Some “seriously beginner grade” questions regarding resistors

[Helmholtz]

That's why theory stuff is so confusing. If the purpose of the cap is to eliminate DC, why would the grid resistor need to exist if there's no DC to destabilize it?

THe 0.05µ cap blocks the plate voltage and the 0.02µ cap is part of the tone low pass filter. As a result there would be no DC path for the grid without the 470k. Without a DC path the grid could have any arbitrary DC potential without having a way to get rid of the undesirable charge. As the grid voltage controls the plate current, plate current would be undefined.

[gui_tarzan]

"THe 0.05µ cap blocks the plate voltage and the 0.02µ cap is part of the tone low pass filter. "

This I understand. The second part is what I don't get. Going back to basics... electricity flows from negative to positive. The grid acts as the valve, controlling how much electricity goes from the cathode to the plate. That's DC voltage but the AC voltage (guitar signal) also flows through the plate on its way to the coupling capacitor. So if the DC is blocked beyond that capacitor, where is the potential DC coming from further down the signal path?

[Helmholtz]

So if the DC is blocked beyond that capacitor, where is the potential DC coming from further down the signal path?

Without a DC current path, the grid just has some static DC potential caused by some tiny static charge. This charge is at least partly caused by electrons hitting the grid and charging it negative wrt the cathode.

[gui_tarzan]

Oh, ok so it's like stray voltage passing through? But if the DC flows from cathode to plate, negative to positive, are you saying the stray electrons could drain off through the 470k resistor to ground? That seems backwards.

[Helmholtz]

.. are you saying the stray electrons could drain off through the 470k resistor to ground? That seems backwards.

Exactly that's how grid leak works. The negative (electron) current flows from grid to ground. In more technical rather than physical terms this corresponds to a positive current flowing from ground into the negatively charged grid.

[gui_tarzan]

Thus "grid leak". It just seems there shouldn't be a positive charge going to the ground, it should be the other way. Electricity is mystifying sometimes.

Thank you for spending the time to explain it.

[uneumann]

There seems to be confusion over resistor basics. It is good to ask these questions if you find them bugging you. Things should be consistent and make sense. If not, there's a problem in your understanding that needs to get cleared up. Good for you to see the problem and ask the questions....

First, understand a resistor by itself - then consider it in context of a circuit.

A resistor is a device that enforces a fixed ratio of voltage across it to the current through it. Put a voltage across a resistor, get a current. Push a current through a resistor, it creates a voltage across the resistor. Period. There is nothing more to it. R = V/I. The resistance value determines the ratio of voltage and current. There is no magic and there are no exceptions.

Your confusion over "current" is understandable. People use the term in two ways. One is for the flow of electrons (negative charges), the other is the flow of "positrons" or positive charges ... Honestly, it doesn't really matter which you use since it only creates a sign or direction change. Don't get hung up on it. Current is charge flowing... period. I find myself rarely worrying about which type of charge it is. Current is a measure of charge flow (Amps). It's analogous to gallons per min - that's enough for 99% of cases.

Current flows (or more accurately, it propagates) through resistors at essentially the speed of light - don't get hung up on "slower" or faster flow due to resistance... that's not a valid analogy.
A resistance determines the amount of current (Amps) and the amount of voltage (Volts) through it or across it at any instant in time.

OK - I'll leave the circuit contexts for others - some good explanations are already above. Get the resistor straight first - then the circuits will be easier to follow.

[Helmholtz]

Positive charge is just a lack of electrons. ( No positrons involved. Positrons are antimatter particles that can't exist/survive in matter.)

A positive current flowing from left to right is indistinguishable from a negative current of same value flowing from right to left.

[Enzo]

The current - electrons if you like - only flows one direction through the tube. We toss around the term AC rather casually. But the current through the tube is always DC. I would say there is an "AC component" to make it less confusing. So there is no case that "AC voltage" isn't blocked while DC is. The grid controls the tube current, so with your signal the current varies from higher to lower and back, but that is just varying DC, not realy AC.

If you allow a grid to float, all those electrons streaming by will occasionally stick to the grid, allowing a charge to build up. That charge can bias the tube. SO a large value resistor will allow any accumulated charge to bled off to ground.

[gui_tarzan]

There seems to be confusion over resistor basics. It is good to ask these questions if you find them bugging you. Things should be consistent and make sense. If not, there's a problem in your understanding that needs to get cleared up. Good for you to see the problem and ask the questions....

Thank you. I probably care too much about the why, but there are things that have been bugging me for a long time.

First, understand a resistor by itself - then consider it in context of a circuit. A resistor is a device that enforces a fixed ratio of voltage across it to the current through it. Put a voltage across a resistor, get a current. Push a current through a resistor, it creates a voltage across the resistor. Period. There is nothing more to it. R = V/I. The resistance value determines the ratio of voltage and current. There is no magic and there are no exceptions.

Your confusion over "current" is understandable. People use the term in two ways. One is for the flow of electrons (negative charges), the other is the flow of "positrons" or positive charges ... Honestly, it doesn't really matter which you use since it only creates a sign or direction change. Don't get hung up on it. Current is charge flowing... period. I find myself rarely worrying about which type of charge it is. Current is a measure of charge flow (Amps). It's analogous to gallons per min - that's enough for 99% of cases.

I have never heard anyone say there is a current/voltage ratio. It kind of makes sense but before I started working on amps many years ago (totally self-taught as you can tell) my understanding of a resistor was to limit the voltage flowing through, like a water faucet. But that was an inaccurate analogy because the water pipe is a specific size, like set voltage, the flow is actually the current. Oh Geez, I just made the connection to the term "current" in a river which is how much water volume is being moved, not the the static amount of water at any given time. When I look back on it now, I see where the current limiting happened in the case of LEDs I was playing with. The resistor was to limit the current drawn by the LED because it could draw too much and burn out.

Current flows through resistors at essentially the speed of light - don't get hung up on "slower" or faster flow due to resistance... that's not a valid analogy.
A Resistance determines the amount of current (Amps) and the amount of voltage (Volts) through it or across it at any instant in time.

Ok, so that's confusing. How can that resistor both limit current and drop voltage at the same time? That goes back to the ratio you were talking about, right? That confuses me as much as having AC and DC on the same wire, not necessarily going the same direction. I was previously only thinking in the hydraulic sense of only being able to push fluid in one direction at a time. But with amps I learned a long time ago that most types of voltage can sit on the same wire at the same time. That was a mind-bender to be honest, and still is to a point.

[gui_tarzan]

The current - electrons if you like - only flows one direction through the tube. We toss around the term AC rather casually. But the current through the tube is always DC. I would say there is an "AC component" to make it less confusing. So there is no case that "AC voltage" isn't blocked while DC is. The grid controls the tube current, so with your signal the current varies from higher to lower and back, but that is just varying DC, not realy AC.

Ok but the signal from the instrument is a sine wave signal which is being amplified through each stage. Since DC supply voltage isn't passing through the capacitor, how is that signal not AC? I know the signal starts out in millivolts but it's still alternating, is it not?

[Dave H]

Ok but the signal from the instrument is a sine wave signal which is being amplified through each stage. Since DC supply voltage isn't passing through the capacitor, how is that signal not AC? I know the signal starts out in millivolts but it's still alternating, is it not?

The guitar signal is AC because its polarity alternates. The tube's plate voltage is modulated DC because it's always positive. The output capacitor removes this DC component converting the output signal back to AC. The DC component is the quiescent (no signal) plate voltage of the tube.

[Enzo]

Current/voltage. It isn't a ratio, it is a relationship. This is what Ohm's LAw says. It is one of THE most fundamental concepts in electronics. It says voltage dropped from one end of a resistor to the other is equal to the current through it times the resistance in ohms. But you can turn it around too. If you know the voltage across the resistor, and the resistance, you can then know the current flowing. And yet another way. If you know the voltage across a resistor, and you know the current flowing through it, you then will know its resistance. This is something you use every day. The formula is simple: V=IxR, which is also I=V/R, and also R=V/I If you know any two things, then you know the third.

Look at a simple triode stage in an amp. If it has a 1500 ohm cathode resistor, and 1.5v on the cathode (which would be across the resistor), then I=V/R or 1.5v/1500 ohms, solving gives 0.001A or 1 milliamp. That is the current through the cathode, and so in this case also through the tube. And what if we have a 100k plate resistor? Those are typical values. The current through the tube is 1ma, so that means the 100k plate resistor must have 1ma flowing through it. Well, plugging that into Ohm's Law, V=0.001Ax100,000 ohms. 100 volts. There will be 100v difference end to end on that 100k resistor. So for example 300v on the B+ end leams 200v on the plate end. Obviously this rules out running the tube on 80v.

OK, you are right, the signal arriving at the next grid would be AC, as long as you have a grid return. I was referring to the tube currents. You were concerning over electrons, so I addressed the currents in the tube. NO grid current flows. You also mentioned AC and DC on the same wire and confused how they could be going opposite directions. That is why I explained they do not, The DC just varies with the pattern of your AC signal.

[gui_tarzan]

The guitar signal is AC because its polarity alternates. The voltage and current in the tube are modulated DC because they are always positive. The output capacitor removes this DC component converting the output signal back to AC.

Yes, alternating current is coming from the instrument. I know the tube voltage is DC but isn't that AC signal what causes the modulation to occur by varying the grid voltage? If that's the case, how does the signal coming out of the coupling capacitor turn back into a sine wave if it's not still an AC signal?

Am I over-analyzing things? I'm just trying to understand things better.

Ok, Enzo answered the advancing signal question and the modulation origin.

I want to process what you guys have told me tonight, I may have more questions tomorrow. Thanks to all of you who have explained things so far.

[Dave H]

Yes, alternating current is coming from the instrument. I know the tube voltage is DC but isn't that AC signal what causes the modulation to occur by varying the grid voltage? If that's the case, how does the signal coming out of the coupling capacitor turn back into a sine wave if it's not still an AC signal?

Yes, the grid voltage causes the modulation. The signal always has the shape of a sine wave but at the plate it also has a DC offset. On the sim below grid V is green, plate V is blue and downstream of the cap is red. You can see that the signal at the plate has a 180V DC offset which is removed by the cap.