Tube Amplifiers
Dan L asked:

From what we understand? a filament that? somewhat heated. Then download a cloud of electrons in a vacuum and moving positively towards the metal plate loaded by a -? And something about the grid between the control the movement of electrons? Where the original signal from the guitar enters? Where exits? As the filament? heated? I also noticed that? are typically four poles that sit out from the bottom, what everyone does? Moreover,? across a closed circuit correct? I 'M under the impression that in the current flow passes through the filament, gets transferred to the plate and then it leaves the flat where? Where to go from l? and the pipe? feels like the 'missing / misunderstanding of m. something. I 'm. also still a bit disconcerting as to which party amplifies the signal. ? the grid? ? the signal from the guitar and the electron current through the separate property of the filament is a two or the same? If separated, leaving the audio signal from the guitar enters and exits?


3 Comments so far

  1. billrussell42 on December 23, 2009 11:21 pm

    The filament is heating by passing current through it.

    Very simplified:
    The control signal is applied to the grid, which modulates the electron flow, and that causes the plate voltage to change, and the signal is taken from the plate.


  2. Ecko on December 26, 2009 4:55 pm

    A typical setup for an output stage is that the negative of the power supply connects to the cathode. Electrons travel through the vacuum to the anode/plate, attracted by the positive voltage there. From the anode the current flows through a transformer primary to the positive of the power supply. The secondary of the transformer is the load, such as a loudspeaker. The cathode is heated by the filament/heater, usually in a separate low voltage circuit. The filament is the cathode too in some older types and battery operated types.

    The voltage between the grid and the cathode controls the anode current. The grid is always kept more negative than the cathode, so it can repel some of that cloud of electrons that is being attracted to the anode. This negative bias keeps the anode current to a comfortable level for the tube. More positive on grid, more anode current. More negative and less current flows. The grid is connected to the cathode indirectly through a high value resistor, so electrons will have a return path.

    The audio signal is coupled to the grid with a capacitor, and is superimposed on the bias voltage. The anode current varies with it. The transformer couples the AC component of the anode current to the load.

    Other points. There may be other grids (usually called screen grids. These improve the performance. The basic three element unit is a triode. Cathode anode and grid. Others are tetrodes and pentodes. Usually the signal from the guitar would be amplified by low power stages so it has enough voltage to drive the output stage. The output stage may be made up of two or more tubes driving the same output transformer. Those pins are the connections to the heaters, cathode etc. Sometimes there is a connection on top, which may be the grid, or the anode. The anode is very bitey if you touch it, as it is several hundred volts and significant current is available.

  3. dmb06851 on December 29, 2009 8:43 pm

    The English “valve” is much more evocative than the American “tube” of what the device does.

    The filament (in older valves) or the heater (in more modern ones) has nothing whatsoever to do with the amplification. It just liberates electrons which make up the current through the valve.

    A valve is used to control the flow of something; gas, liquid or electrons.

    The control in a gas or liquid valve is exerted through a knob/wheel (for instance).

    “Where does the original signal from the guitar enter?”

    Control of the current through an electron tube is by the voltage on the grid with respect to that on the cathode.

    Waggle that voltage around and you waggle the value of current through the valve.

    That change in current produces a change in voltage at the anode, and that voltage is an inverted and larger copy of the voltage at the grid.

    “Where does it ( the original signal from the guitar) exit?”

    It doesn’t. It is used solely as the control. Once it has been presented to the grid of the first valve it has done its job.

    What comes out of the valve, or any subsequent valve/s, is a larger copy of the input signal. Sometimes inverted, sometimes not.

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