Cathode Bias - can some help translate??

[ryanuk]

Hi

I would like to learn the theory of tube amplification and characteristics of tubes in order to design and test a basic tube preamp. Its gonna be a long road and I'm just starting out. I've got some experience. Built some pedals and basic amp kits but I'm ignorant to a lot of the theory.

I am reading about common cathode stages using a triode. Im beginning to read about load lines and how these can determine gain/headroom characteristics but getting caught up on some of the terminology on cathode biasing. Specifically what referencing to ground means. Its funny, I've come across this in op amp circuits and guitar fx inputs but now questioning my understanding.

In the common cathode, cathode biased circuit I understand that;

1. This form of biasing places the grid at a negative potential difference to the cathode. A DC negative voltage is necessary to bias the tube.
2. The cathode has a resistor to ground. I guess this forms a voltage divider with the tube and load resistor.
3. A voltage is placed at the cathode due to the voltage drop across the cathode resistor - lets say +15v.
4. The grid needs to be connected to ground for this to work but without shunting the AC input signal (eg guitar) to ground, so a grid leak resistor to ground is placed. Online explanations suggest that there is no voltage drop across this resistor as no current flows through it.

If the grid was directly connect to ground, Id understand how this works ie. the cathode to ground potential is +15v, so inversely, the grid to cathode potential is -15v. But the grid leak resistor is placed between grid/ground so there is no direct. So I mean confused.

I've watch some you tube videos and searched here and online. I'm still confused.

Is anybody willing to help clarify?

[Manfred]

That in a good book to learn the basics:
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

[ryanuk]

Absolutely! I have that document and reading through it. Its the info on pages 12/13 causing my confusion;

"The grid-leak resistor now pulls the grid down to ground (zero volts) ..."

I guess its how this works/what this means.

[Manfred]

Absolutely! I have that document and reading through it. Its the info on pages 12/13 causing my confusion;

"The grid-leak resistor now pulls the grid down to ground (zero volts) ..."

I guess its how this works/what this means.

With the ideal tube, no grid current flows in, so there is no voltage drop at the grid resistor and the grid is pulled to the ground.
In the real tube, a very small grid current flows, so there is a very small and negligible voltage drop of a few Microvolts at the grid resistor.
The voltage drop at the cathode resistance then appears with reversed polarity between grid and ground.

[ryanuk]

Thanks for this

I think I'm beginning to understand.
Found some useful YouTube videos on pull down resistors which helped.

So (in an ideal tube) the grid is a dead end so stunts current flow?

But if there is a tiny drop over the grid resistor, essentially both cathode and grid are referenced to ground but biased to leave the cathode more positive?

[Manfred]

So (in an ideal tube) the grid is a dead end so stunts current flow?

The electrons flowing from the cathode to the anode have a negative charge.
The negative voltage between the grid and the cathode forms an negative electric field between the grid and the cathode
which reduces the electron flux depending on the voltage level.
The generation of grid voltage with the cathode resistance is the so-called automatic biasing,
This means that after starting the anode current is increased until the appropriate grid voltage is reached.

But if there is a tiny drop over the grid resistor, essentially both cathode and grid are referenced to ground but biased to leave the cathode more positive?

A tiny positive voltage drop at the grid resistor reduces the negative grid voltage insignificantly,
but this changes with a defective tube where the vacuum loses,
then a grid current flows which produces a noticeable voltage drop at the grid resistor.
The negative voltage between grid and cathode is reduced and the operating point of the tube stage is shifted.