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Old 7th May 2009, 09:18 PM   #1
losacco is offline losacco  United States
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Default Starting to understand.....

Ok, a newbie has a question. Iíve been reading a lot and have a good understanding on how a triode works. Now Iím stuck, Iím not understanding how the signal(music) and other current and voltages are separated or not separated in the circuit. What path does the signal take? I know itís applied to the grid to very the electron flow from cathode to plate but what is the relationship between the increased electron flow from cathode to the plate and the signal(music) on the grid? It seems that the signal stops at the grid. How does the signal get amplified?

I know there is probably not a simple answer but any help would be appreciated.

Patrick
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Old 7th May 2009, 09:34 PM   #2
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Hi Losacco

As you have well noted, varying the voltage at the grid changes the current conducted between plate and cathode. That is, a variation in voltage at the grid produces a proportional variation of current in the tube. With plain current one can't do much, therefore one inserts a resistor between the power supply and the plate (for example). When the current between plate and cathode varies (as consequence of changes of the voltage at the grid), this current has to go through the resistor... now you apply Ohm's law: V=R*I and you see that with a fixed resistor R and a varying current I you get a varying voltage V across the resistor: when there is little current the voltage across the resistor is low, when there is lots of current, the voltage across the resistor is high.

Hope this helps! Erik
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Old 7th May 2009, 11:25 PM   #3
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Quote:
It seems that the signal stops at the grid. How does the signal get amplified?
Yes, there is (usually) no current between the grid and either the cathode or the plate. The grid has a charge which is (usually) negative with repect to the cathode. This causes the plate current to be reduced. The stronger the negative charge on the grid, the less the plate current.

If the grid is made too strongly negative, the plate current will be cut off altogether. If the grid is not negative enough, then too much current will flow and the tube will be damaged. So, a negative grid bias is established about midway between these two extremes.

By varying the negative grid voltage within these limits with a signal, you can achieve a variation in the plate current that is in time with the input signal. The relationship between the two will be reasonably linear, but just how linear depends on the tube design. (Some tubes that were not designed for audio amplification are not very linear at all.)

The amount of the variation in plate current per given signal on the grid (the tube's sensitivity) depends on the tube design too, and is expressed in mA/v or uA/v. This parameter is known as the transconductance.

Erik has explained how the varying plate current is turned into a varying output signal voltage by means of a plate resistor. The amplification (gain) achieved is the ratio of output signal amplitude to input signal amplitude. Another tube parameter, the amplification factor (mu), tells how much amplification would be obtained if the plate load resistance were infinite. In practice, using a resistor as the load, the resistance will never be infinite but there is a simple formula to calculate the gain based on the actual load resistance used.
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Old 8th May 2009, 12:53 AM   #4
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I just happen to have gotten stuck on my studies last night when I got to the point about the load resistor converting current to voltage swing. I got a little lost on the "how" part. What is really happening here? how does the resistor come into to play? I need this detailed to really understand whats going on.
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Old 8th May 2009, 01:03 AM   #5
SY is offline SY  United States
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Think of a valve that controls the flow of high pressure water. With very little force, you can modulate the pressure of the water by turning the valve to and fro.

In the case of a tube, electrons are emitted from the cathode. Because of the gradient of the electric field set up between cathode and plate when the plate is positive with respect to the cathode, the electrons are attracted plate-ward. When a grid is interposed (generally much closer to the cathode than to the plate), the field that the cathode "sees" is much diminished since the grid is usually negative. Grid more negative, lower field strength at the cathode. Lower field, less electron flow.

Now the function of the load resistor is to turn the variation in current into a variation in voltage.

This is very simplified and you'll have to go a few more steps to get why a constant current source replacing the resistor maximizes gain and linearity, but it's at least a start toward visualizing what physically happens.

Hopefully, you've got some good books to help you along. No secret, my favorite basic text is Morgan Jones's "Valve Amplifiers."
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Old 8th May 2009, 01:14 AM   #6
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New to tubes?
See the thread at the top for on-line information.
On Line Tube Learning for newbies....
Cheers,
Ian
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Old 8th May 2009, 02:39 AM   #7
losacco is offline losacco  United States
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Wow, thanks to you all for your comments. Your explanations cleared a lot up for me.

I have one more question before I start to read up on biasing. Ok, the increased current flows towards the plate and out of the tube, right? I understand how the resistor causes the change in current to give an increase in voltage (ohms law); my question is, on some schematics, it seems the output signal is taken off before the resistor. What am I missing?
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Old 8th May 2009, 03:08 AM   #8
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Maybe this will help. Another way to think of a valve is as a voltage controlled variable resistor. As the voltage on the grid is varied, so is the current passing between anode and cathode. If the cathode is grounded, and you have one fixed resistor between anode and B+, then you got a voltage divider. The voltage value in between the resistor and anode will vary accordingly, as the current varies through the "two resistors", so to say.


B+ ---- R1--------- Anode Grid Cathode ----- Ground

and equivalent circuit

B+ ---- R1---------R2 variable resistor ------- Ground

HTH
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Old 8th May 2009, 03:11 AM   #9
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Patrick,
One of the wonderful things about questions from folk trying to understand things from first principals is that they often, as in the case here, ask a critcial question.

You ask "what am I missing?" - The answer is the power supply.

The top of the anode load resistor is held at a fixed positive DC voltage.
The anode will sit at a voltage somewhat lower than that, due to the "idle" or "bias" or "steady state" DC current through the tube.

This anode voltage will move up and down in response to the signal current which is imposed upon the "idle" current.

The grid signal voltage causes a tube signal current which develops a signal voltage across that load resistor.

That signal voltage at the anode will be an accurate amplified representation of the grid signal voltage - ASSUMING that the power supply can hold the supply voltage to the top of the anode load resistor constant, while signal current is being drawn from the power supply. We therefore note that as far as AC signal is concerned that the power supply is "in the loop".

Note that as the grid swings positive with signal, the tube current increases, the voltage drop across the anode load resistor therefore increases and teh anode voltage which is power supply volts less the anode load resistor drop DECREASES. That is the tube stage amplifies AND inverts the signal.

The "loop" - which way we view the "loop" depends upon whether you go with practical physics or traditional definitions arrived at before the practical physics were understood. Positive Current flow was defined as from positive to negative long before it was understood that what is actually happening is that electrons flow from negative to positive to make up that current. We've been living with that "blunder" for nearly 200 years.

Physically, in the tube, electrons are emitted by the cathode and are attracted up toward the positive anode. The grid sits in the electron flow between cathode and anode and controls that flow.
Due to that 200 year old "blunder" we call this positive current flow from anode to cathode.

If we stick to the CURRENT FLOW definition then the loop is.

From Power Supply positive
down through the anode load resistor into the anode
down through the tube and out of the cathode
through any cathode bias resistor to 0V
AND back through the power supply to the Power Supply positive.

That is why for HIFI the power supply is as important as all the fancy amplification circuits. Power Supply defects show up in the signal voltage.

A bit of a rave but I hope useful.
I note as I type someone else has just posted - hope I'm not saying all the same things.

Cheers,
Ian
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