Triode and Phase

[J Rindt]

Seems like I am always the one to ask the dumb-shit questions.....but what can I say.?
I checked all my books:
Jack Darr
Merlin
Reosenblitz
Some of the old Navy articles
...maybe I am missing it.
They all mention that when a signal enters the grid of a triode, it exits the plate out of phase. But is there an explanation as to what causes the Phase Shift.?
Thank You

[sluckey]

They all mention that when a signal enters the grid of a triode, it exits the plate out of phase. But is there an explanation as to what causes the Phase Shift.?

Specifically, that would be a 180° phase shift. It's just the nature of the beast.

Here's the really easy explanation...

Put a variable voltage on the grid. Can be an AC signal, battery connected thru a pot, whatever. As the voltage on the grid increases in a positive direction, the current thru the tube will increase. This current increase will cause an increased voltage drop across the plate load resistor which will leave less voltage to be dropped across the tube. The net result is that the plate voltage will decrease, or become less positive. IOW, an increasing positive voltage on the grid causes a decreasing positive voltage on the plate. That's the phase shift.

Just the opposite happens when you decrease the voltage on the grid.

[HotBluePlates]

They all mention that when a signal enters the grid of a triode, it exits the plate ...

EDIT: Sluckey said it shorter, typed faster... 

Are you ready to really understand what is happening? If yes, then do yourself a favor and delete from your brain the part about "a signal enters ... a signal exits." We will not get far thinking about a tube in this manner, even though it is often said that way in simplistic explanations.

If you heat a tube's cathode and apply a large positive voltage to the plate, a large current flows between the plate and cathode; this is a diode (like a rectifier tube).

If you want to control the current flow with a steady voltage applied to the plate, you can insert a grid between the cathode and plate (usually closer to the cathode). This makes a triode, and a small voltage (relative to the plate) on the grid is much more effect at controlling the current than a change of plate voltage (the ratio of the two causing equal changes is the tube's Mu, or amplification factor).

All voltages we're talking about are relative to the tube's cathode. If we say "grid voltage change" it is really a change in the voltage measured from cathode to grid; if we say "plate voltage" it's really the measured voltage from cathode to plate.

Amplification Example
Pretend we have a 12__7 triode stage with a 100kΩ plate resistor, a supply voltage of 300vdc and a grid voltage of -2v which causes an idle current of 1mA. Let's also assume there is no cathode resistor; the cathode is grounded and is at 0vdc. Ohm's Law says that the 1mA through the 100kΩ plate load resistor causes a voltage drop of 1mA * 100kΩ = 100v across the resistor, leaving 300v - 100v = 200vdc on the 12__7 plate.

Now let's say the grid voltage changes from -2v to -1v, and that change (+1v) causes the tube to pass 1.5mA. What is the voltage drop across the plate load resistor? 1.5mA * 100kΩ = 150v, which leaves 300v - 150v = 150vdc at the plate of the tube. So a 1v increase of grid voltage caused the tube to pass a current which created a -50v shift in plate voltage due to the voltage drop across the plate load resistor (from 200vdc to 150vdc).

Let's continue by saying the grid voltage changes from -2v to -3v, and that change (-1v) causes the tube to pass 0.5mA. What is the voltage drop across the plate load resistor? 0.5mA * 100kΩ = 50v, which leaves 300v - 50v = 250vdc at the plate of the tube. So a 1v decrease of grid voltage caused the tube to pass a current which created a +50v shift in plate voltage due to the voltage drop across the plate load resistor (from 200vdc to 250vdc).

  - At all times, a tube voltage change which causes an increase in tube current also causes an increased voltage drop through resistances connected to the tube. Because the plate load resistor is connected from a fixed supply voltage to the plate, an increased voltage drop causes a decrease in plate voltage. This is the reason for the apparent polarity change.

  - Consider the same increase in tube current causing an increased voltage drop through a resistance connected between the tube's cathode and ground. Because the cathode resistor is connected from the 0vdc reference point (ground), an increased voltage drop causes an increase in cathode voltage. A rising grid voltage would have caused the increase in tube current causing this increased voltage drop. This is the reason for the apparent same-polarity output at the cathode.

So there is no magic inside the tube, except in the manner in which a grid can allow a small applied voltage to control a relatively large current through the tube. Everything else we observe (voltage gain, signal polarity) is really just a byproduct of the interaction of circuit resistances with that tube current.

[J Rindt]

sluckey
HPB
10-4
Thanks for pointing out the obvious to me.
The plate is "out of phase" for the same reason the Cat is in phase.
Lord help me...I am just not that bright. I have never been an Intuitive Learner (if there is such a phrase) I have always needed a Textbook, or visual explanation.
I guess what I am saying is that I learn by the lowest common denominator.
Christ.
Crystal Clear
Thank You Both

[eleventeen]

This is quite the excellent and short video presentation of the goings on if you like the visual. The video does not discuss phase, per se, though it does show it very well. Well worth watching a few times. Perhaps augmented by Sluckey's and HBPs simple and more complex explanations, you will get a better picture.

http://vimeo.com/33244955

This, the phase relationship (and the implications) can be a hard thing to get into your head but is critical to understanding triodes (and thereby all common tubes----or transistors) when you encounter them in real-world circuits and try to make them work or troubleshoot them. Lots of times we simply do not care about the input : output phase relationship as in the first stage of a guitar preamp.

Watch the video a few times.

This can be sort of a tricky concept to get into your head, it reminds me of studying music theory. You start off studying scales....chords....keys....and they don't seem to connect in any coherent way and it seems like a big struggle to keep track of all the particulars...then one day, you start to understand that they are (and the ways they are) all tied together and are all kind of the same thing.

[tubeswell]

Electrons flow from the cathode to the plate (because the plate is connected - via the plate resistor - to a High Tension voltage). The grid is in between the cathode and the plate.

When the grid voltage goes more negative it decreases the electron flow to the plate, so there is less plate current. Ohm's Law states that Voltage = Current x Resistance, therefore if resistance stays constant, then lowering the tube current will decrease the voltage being dropped across the plate resistor that is in series with the tube. Seeing as how the plate resistor is connected to a fixed source High Tension voltage, the actual plate voltage goes more positive (relative to the High Tension source). (i.e.; say the HT voltage is 350VDC - as tube current decreases, the voltage at the plate-side of the plate resistor gets closer to 350VDC, because there is  less voltage dropped across the plate resistor)

And vice-versa; when the grid voltage goes more positive, the tube current increases, and (therefore Ohms Law kicks in) the voltage dropped across the plate resistor increases, so the plate voltage goes more negative (relative to the High Tension voltage source). So the grid is in opposite phase to the plate.

However at the cathode end of the tube (where you have a cathode resistor in series with the tube), as the grid voltage goes more negative (and you therefore decrease the tube current) the voltage across the cathode resistor also decreases (because the cathode resistor is grounded at one end and the ground is sitting at 0VDC). So the cathode voltage moves in the same phase as the grid voltage. Whereas the plate voltage moves in opposite phase to the grid voltage

[J Rindt]

Very well said everybody.
.....and thanks for the video link.
best

[PRR]

The same is true for "all" 3-leg amplifiers with both voltage gain and current gain.

Junction transistors, FETs.

E.M.Cherry's book treats them all the same.