Grid resistor location

bluegti · 16th March 2010, 03:50 PM

When building an amplifier with a fixed resistor (as opposed to using a 100K pot), where should the resistor be located?

Does it make sense to connect the 100K resistor from the center of the RCA jack to the ground on the jack?

Or, is it better to have the resistor connected to the terminal on the tube socket?

TheGimp · 16th March 2010, 03:52 PM

Bests to terminate the grid of the tube by putting the resistor at the tube socket.

If you were using a pot, it would be good to add a series grid-stop resistor at the tube socket (grid).

bluegti · 16th March 2010, 04:39 PM

Is a grid-stopper necessary if you have the grid resistor connected to the socket?

TheGimp · 16th March 2010, 04:57 PM

If it oscillates you do. If it does not oscillate, it still may be unstable and the gridstop resistor will help.

I don't know of any hard fast rules about them, othere than a recommendation to always use them with high frequency tubes (TX tubes used for audio purposes).

I've seen a Marshall JCM800 with the high gain input 12AX7 oscillating. A gridstop resistor helped tame it.

Here is a good explaination on grid stop resistors:

Grid Resistors

m6tt · 16th March 2010, 07:38 PM

Grid stoppers (in series with grid) will use the miller capacitance to form a low-pass circuit. You can easily set this for some arbitrary value above hearing, such as 35khz. It will reduce the chances of high frequency oscillation.

The grid resistor (grid to ground) is what the voltage to control the tube is developed across. Ideally, its ground and the cathode ground are identical. Any differences will be amplified as hum. Any long cable between the grid resistor and the tube pin may be seen as a series inductor and/or parallel capacitor (shielded cable). This may destabilize your amp, maybe not. I have seen RCA jacks with the Rgk on the jack and a wire to the pin in vintage equipment that was fairly stable.

Personally, I use a grid stopper at the input of every amp I make, and recently often at the power tubes' grids as well if necessary. The correct value depends on the tube being used, but for 'ax7, around 47k seems to have little effect on audio. Ideally use the smallest possible value that prevents oscillation, if you want to maximize Q.

16th March 2010, 03:50 PM	#1
bluegti diyAudio Member Join Date: Nov 2004 Location: Northern Virginia	Grid resistor location When building an amplifier with a fixed resistor (as opposed to using a 100K pot), where should the resistor be located? Does it make sense to connect the 100K resistor from the center of the RCA jack to the ground on the jack? Or, is it better to have the resistor connected to the terminal on the tube socket?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Value of grid-leak resistor	hkoetz	Tubes / Valves	4	11th January 2009 04:15 PM
Location of voltage-dropping power resistor	hollowman	Power Supplies	21	5th December 2008 08:13 PM
grid-stopper resistor	krusty75	Tubes / Valves	2	7th March 2008 12:24 AM
screen-plate resistor and grid leak resistor	metebalci	Tubes / Valves	4	26th February 2004 03:18 AM
Amplifier Feedback Resistor Location?	thylantyr	Solid State	22	30th April 2003 03:59 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

16th March 2010, 03:52 PM	#2
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	Bests to terminate the grid of the tube by putting the resistor at the tube socket. If you were using a pot, it would be good to add a series grid-stop resistor at the tube socket (grid).

16th March 2010, 04:39 PM	#3
bluegti diyAudio Member Join Date: Nov 2004 Location: Northern Virginia	Is a grid-stopper necessary if you have the grid resistor connected to the socket?

16th March 2010, 04:57 PM	#4
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	If it oscillates you do. If it does not oscillate, it still may be unstable and the gridstop resistor will help. I don't know of any hard fast rules about them, othere than a recommendation to always use them with high frequency tubes (TX tubes used for audio purposes). I've seen a Marshall JCM800 with the high gain input 12AX7 oscillating. A gridstop resistor helped tame it. Here is a good explaination on grid stop resistors: Grid Resistors

16th March 2010, 07:38 PM	#5
m6tt diyAudio Member Join Date: Aug 2007	Grid stoppers (in series with grid) will use the miller capacitance to form a low-pass circuit. You can easily set this for some arbitrary value above hearing, such as 35khz. It will reduce the chances of high frequency oscillation. The grid resistor (grid to ground) is what the voltage to control the tube is developed across. Ideally, its ground and the cathode ground are identical. Any differences will be amplified as hum. Any long cable between the grid resistor and the tube pin may be seen as a series inductor and/or parallel capacitor (shielded cable). This may destabilize your amp, maybe not. I have seen RCA jacks with the Rgk on the jack and a wire to the pin in vintage equipment that was fairly stable. Personally, I use a grid stopper at the input of every amp I make, and recently often at the power tubes' grids as well if necessary. The correct value depends on the tube being used, but for 'ax7, around 47k seems to have little effect on audio. Ideally use the smallest possible value that prevents oscillation, if you want to maximize Q.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)