the pot?

Don't do that. One resistor is cheap, and it saves you from noises: no ideal pots exist.

bequerel · 24th April 2009, 08:19 PM

A grid choke would serve the same function, and maybe sound better as well, right?

SY · 25th April 2009, 01:09 AM

Yes and no. In that order.

24th April 2009, 10:19 AM	#1
bequerel diyAudio Member Join Date: Sep 2003	When to use a grid leak resistor? I have studied a lot of schematics lately to try to find a rule regarding when to use a grid leak resistor. There are a lot of eg. 12b4a linestage scematics floating around here at diyaudio.com. Some use a grid leak resistor, and some not. What is the deal? __________________ Thanx!

24th April 2009, 10:50 AM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	There has to be some sort of DC path to ground or to to some reference voltage that has a DC path to ground. So, there's always some sort of grid leak if the grid isn't returned through a transformer or set at some reference voltage. __________________ If there's a sucker born every minute, where do the rest of them come from?

24th April 2009, 11:27 AM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	In that case, the pot IS the grid leak. As a matter of course, I will always use a resistor from the grid to ground anyway, just in case there's an intermittent between the wiper of the pot and its resistive track. It's not needed in normal operation, but could potentially save all of the downstream equipment in case of a fault. __________________ If there's a sucker born every minute, where do the rest of them come from?

24th April 2009, 08:19 PM	#8
bequerel diyAudio Member Join Date: Sep 2003	A grid choke would serve the same function, and maybe sound better as well, right? __________________ Thanx!

25th April 2009, 01:09 AM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Yes and no. In that order. __________________ If there's a sucker born every minute, where do the rest of them come from?


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24th April 2009, 06:38 PM	#6
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	I believe the deal is that a floating grid will collect charge. This collected charge causes the grid voltage to rise, thus, eventually resulting in positive grid-cathode voltage. This will cause a large current to flow in the tube, which will likely destroy it. The grid leak resistor is there to bleed off the charge. An input volume pot is likely to serve the same function, but a 1Mohm resistor is not that expensive so why not just put it on there and be done. There are some cases where the grid leak isn't strictly needed. But you will always need a DC path from the grid to some sort of ground. ~Tom

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