Capacitor between Grid Leak and Grid Stopper in preamp ok ?

[walkman]

Hi all,

Grid leak resistors are generally seen on guitar amps.  If they are large enough they can be used for Grid leak bias.

My understanding is  as follows…

Their purpose is to provide a DC path to ground for charge that may accumulate at the grid. And if there is a large signal and depending on the cathode bias voltage and the tube type,  current can be drawn through this resistor and potentially affect the bias of the tube. This may also present a DC voltage back to the guitar, which though small may be noticeable with either a scratchy volume or tone pot.

A .022 cap at the input before the a grid leak resistor would prevent DC being present at the guitar, however would not prevent any grid current. A grid stopper can help limit grid current, though if it is too large would affect the frequency response of the tube due to miller capacitance. This is less of a concern for small signal pentodes.

How would a the addition of a capacitor between a 1M input resistor /grid leak and the grid stopper affect the tube?

(I’m assuming this would block any DC back to the guitar and prevent any grid current flowing back to ground  and affecting the bias of the tube.)

Is it really necessary to have a path to ground to allow for the build up for a preamp tube, either triode or pentode if the tube is cathode biased? 

At the second gain stage  in a preamp is grid current is more of a concern due to a larger signal than at  an input stage or is the drawing of grid current generally not an issue with the use of large enough grid stoppers?

Please forgive my misunderstanding or ignorance, as i have tried to understand this a best possible, and am looking for some friendly discussion 

[brewdude]

I think that the capacitor to keep dc off of the guitar’s pots could be placed in series from the input jack and to the typical grid leak and grid stopper. 

[pdf64]

There should be a resistive path between a control grid and its cathode, the grid’s DC voltage must refer to the cathode.
A grid leak resistor provides such a current path between the grid and cathode.

For a grid stopper to function as a grid stopper (ie help stop oscillation) it should be mounted at the valve socket terminal. Such that the conductor length (and hence parasitic inductance) between the resistor body and the grid pin is minimised. 
A signal’s source impedance acts to limit grid current when the grid is forward biased (eg overdriven). A ‘grid stopper’ can be used to increase the signal’s source resistance. Though for this function, higher resistance values are generally used than for a purely grid stopping function.
Forward biased grid current might be seen as being a different thing than normal grid current.
The former is externally sourced, eg positive signal peaks that are greater than the bias voltage. Whereas the latter is generated within the valve.

[tubeswell]

Yes you need a DC-blocking cap between the input jack and the grid leak resistor and 10nF is ample.

[walkman]

What will happen if there were no dc path to ground ? Would the tube eventually self-destruct ? Or stop working correctly? 

Edit. this post on Music Electronics Forum , would indicate that it wont harm a preamp tube and current is limited by the anode resistor. (Posts 5 & 6 by Kevin O Connor and Mike Sulzer)

https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/44028-no-grid-leak-on-input-stage

[PRR]

....A .022 cap at the input before the a grid leak resistor would prevent DC being present at the guitar, however would not prevent any grid current. ..... How would a the addition of a capacitor between a 1M input resistor /grid leak and the grid stopper affect the tube?....

The question seems to be drowning in ambiguity. A circuit with five parts can't be described with "before" or "between", unless you run 5,000 words (like patent descriptions, and even then they provide a diagram).

Is this what you mean?

[walkman]

Is this what you mean?

Yes it is, though not necessarily for a 12ax7 at that voltage.

[sluckey]

The tube cannot work properly without a dc path between the grid and cathode. Bias voltage cannot be established.

[tubeswell]

What will happen if there were no dc path to ground ? Would the tube eventually self-destruct ? Or stop working correctly? 

The tube needs a DC path to ground to achieve bias - the grid leak resistor provides this. In grid-leak bias, this resistor is high (~5M-10M) resistance to inhibit the grid leak function enough to achieve bias, but not kill off the grid leak altogether. The blocking cap needs to go between the input jack and the grid leak resistor (as I said earlier)

[walkman]

What will happen if there were no dc path to ground ? Would the tube eventually self-destruct ? Or stop working correctly? 

The tube needs a DC path to ground to achieve bias - the grid leak resistor provides this. In grid-leak bias, this resistor is high (~5M-10M) resistance to inhibit the grid leak function enough to achieve bias, but not kill off the grid leak altogether. The blocking cap needs to go between the input jack and the grid leak resistor (as I said earlier)

Yes i understand that placing the cap between the input and the grid leak would be sufficient to prevent any Dc flowing back to the guitar if it were present.

I’m not entirely clear why though the dc path to ground is needed to bias the tube.

With no dc path to ground the would the grid  have an electrical charge that will not be the same charge as the cathode or the anode or screen if it is a pentode. Is it appropriate to call that a ‘floating’ grid?

If charge increasingly sticks to the ‘floating’ grid and drives it more ‘positive’ than the charge at the cathode the suggestion in link above is that the current in the may increase but that would be limited by the anode. The Tube may not operate as intended or expected, however would it still work to some extent ? Would the tube find its own ‘ floating bias’ ? Is that even an appropriate term?

Is the bias the tube establishes the voltage difference between grid and cathode or is bias in some ways seen as a ‘proxy’ to determine or set the current through the tube.

Please forgive my misunderstandings perhaps i am just trying to look at this from a different perspective, and if i have got it completely wrong i will need to learn more to better understand.

[PRR]

The floating grid can drift to two different operating points. It may turn full-on or nearly full-off.

Nearly full off is not a good operating point.

Full-on is not dangerous because, as you say, plate resistor. And it happens to be a maximum-gain condition. However it overloads even more easily than you would think from the gain. It has been done in condenser microphones (less diaphragm loading), but guitar levels are higher.

With unselected tubes and high signal levels, the high current high gain condition is liable to toggle-over to the nearly cut-off condition ("faint") and stay there for moments or minutes.

Are you trying to avoid the cost of a resistor?

[walkman]

Are you trying to avoid the cost of a resistor?

No not trying to save money building amps! resisting the tendency to buy them by building !

I have built an amp with a pentode front end ( and second stage with low input). My guitar was making a scratchy sound on the tone pot on the first (high gain) pentode with this amp so i guessed it may be grid current.

Instead of desoldering the grid leak on the input jack, I added a .022 cap between the input jack and the grid stopper. I didn’t notice anything I didn’t like after adding the cap, though perhaps it was a bit mellower with less gain. So I wanted to understand what was happening. I have also tried it on the second stage I wondered if grid current was messing up the bias and causing a some unwanted fizzing / distortion on the trailing end of the note.

After this discussion I will now add a .022 cap into a jack plug connector at the end of a guitar lead, and bypass or switch out the cap between the grid leak and the grid stopper and see how that goes.