Rk vs Rg voltage

[EtherealWidow]

Ok. So I have a question concerning when both of these are used in a tube circuit. Why is it that when they are both used, the internet has told me that Rk drops enough voltage to bias the tube by making the cathode positive relative to the grid, but Rg serves as a reference point to ground. That doesn't make sense to me. Why doesn't Rg drop voltage making the grid more positive? I understand Rg's purpose in a grid leak biased stage, but not in a cathode biased stage. I also understand that when used with a coupling capacitor from the previous stage it forms a high pass filter, and when it's in the very first gain stage it serves as the load that the pickups see, but I don't get how it serves as a reference to ground and doesn't drop any voltage. Tried to look it up on other forums, but it seems like they don't know what they're talking about, even if I'm not really sure what I'm talking about.

[martin manning]

Under normal circumstances there us virtually no current flowing into or out of the grid, so there is no noticeable voltage rise from ground across the grid leak resistor. There may also be a series grid resistor (often called a grid stopper) that forms a low-pass filter in combination with the input capacitance of the tube.

[VacuumVoodoo]

Cathode current is a mathematical construct that defines current direction opposite to factual movement of electrons. This assumes electrons to be negatively charged particles and not waves.
Grid leak current would not exist if the tube was perfectly evacuated of all gas residues. Thus there would be no current flowing in the Rg and grid is thus on ground potential.There's always some gas residue in the tube. Gas atoms become ionized and this is called grid leak current because grid offers the only escape route to the gas ions.
Confusing? It's meant to be

[Reeltarded]

Charged particles and poles. I'm scared.

[pdf64]

Grid leak current would not exist if the tube was perfectly evacuated of all gas residues

My (cursory) understanding is that the cathode emits electrons due to being heated (the use of suitable materials greatly enhances this effect); as the grid is very close to the cathode, it may in turn heat up due to heat radiated from the cathode, and thereby the grid may emit a few electrons even if the vacuum was good.
Pete

[vibratoking]

Take a look at this from Merlin. A full explanation. Biasing and grid leak current on pages 11,12,13. Pay attention to what Voodoo said about the direction of current convention vs electron flow. Imaging the IR drop and you will understand why the grid voltage is negative compared to the cathode in that configuration.

[EtherealWidow]

Well, I had to revert back to drawing a basic triode gain stage with the plate connected to the cathode of a tube diode and the triode cathode connected to the plate of a tube diode instead of ground. Just kind of helps me understand what's going on. When I look at the schematic that way and understand that with an Ra of 100k, Rg of 1M, and an Rk of 1.5k to bias a 12AX7 at 1mA of quiescent anode current, most of that 1mA is going to go through the cathode, being that the other path is over 600 times more resistant. That's a pretty tiny amount of current.

I've been reading the Valve Wizard's free PDF on triode gain stages. That's partly why more questions keep cropping up. To be honest, his section on grid leak resistors confused me really bad. I hate it when people bring conventional current into the discussion because it completely flies against my "actual electron flow" mental visualization. But, from what the valve wizard was saying in the beginning of that free PDF, current has no direction because it's not actual physical particles and it's just a purely conceptual mathematical change in flux blah blah blah, even though electrons are physical matter, whatever.

Amps are made of fairy dust. Understood.

[JazzGuitarGimp]

I took a basic electronics class my first year in college (it was required for my major - recording studio engineering). This was 1976. My professor explained that current and electrons flow in opposite directions. I've been confused ever since.

[vibratoking]

I've been reading the Valve Wizard's free PDF on triode gain stages. That's partly why more questions keep cropping up. To be honest, his section on grid leak resistors confused me really bad. I hate it when people bring conventional current into the discussion because it completely flies against my "actual electron flow" mental visualization.

Sorry, I didn't know you were already reading that. Conventional current is used in almost all circuit theory. Thank Ben Franklin for the confusion. Grid leak should make sense in spite of convention. The point of the grid leak is to keep the grid from building up charge and changing potential. Regardless of convention, electrons are attracted to higher potential. Electrons are attracted from the grid to ground, so ground must be at a higher potential than the grid. You can think of the IR drop across the grid leak resistor in that way,

But, from what the valve wizard was saying in the beginning of that free PDF, current has no direction because it's not actual physical particles and it's just a purely conceptual mathematical change in flux blah blah blah, even though electrons are physical matter, whatever.

If you don't want to understand it, then ignore it. It really doesn't matter in a guitar amp.

[EtherealWidow]

You know, I can understand current as in the flow of electrons moving one way, and then the "holes" that cause the electrons to be shifting to be moving in the opposite way.

I'd like to be able to understand what's going on in an amp better so that I can design/troubleshoot my own circuits instead of just copying off everyone else all the time, but I find that to gain a better understanding I need to first have an intimate knowledge of Teletubbies, the Large Hadron Collider, and Beowulf in the original Old English. That's not even the math portion.

[NickC]

..... I need to first have an intimate knowledge of Teletubbies, the Large Hadron Collider, and Beowulf in the original Old English..............

I had to read Le Morte d'Arthur in old English in college. Oddly enough, the story is the same in old-English as it is in modern translation.

[Tillydog]

... To be honest, his section on grid leak resistors confused me really bad...

For grid leak biasing, we acknowledge that there is some miniscule current flowing in the grid.

For cathode biasing ignore that. Keep it simple and assume that no current flows to or from the grid [1].

You know that the voltage of the grid relative to the cathode is the important factor in making the valve/tube work, but how do you know what voltage the grid is at? If there's no DC path from the rest of your circuit to the grid, then you don't - the grid voltage could be anything - there's no 'defaut' zero voltage.

So what you need is a resistor here (the 'grid leak' resistor) to establish a DC path from the grid to your circuit at whatever voltage you want the grid to be - in this case ground. The resistor can be as high or as low as you like, except that it will also be a load on the AC signal. Essentially, in the absence of a signal, no current flows through it and there is no voltage across it, it's just there to tether the grid to some known voltage.

You could go as high as you like on the value, except that assumption [1] begins to break down. Of more practical concern is how this resistor interacts with the coupling cap for AC signals and affects the frequency response of the stage.


So now you're even more confused!

[EtherealWidow]

Actually, that response really helped, along with having to draw out the circuit like I did and realizing that such a seriously small amount of current goes through that resistor. Thanks y'all!

[VacuumVoodoo]

You know, I can understand current as in the flow of electrons moving one way, and then the "holes" that cause the electrons to be shifting to be moving in the opposite way.

That's it. "Hole current" is what the conventional current is. It's also the most difficult concept to grasp in "Introduction to Semiconductors" class.

[jjman]

Essentially all of the ac signal current of the previous stage flows thru the grid leak resistor. Essentially no dc current flows thru the grid leak resistor since there is essentially no dc voltage on either end of it with cathode bias. Even grid bias has no dc voltage DROP on the grid leak resistor, so no dc current.

Realizing that essentially no (dc or ac) current flows into the grid is an "aha" moment with regard to understanding tubes. Rather, the (ac) voltage signal is merely "on" the grid like there is ac voltage "on" the hot wire in your wall outlet that has nothing plugged into it.

Yes grid current exists during clipping and class A2 and AB2 but the "no-grid-current" scenario is best understood 1st.