grid grounding resistor for 12ax7

[sbirkenstock]

Hi there,

it seams that with most 12ax7 (at least with BF Fender amps) the grid is grounded with a 1 meg resistor. I guess that this is mainly to create a ground reference for the grid? If that is the case, what resitor values would be reasonable for this application?
The lower the resistance, the more signal is loost to ground, if it is too high, does the ground referencing still work?

The grid of the second stage basically grounded via the volume pot.
The lower the volume, the lower the grid grounding resistor.
So if the volume is all the way down, the grid is directly grounded?

With most amps the volume comes up pretty quickly. If you want to adjust the amp to really low volume, you need to adjust the volume pot really carefully.
So I was thinking to build in (via a switch as a "low" switch) an extra resitor, kind of creating a "gear box" for the volume pot.

So if I add another resistor between the wiper of the vol pot and the grid, the ration should change?
But basically I have 3 resistors now. I still have the voltage divider between the signal input and the ground towards the grid.
And than I would have another resistor to the grid. Does this make another voltage divider between the grid and the resistance between the pots wiper and the ground connection?
Can you give me some info on that?
Or would the extra resistor have to be before the signal input of the pot?
That would increase the "max total grounding resistor" to 2 meg, if I use a 1 meg resistor.

Any hints very much appreciated!

Best regards,

Stephan

[Stevem]

The amount of current driving the signal off of your pickups is so small that the signal loss on that first gain stage is very little and the relative voltage relationship of the grid to cathode set up by that 1 meg resistor is far more important !

[martin manning]

it seams that with most 12ax7 (at least with BF Fender amps) the grid is grounded with a 1 meg resistor. I guess that this is mainly to create a ground reference for the grid? If that is the case, what resitor values would be reasonable for this application?

The lower the resistance, the more signal is loost to ground, if it is too high, does the ground referencing still work?

The grid resistor creates a ground reference and provides a path for electrons that strike the grid to "leak" away, thus preventing the grid from developing a negative bias voltage. Typical values are 500k to 2M. Large values (~5M) can create enough negative grid bias that the cathode can be connected directly to ground.

The grid of the second stage basically grounded via the volume pot. The lower the volume, the lower the grid grounding resistor.
So if the volume is all the way down, the grid is directly grounded?

Yes.

With most amps the volume comes up pretty quickly. If you want to adjust the amp to really low volume, you need to adjust the volume pot really carefully.
So I was thinking to build in (via a switch as a "low" switch) an extra resitor, kind of creating a "gear box" for the volume pot...
...Or would the extra resistor have to be before the signal input of the pot?
That would increase the "max total grounding resistor" to 2 meg, if I use a 1 meg resistor.

You would have to place the resistor in series with the pot's input, which increases the resistance above the wiper. If you add resistance there you will lower the corner frequency of the HP filter formed by the pot and the capacitors upstream (not a bad thing at low volume), and also reduce the high frequency response of the following stage due to the combination of the increased series resistance and following stage's input capacitance (not so good).

[Structo]

Are we talking about the grid leak resistor (1M) at the input?

I'm just trying to get the nomenclature correct.

[sbirkenstock]

In a BF Fender several 12a?7 grids are grounded with 1 meg
The input one is, as well as the 12at7 for the reverb, also the phase splitter is grounded with 1 meg (here with extra +27K + 47), the second stage has 1 meg as well if the volume pot is all the way up.

The 5 meg grounding was done on Fender A or B series?
So also grid current plays a role.

A friend of mine who is a pro says, you can warm up an amp by putting a 220k resistor in front of the grid of the second stage, with a bypass cap, but I can´t remember the cap value.

thanks a lot!

Stephan

[martin manning]

You will only see 5M or so grid leak resistors on older Tweed designs (and a few others), typically on the input stage. This resistor sets the input impedance (high), and some people like that sound.

[matt h]

(deleted)

[bruce egnater]

Typical "standard" value you will see is 1M. There have been others lower and higher. Ampeg used 5.6M. This is the resistance your guitar sees looking into the input so it will affect tone. The load your pickups see are the parallel combination of the pots in your guitar and the grid leak resistor in the amp so it is really not "that" big a deal. The higher the value, the more theoretical noise could be created.