Typical Triode Amplifer

 

The structural pieces(everything not a tube) pictured in the figure below provide the environment and conditions necessary for the tube to it's job which is to amplify the input signal.  Keep in mind the figure shows a Triode which is not necessarily a single tube.  There are for example two triodes inside one 12AX7 tube. But there is only one triode in a 6AB4 tube. 

 

Figure 1: Typical triode amplifier showing typical Fender values

 

Fender denotes this in their schematics generally by the use of letters in their numbering. The first triode is referred to as V1A, the second triode is V1B in most Fender schematics.

 

In the Champ they are not numbered, just drawn, but there are two circuits like the one in Figure 1 found in the Champ 5f1 we are reviewing.  Both circuits are in the Input Section with each being generically referred to as a numbered stage of the Input Section.   Stage 1, stage 2, etc..  The only difference from Figure 1 is the Champ stages lack a 25uf/25V bypass capacitor. That innovation came after the Champ. 

 

Figure 2: Stages 1 and 2, the Input Section of a Champ 5f1

 

Notice in Stage 2 there is an extra connection, one not shown in Figure 1.  It's immediately above the Bias Resistor.  We'll say more about that connection when we post about the Output Section of the amplifier. If you're curious now, it's the negative feedback connection coming from the speaker output and run through a 22K resistor.  Figure 3 below shows it's placement. 

 

Figure 3: Negative Feedback Connection Champ 5f1

 

Getting back to Figure 1 now, we'll address of each of the structural pieces briefly touching on it's role in creating the necessary environment for the tube to amplify properly. For your reference the next diagram illustrates the pinout and historical letter designations for the triodes inside a 12AX7 (or 12AU7, 12AT7, 12AY7). 

 

 

Figure 4: 12AX7 Pinout

 

Heater 

 

  • The historical letter designation for a heater is H.  
  • The purpose of a Heater is to provide an indirect heat source to heat the Cathode.

 

In the 12AX7 pinout shown in Figure 4 the heater of the tube runs through pins 4 and 5.  The 12 in 12AX7 denotes the heater voltage required for the tube, 12.6 Volts. But don't all those old Fender schematics have a 6.3V transformer winding for the heaters?  Why don't they use 12.6 Volts?  The flippant answer is because 6 comes before 12.  But it's also the best answer to illustrate the point.

 

Before there were tubes starting with a 12 there were tubes starting with a 6.  There were a lot of tubes starting with a 6. In fact by then 6.3V tubes were almost ubiquitous .  All those tubes required 6.3V transformer windings which meant that transformers with a 6.3 Volt winding were the most common place. They were the least expensive to build with and Leo Fender was a business man.  

 

Using a tube, like a 12AX7, that contains two triodes with each needing 6.3V heaters was also a good business decision.  It provided the best of both worlds to Fender and other builders. A smaller number of tubes were required so less wiring needed to be done and that combined with needing the least expensive transformers all adds up to less expensive amps to build and sell. 

 

Because of all of that when the heaters are wired up on 12AX7 tubes, pins 4 and 5 of the 12AX7 are tied together on one end of the heater source, with the other end of the heater tied to pin 9. This provides each triode with the 6.3V heater it needs.  However, if you have a 12.6V heater source you can simply tie one end to pin 4 and the other to pin5. See the article "Power Supply Basics" for more information on power supplies. 

 

Cathode

 

  • The historical letter designation for a Cathode is K.  
  • The purpose of a Cathode is to provide a source of electrons for the Anode's consumption. 

 

A Cathode is nothing more than a source from which electrons may emit.  When it's heated electrons emit off of the Cathode and are charged such that they are attracted to the Anode.  That's it, that's all a Cathode is for and all you really need to know about it. When heated it emits electrons towards the ready to receive them Anode. 

 

I'm being vague here about the charge of the electrons and their attraction on purpose.  You can thank Ben Franklin for that one.  The larger point here is more important and the details will only serve to confuse an early learner.  So, whether it's technically correct or not think of it this way... Current flows from the Cathode to the Anode when electrons emit off of the Cathode.  

 

The goal is to make the electron flow from the Cathode to Anode take the shape of the signal you are inputing to the tube from your guitar(or the previous stage), only larger.  You want to amplify your input signal.  You can do this by controling the electron flow from the Cathode to the Anode.  And to do that you need to put something in between the Cathode and the Anode that you can then use to control the flow.

 

That thing is the Grid.  

 

Grid

 

  • The historical letter designation for a Grid is G.  
  • The purpose of a Grid is to provide a means of control over the electron flow to the Anode. 
  • A tube can have more than one Grid in it.  It depends on the tube type, Triode, Pentode, etc.

 

A literal grid wire mesh is placed between the Cathode and the Anode in vacuum tubes.  It's used to impede and control the flow of electrons from the Cathode to the Anode by biasing it(charging) to be slightly negative with respect to the Cathode's charge.  

 

Usually the Grid is left at zero VDC and is made more negative than the Cathode by the presence of the Cathode's Bias Resistor which makes the Cathode's voltage sit above the Grid's.

 

SIDE NOTE:  When you hear or read the phrase "Cathode Biased Tube or Amp", this is what they are talking about.  A cathode biased tube makes the Grid negative with respect to the Cathode because it raises the Cathodes voltage above 0 VDC.  The other way to make the the Grid negative with respect to the Cathode is to apply a fixed negative voltage to the Grid itself and that's called a "Fixed Biased Tube or Amp". 

 

That's what a large part of Figure 1 above is all about.  It's all about setting up the tubes bias point so we can then manipulate it with our AC signal.  A Grid's bias point determines the amount of electron flow from the Cathode reaching the Anode at any given point. Where the tube is biased will change with our AC signal so the electron flow effectiviely matches our signal. 

 

Now, those last few paragraphs say a huge amount so take some time to review them and think about them if you're an early learner.  If you can wrap your head around that conceptually then you understand what the vacuum tube is doing and basically what we are doing to manipulate it.

 

How the signal gets amplified is determined by a number of things, in addition to the tube design itself, but chief among them is the Plate resistor value.

 

However it gets amplified, the electron flow the Grid is letting through needs to go somewhere. 

 

It goes on the Plate as an AC signal so it can be consumed by the next stage.  Literally.   

 

Anode/Plate

 

  • The historical letter designation for an Anode is P.
  • The purpose of a Plate is to provide enough voltage (called B+) to make it attractive to the electrons emiting from the Cathode and passing through the Grid and to provide a plate for the amplified AC signal to be served on to the next stage. Pun intended, of course.  

 

The Anode is used to attract electrons emmited from the Cathode. Or at least those that get through the Grid which is allowing a copy of the guitar input signal, an AC signal, through it based on our varying it's bias voltage.  

 

Plate Resistor

 

How much that signal is amplified is largely determined by the Plate Resistor value which determines the level of B+ voltage present on the Anode.  The greater the voltage present the greater the amplificaion. The larger the Plate Resistor the larger the voltage.

 

B+ voltage for a preamp section like the one in Figure 1 is a high, positive DC voltage in the range of  about 130 to 250 volts.  Fender uses lower voltages and Marshall uses higher ones.  

 

So the Anode has both DC voltage (B+) and AC voltage (the amplified electron flow of our input signal) on it. We want the AC voltage to go onto the next stage of amplification or output but we don't want to allow the B+ onto the next stage.

 

That's what a Coupling Capacitor is used for.  Block the high voltage DC and pass the low voltage AC.

 

Coupling Capacitor 

 

  • The purpose of a Coupling Capacitor is to block the high DC voltage (B+) present on the Anode while passing the AC voltage also present on the Anode.  
  • It couples AC, and only AC, from one stage to the next.

 

A coupling capacitor's voltage rating is usually between 400 and 600 VDC. The presence of high voltage B+ on the Anode is the reason for this.  

 

A couling capacitor's value is chosen based on it's presence in the context of whatever the next stage is (amplification or output).

 

For example in Figure 2 above the first stage coupling capacitor, C1, forms a high pass filter with the second stage resistor, R6(a potentiometer).  That's why the pot on a Chanp acts as both a tone and volume control.

 

The remaining parts of Figure 1 above, Grid Leak Reistor, Cathode Bias Resistor and Cathode Bypass Capacitor all relate to configuring the tubes Bias, for the most part.  Remember we want the tube biased such that the Grid is slightly negaively charged with respect to the Cathode. 

 

Grid Leak Resistor 

 

  • The purpose of a Grid Leak Resistor is to provide the Grid with a reference to Ground, to keep it at zero VDC.

 

Cathode Bias Resistor 

 

  • The purpose of a Cathode Bias Resistor is to raise the charge of the Cathode above that of the Grid. It makes the Cathode more positive than the Grid. Alternatively said, it makes the Grid more negative than the Cathode, which is the point.

  • It can affect gain as well since it determines how much above the Grid the Cathode sits. The greater the difference the greater the flow. 

 

Cathode Bypass Capacitor

 

  • The purpose of a Cathode Bypass Capacitor in the context of tube biasing, is to provide finer control over the Cathode's charge.
  • It provides an AC bypass to ground for any unwanted signal reaching the Cathode  

 

Normal values for the Cathode Bypass Capacitor are in the range of 25uf/25V up to about 50uf/50V.  You will will sometimes see them in the 80uf range but not normally.