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Old 17th November 2010, 07:55 PM   #1
jow is offline jow
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Default Triodes and current flow

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
 
Old 17th November 2010, 08:12 PM   #2
SY is offline SY  United States
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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.
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Old 17th November 2010, 08:29 PM   #3
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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
 
Old 17th November 2010, 08:30 PM   #4
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What about CCS?
 
Old 17th November 2010, 08:32 PM   #5
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Quote:
Originally Posted by jow View Post
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.
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Old 17th November 2010, 08:33 PM   #6
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Quote:
Originally Posted by MrCurwen View Post
What about CCS?
What about it?

In a triode stage, if the current is held constant in the plate circuit, the plate voltage will swing up and down.
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Old 17th November 2010, 08:40 PM   #7
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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?
 
Old 17th November 2010, 09:03 PM   #8
DF96 is offline DF96  England
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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.
 
Old 17th November 2010, 09:04 PM   #9
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No, that's not correct. Think of, for example, of a signal that cycles sinusoidally between 9V and 11V. Mathematically, that can be written as 10 + sin(wt), where w is a constant (2*pi*sine frequency). That is, the signal is a linear sum of a DC component (the 10) and an AC component (the sine term).
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Old 17th November 2010, 09:04 PM   #10
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Pure AC requires changes of direction. Here we have AC and DC combined, as SY explained.
 

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