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Triodes and current flow

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jow

Member
2010-11-11 9:04 pm
I have been reading the NEETS modules to get a basic understanding of electronics and electricity prior to attempting a DIY tube amp. I have hit a road block in my understanding of triodes that I can't seem to solve.

According to the NEETS modules, the triode has DC current flowing from the cathode to the plate and that this is not reversible. Current flows from the cathode to the plate even when no input signal is applied to the grid (quiescent).

An AC audio signal applied to the grid causes the the current from the cathode to the plate to increase or decrease relative the the positive or negative voltage of the AC signal. It is implied in the NEETS modules that there is an increased AC signal from the plate.

My confusion is this. If current can only flow from the cathode to the plate, then the increased signal off the plate must be an oscillating DC current. In AC the movement of electric charge periodically reverses direction. As the current can't reverse direction through the triode it must only be DC.

My question is what is the current flow that would allow an AC input signal to the grid to become an increased AC current from the plate.

Sorry if this is really basic question. I'm just not sure why I am having such a difficult time understanding it.

Thanks in advance

Jason
 
First, what flows from cathode to plate is electron current (negative to positive). "Conventional" current (positive to negative) flows the other way.

OK, now your question. Tubes are generally set up to pass a constant current with no signal applied. When you apply signal, the current increases and decreases, but will never reverse. For example, let's say you have the idle current of a tube set to 10mA. A typical signal that would cause a 1mA AC current change will then cause that current to move between 9 and 11 mA.
 

jow

Member
2010-11-11 9:04 pm
Thank you for the reply. I understand everything you stated. My confusion is the types of current being used.

1. Is the signal being applied the the grid AC or oscillating DC?
2. If an AC signal is applied to the grid does it not produce an oscillating DC?
3. If oscillating DC is produced then preamp like 12AX7 would provide DC to the grid of the main tube.

I hope this clarifies where I am get confused

Jason
 
Thank you for the reply. I understand everything you stated. My confusion is the types of current being used.

1. Is the signal being applied the the grid AC or oscillating DC?
2. If an AC signal is applied to the grid does it not produce an oscillating DC?
3. If oscillating DC is produced then preamp like 12AX7 would provide DC to the grid of the main tube.

I hope this clarifies where I am get confused

Jason

"Oscillating DC" just means AC and DC added together. The circuit coupling (usually RC coupling in preamps) will remove the DC component leaving the AC.
 

jow

Member
2010-11-11 9:04 pm
There lies my confusion. In order for there to be AC the current flow must reverse for half a cycle. If the current can only flow in one direction (like through a tube) then the produced current must be DC. The fact that voltage oscillating should not define it as AC, as a reverse in current direction would be required. Is this correct?
 
Introducing CCS into the discussion will simply confuse the OP. CCS does not hold current constant, but nearly so.

The signal applied to the grid can be AC (e.g. as in audio amp) or DC (e.g. as in a computer). It varies the anode current, and hence the anode voltage. This may be directly connected to the next stage, but more common is to use a coupling capacitor to pass the AC only.
 
There lies my confusion. In order for there to be AC the current flow must reverse for half a cycle. If the current can only flow in one direction (like through a tube) then the produced current must be DC. The fact that voltage oscillating should not define it as AC, as a reverse in current direction would be required. Is this correct?

It is AC about your reference voltage (the superimposed DC). That would be your bias voltage. AC only requires an actual current flow in the opposite direction if your reference is ground. If you elevate the reference (bias) it is still AC, just superimposed on a DC voltage as SY stated, but is the same in function. If you don't elevate the bias voltage reference, you can then only swing in one direction because a triode is still a diode at heart.
 
Introducing CCS into the discussion will simply confuse the OP. CCS does not hold current constant, but nearly so.

Agreed, CCS is a red herring and the OP ought to not worry about that. BUT... even with a perfect CCS and no variation in plate current, the plate voltage will still swing at mu times the grid voltage. Draw a perfectly horizontal load line on the tube curves to convince yourself.
 

jow

Member
2010-11-11 9:04 pm
Just to make sure I understand. AC is either current that reverse direction or voltage that oscillates?

So one could say that if I take a simple circuit with battery as its source followed by a potentiometer then a resister. If i constantly adjust the pot the voltage produced between it and the resister would be considered AC?
 
To the OP: The coupling capacitor connected to the plate removes the DC component of the DC+AC signal found at the plate, and in fact you now have bonafide alternating current referenced to ground if the output side of the cap is returned to ground through a resistor. (That is the voltage swings symmetrically above and below ground.)

Another way of looking at it is the capacitor allows you to remove the quiescent dc voltage present at the plate with no signal applied from the output - all you see at the output side of the capacitor then is the ac component varying about the baseline which at the plate is the static DC value, and at the output side of the cap is ground. The peak to peak amplitude hasn't changed - what has is the point of reference..

I've never realized actually how hard this might be to explain..

Hopefully I have not made this even worse.
 
Last edited:
Just to make sure I understand. AC is either current that reverse direction or voltage that oscillates?

So one could say that if I take a simple circuit with battery as its source followed by a potentiometer then a resister. If i constantly adjust the pot the voltage produced between it and the resister would be considered AC?

It would be considered a sum of AC and DC. You can separate out one component or the other, as Kevin alluded to, but the TOTAL signal is the sum of a DC component and an AC component.
 
Just to make sure I understand. AC is either current that reverse direction or voltage that oscillates?

What's the difference, if you jump on say 1 feet below an ocean level surface, or on a mountain? Do your movements change direction, or not?
Yes, they do. But in the first case you move below and above "Zero", in the second one you move way above it.
 

jow

Member
2010-11-11 9:04 pm
It would be considered a sum of AC and DC. You can separate out one component or the other, as Kevin alluded to, but the TOTAL signal is the sum of a DC component and an AC component.

Sorry I'm so slow on this but can you explain further. In the example I gave no AC source is supplied. It is a battery, pot and resister in a series circuit. The constantly changing pot producing a changing voltage between the pot and resister would be considered AC over DC?

If I apply a circuit connecting between the pot and resister, through a cap then to ground I will have an AC only current after the capacitor? Will the filtered AC have alternating current (reversing directions)?

Thanks
 
The point is that AC is alternating around a reference. Not necessarily ground. If it all stays in the positive or all in the negative (in relationship to ground), then it is alternating still, but about the reference only and the current may all flow in the same direction when then referenced to ground, but really alternates when referenced to the reference voltage.
If your reference is 5 volts, 3 volts is the same as -2 volts. There just is no difference, except what you are referencing to.
 
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