Question about current flow in tubes

[ricohman]

I found this post on another board today and it has me a bit confused. Am I to understand that electrons flow from the cathode to anode and current then flows from the anode to the cathode?
I think I am more confused now that I read this.

All tubes work by controlling the flow of electrodes forced off a cathode by either thermal energy or electric field concentration. The thermal ones are the only ones that we need to think about.

The thermal emission makes a cloud of electrons around the cathode. A positive charge on the plate attracts the electrons to it, and current will flow from plate to cathode because the negatively charged electrons are flowing from cathode to plate. Current will not flow the other way because there are no positively charged particles to carry the currrent. (OK, OK positrons exist, but as antimatter, they're incredibly rare here, and no tubes ever produced on earth use them.) We have a thermionic diode.

If we insert a grid between cathode and plate, we can choke off some of the electron flow by making the grid negative with respect to the cathode. That repells the electrons back to the cathode space charge (notice how I snuck in that technical term?) either partially or completely depending on how negative we make it.

The triode suffers from poor high frequency response because of the capacitance betweel plate and grid. If we put in a screen grid between the grid and plate, we can literally shield the plate from the grid by introducing an effectively grounded surface between the grid and plate. The screen grid also helps accelerate the electrons toward the plate, and as a result of its screening and evening the flow of electrons, the flow of electrons is much less affected by the plate-to-grid or -cathode voltage, so the plate characteristics of the tube show a flatter current vs grid voltage, less affected by plate voltage variations, in addition to high frequency response. We have a tetrode.

The tetrode is great, but with low plate voltages, the electrons flying into the plate are hitting hard enough to knock other electrons back off. This is secondary emission, and it gums up the works at voltages of about 100V and less. Inserting a coarse grid near the plate, but with the voltage of the cathode forces any secondary emission electrons back into the plate. This removes the secondary emission kink in the plate characteristics at low plate voltage, and we now have a pentode.

Interestingly enough, in the UK, the beam-power form of the pentode is called a "kinkless tetrode" and this is what accounts for the "KT88" and other KTxx numbered Brit tubes.

 

[dcgillespie]

From a scientific standpoint, all electrical charges flow from a negative potential to a positive potential. Therefore, electrons and current flows from the cathode to the plate.

In the "olden days" it was actually taught that current flowed from the plate to the cathode -- which was done for the ease of teaching (apparently). With the introduction of SS electronics, the symbols and thought process didn't work with the old material, requiring those still working with vacuum tubes to "rethink" how the current actually flowed through the circuit.

With the near demise of tubes, the problem nearly corrected itself, but now with the resurgence of tubes -- and some of the old timers still around who were taught the wrong way -- some confusion exists again.

The bottom line is that electron flow and current flow are one and the same. You can't have one going one direction, and one the other. Therefore, electron and current flow always originates and flows from the cathode, moving towards the plate.

I hope this helps!

Dave

 

[goldleaf]

I recall a physics teacher trying to get around this issue by teaching us that current in transistors was created by the flow of "holes". I used to laugh at that.

 

[ricohman]

Thanks Dave for clearing this up. After I read that I thought I had missed something.

 

[ferninando]

goldleaf is right about that

 

[gadget73]

In the "olden days" it was actually taught that current flowed from the plate to the cathode -- which was done for the ease of teaching (apparently).

Dave

never understood that one at all. Actually for years I'd been told that electricity flowed from positive to negative, which is why I flatly did not understand how tubes worked. Once I finally learned that it actually goes the other way, then suddenly tubes made sense.

 

[sebrof]

I recall a physics teacher trying to get around this issue by teaching us that current in transistors was created by the flow of "holes". I used to laugh at that.

The military schools used to, and still may, teach "Hole Flow" theory; electrons move in one direction causing the "holes" created by the vacated electrons to flow in the other direction. Hence current flowing from neg to pos. It's all the same as long as you keep to the same theory from beginning to end. I don't remembering having any trouble re-learning when I left the military and went to college.

 

[wa2ise]

Actually for years I'd been told that electricity flowed from positive to negative, ... Once I finally learned that it actually goes the other way, then suddenly tubes made sense.

Blame Ben Franklin for that. When he flew his kite in a thunderstorm (urban legend?) he took a guess as to which way the current flowed. And he had a 50 50 chance but got it wrong. It wasn't until physicists figured out atoms did it become known that Ben got it wrong. But it was too late to correct it, so we have to live with it. All electrical engineering students have to come to terms with it. As do math students have to learn that imaginary numbers are not bogus or fake numbers, and why the d's don't cancel in d/dx in calculus class. :scratch2: "Well, it looks like a fraction..."

 

[Mike Stone]

Blame Ben Franklin for that. When he flew his kite in a thunderstorm (urban legend?) he took a guess as to which way the current flowed. And he had a 50 50 chance but got it wrong. It wasn't until physicists figured out atoms did it become known that Ben got it wrong. But it was too late to correct it, so we have to live with it. All electrical engineering students have to come to terms with it. As do math students have to learn that imaginary numbers are not bogus or fake numbers, and why the d's don't cancel in d/dx in calculus class. :scratch2: "Well, it looks like a fraction..."

I'm totally with you on this one wa2ise. I always thought current flowed from positive to negative as well. I've asked 20 guys and got 20 answers

 

[Mike Stone]

From a scientific standpoint, all electrical charges flow from a negative potential to a positive potential. Therefore, electrons and current flows from the cathode to the plate.

In the "olden days" it was actually taught that current flowed from the plate to the cathode -- which was done for the ease of teaching (apparently). With the introduction of SS electronics, the symbols and thought process didn't work with the old material, requiring those still working with vacuum tubes to "rethink" how the current actually flowed through the circuit.

With the near demise of tubes, the problem nearly corrected itself, but now with the resurgence of tubes -- and some of the old timers still around who were taught the wrong way -- some confusion exists again.

The bottom line is that electron flow and current flow are one and the same. You can't have one going one direction, and one the other. Therefore, electron and current flow always originates and flows from the cathode, moving towards the plate.

I hope this helps!

Dave

I wanted to also thank you Dave for the info above. I've always wondered what the difference between cathode current and plate current. I see resistors sometimes connected to both sides. Very helpful! Mike

 

[gadget73]

Cathode current is the total of all current flow in the tube. The vast bulk will be cathode to plate, but with pentodes there is also some going to screen, and an infintensimally small amount also goes to the control grid, though generally not enough to measure or care about. If there is enough to care about, the tube is probably bad.

The resistor thing is just about voltage drop. On the cathode side, its typically used to create the bias the tube needs in order to control current. The grid must be negative with respect to the cathode. You can do that by applying negative voltage to the grid, or allowing the cathode to be positive. Current flow through the resistor will make the cathode positive, and a resistor is simple and very cheap way of doing this compared to a bias supply. It can have problems in power stages but for small signal tubes it works extremely well.

The plate resistor is also for drop, but in the opposite direction. Tubes are sort-of like a voltage controlled variable resistor. When the grid becomes less negative, current flow increases, almost like a resistor getting smaller. When the grid becomes more negative current flow decreases like a resistor getting larger. To make this variable current amplify voltage, it has to create a voltage change. Thats where the plate resistor comes in. Higher current flow = more voltage drop on the resistor, less current flow = less drop. The signal that comes out of the plate ends up being a larger amount of voltage, but opposite in phase to what went into the grid. When the grid gets less negative, the plate becomes less positive.

the cathode actually goes in the opposite direction here, as the plate voltage becomes less positive with more current flow, the cathode becomes more positive.

 

[Mike Stone]

Cathode current is the total of all current flow in the tube. The vast bulk will be cathode to plate, but with pentodes there is also some going to screen, and an infintensimally small amount also goes to the control grid, though generally not enough to measure or care about. If there is enough to care about, the tube is probably bad.

The resistor thing is just about voltage drop. On the cathode side, its typically used to create the bias the tube needs in order to control current. The grid must be negative with respect to the cathode. You can do that by applying negative voltage to the grid, or allowing the cathode to be positive. Current flow through the resistor will make the cathode positive, and a resistor is simple and very cheap way of doing this compared to a bias supply. It can have problems in power stages but for small signal tubes it works extremely well.

The plate resistor is also for drop, but in the opposite direction. Tubes are sort-of like a voltage controlled variable resistor. When the grid becomes less negative, current flow increases, almost like a resistor getting smaller. When the grid becomes more negative current flow decreases like a resistor getting larger. To make this variable current amplify voltage, it has to create a voltage change. Thats where the plate resistor comes in. Higher current flow = more voltage drop on the resistor, less current flow = less drop. The signal that comes out of the plate ends up being a larger amount of voltage, but opposite in phase to what went into the grid. When the grid gets less negative, the plate becomes less positive.

the cathode actually goes in the opposite direction here, as the plate voltage becomes less positive with more current flow, the cathode becomes more positive.

Gadget73, I've received a few explanations for this, but you put it more succinctly than anyone else. Thank you for this explanation! Really Awesome. Mike

 

[kward]

Wow, old thread!

“hole flow” was an actual principle discussed in college semiconductor courses. Holes being the location in the outer valence band that used to contain the electron that was sitting there but now is drawn via the electrical field to fill the hole in the valence band of next adjacent silicon atom. So in essence the hole moves in the opposite direction of the electrons. But as we know there is no actual hole particle and there is no actual hole movement. I feel it was taught this way to conveniently describe semiconductor behavior given the solid lattice structure of the composite (doped) silicon material.

As mentioned above, I wonder if that was part of the confusion, when armed with these modern constructs, to go back and try to re-explain the behavior of tubes. We all thought we perfectly understood how tubes behaved until transistors came along and fouled things up

Just kidding of course and I have also become less adverse to using “sand” in my personal building of tube audio circuits if those transistors are not directly in the signal path. Sometimes transistors can marvelously enhance tube circuits. E.g auto bias circuits, power supply regulation, low impedance rectification, and etc.

 

[Mike Stone]

Wow, old thread!

“hole flow” was an actual principle discussed in college semiconductor courses. Holes being the location in the outer valence band that used to contain the electron that was sitting there but now is drawn via the electrical field to fill the hole in the valence band of next adjacent silicon atom. So in essence the hole moves in the opposite direction of the electrons. But as we know there is no actual hole particle and there is no actual hole movement. I feel it was taught this way to conveniently describe semiconductor behavior given the solid lattice structure of the composite (doped) silicon material.

As mentioned above, I wonder if that was part of the confusion, when armed with these modern constructs, to go back and try to re-explain the behavior of tubes. We all thought we perfectly understood how tubes behaved until transistors came along and fouled things up

Just kidding of course and I have also become less adverse to using “sand” in my personal building of tube audio circuits if those transistors are not directly in the signal path. Sometimes transistors can marvelously enhance tube circuits. E.g auto bias circuits, power supply regulation, low impedance rectification, and etc.

I love your comment about transistors messing everything up Transistor gear was cool when I was in high school and into my career, but the first time I heard a tube system, using Conrad Johnson gear, I fell in-love with tubes.
I've also read about some of the "hybrid" preamps and amps that you mention above. I bought an old Moscode amp years ago with tubes and mosfets, if memory serves. P.S. I still like transistors but Tubes have cleared the Path! Mike

 

[oldflame]

In the earlier days of solid-state electronics repair, I remember an old joke about a customer's broken transistor radio that had a hole in its case. The customer was told that too many of the transistor's electrons escaped through the hole and there weren't enough left for the radio to work.

 

[Mr Boochie]

Current is not one electron passing down the wire (or through a vacuum) but the electromotive force of all the electrons caused by a voltage differential. I was taught it's like a pipe filled with marbles. If you cause a voltage differential (potential) across the wire, it's like pushing the marble at one end, and nearly instantly, the marble at the other end falls out of the pipe --- regardless of how long the pipe is (minus friction or electrical resistance in electricity).

 

[oldflame]

I'm awfully sorry Mr. Boochie but I also remember an old joke about losing too many marbles. I'm off to try collecting some of mine with a nap. (good current explanation, BTW)

 

[egellings]

never understood that one at all. Actually for years I'd been told that electricity flowed from positive to negative, which is why I flatly did not understand how tubes worked. Once I finally learned that it actually goes the other way, then suddenly tubes made sense.

A hot cathode boils electrons off it-negatively charged particles. They all head for a positively charged plate. The electron current is thus from the cathode to the plate. The trick is whether you define the current as where the electrons go physically, or in the opposite direction. Big thing is to be consistent. Choose either direction and stick with that for any analysis. Both approaches can lead to a valid result.

 

[Mr Boochie]

I'm awfully sorry Mr. Boochie but I also remember an old joke about losing too many marbles. I'm off to try collecting some of mine with a nap. (good current explanation, BTW)

Much thanks! My marbles have migrated to the North Pole.

 

[gadget73]

I was taught it's like a pipe filled with marbles. If you cause a voltage differential (potential) across the wire, it's like pushing the marble at one end, and nearly instantly, the marble at the other end falls out of the pipe --- regardless of how long the pipe is (minus friction or electrical resistance in electricity)

so a Saginaw box. Everyone knows about the inner fine points of recirculating ball steering boxes right?