A look inside an amp.

All of the how-to's here are referred to as my "trademarked" 'Kinda-How_to's'. Meaning, they will not get very technical, and avoid high math. The reason for this is that the average hobbiest (hopefully) will be able to walk away and know enough to do it, and 'kinda' how to explain it... after all, its a hobby, right?

This is a pretty big task for me, and will take a while to complete. I will explain an amp, from input to output in a fair amount of detail.



And where do we start? The inputs!
The input(s) is where you plug your guitar into the amp. More often then not, there are 2 inputs. Sometimes only one, sometimes four, sometimes 2 per channel, etc. We are going to focus on a single channel amp with 1 input. The majority of principles can be applied to other tube amps.

A typical HI/LOW input jack:

As you can see, one input is labled HI, and the other one LOW. This can cause some confusion, and for good reason. Some manufacturers mean, the HI input will produce a 'higher' signal (more current), and LOW will produce a little less current, or a little weaker signal. While others mean, use the HI input for your stronger signal devices (humbuckers) and your weaker signal devices (single coils) into the LOW. You might think that those two lines of thought are completely opposite.. you're right. So when in doubt, double check your amp manual. From here on out, we are going to go by the first option - HI producing a stronger strength signal and LOW producing a lower strength signal. That line of thinking tends to be more conventional.




Kinda-HOW-To - Where does the input go? The first TUBE!

The majority of guitar amps use a 12ax7 for V1 (V1 denotes 'valve 1', then V2, etc.). Some amps use a 12at7, or maybe a less conventional tube. We are just going to stick with the good old 12ax7. Before we get to the input, lets take a quick look at the 12ax7.

In a tube amp, almost always, you can think of it in 2 sections, pins 1,2,3 & 6,7,8. The other pins (4/5 & 9) are the heaters (more on heaters later.) Pins 1 & 6, is where the tube gets its power - this is the anode, or sometimes refered to as the plate. The high voltage in the amp is sent to these pins usually through a 100K resistor. Pin 2 & 7 are the grids, or screen grids, - think inputs for these pins. Pins 3 & 8 go to ground through a resisitor & capacitor in parallel, or sometimes just a resistor. These pins are the cathodes. As I said, you can think of these as 2 independant sections - becuase they are. You can conbine them to work together, but usually in our amps, each triode ( a triode is the set of 3 pins 1,2,3 and the other set 6,7, 8) is used individually. Sometimes, you will see a design that does not make use of a triode on one of the pre-amp tubes - and nothing will be connected to say pins 6,7, & 8 while 1,2, & 3 will be in use.

NOTE:Do keep in mind, this is not designed nor intended to be complete information - there is a LOT more that goes on inside this tube, but I'm aiming this page at begginers to tube amps. Also, I want to point out, that in my mind, I am picturing a point-to-point hand build amp in my mind, so not everything will apply to a curcuit board based amp (wires, etc...) - remeber, principles can be applies, but these are just kinda-how-to's. Ok.. lets move on.

So... from the section above regarding the input jacks, the signal is sent into the amp. Generally, through a shielded wire, it is then conected to pin 2 (or maybe pin 7 depending on what half of the tube you are using). Sometimes a resistor is needed to be inbetween our input on pin 2 & the wire coming from the input jack - this resistor will rid the amp of any(most all) unwanted RFI - radio frequency interference. Yes, your amp, if not shielded properly, can pick up radio stations. Also interference from floresant lights and a few other things (also called EMI or electromagnetic interference). I usually use a 68k resistor. The choice of a 68k is a little on the high end of common practice. So.. up to this point, here is what we have.





Kinda-HOW-To - Select a cathode bypass capacitor!

Here we are going to discuss cathode bypass capacitors. There will always be exceptions to the rules, but this will cover and apply to most guitar amps. If you look at a schematic, you will notice there is more often then not, a resistor and capacitor in parallel connected to the pins 3 & 8 of most every 12ax7 (12at7, etc.) preamp tube. Pins 3 & 8 are the cathodes of the tube. These resistor and capacitor go to ground. The resistor (Rk) is the cathode resistor, and the capacitor (Ck) is the cathode bypass capacitor. The capacitor has a noticeable effect on the tone produced by the preamp. In most amps, you will find values of this capacitor as high as 25uf down to 1uf and sometimes even lower. The higher values of this capacitor causes more gain produced by the tube. Also, the higher the value the more strength the lower frequencies will have - thus producing a 'fuller' sound.

Here is where personal preference comes in. Some people want more higher frequencies and less lower frequncies to come thorugh, and thus, lower capacitor values are used. Also, in some (not so traditional) designs, if the capacitor value is too large, it will cause excessive -and unwanted - gain. I have seen some very high gain (almost unstable) desgins to where the bypass capcitor is completely omitted from the design and only the cathode resistore is present.

If you are building an amp, and you want to experiment, changing this capacitor is a good way to start fine tuning the amp for tone.

and our drawing that will end up being a full preamp.




Kinda-HOW-To - The anode (or plate) on V1 - HIGH voltage!

Looking at almost all guitar amp preamps, connected to pin 1 & 6 will be a 100K resistor. The 100k resistor is pretty much a standard. This 100K resistor is the feed for the high voltage, or "B+". This high voltage can be in the neighborhood of 200 to 400 VCD (depending on the amp). This is some of that lethal voltage inside amps that you hear about. Anyway, pin 1 (& 6) can also be thought of as the 'output' for the triode. Here is our picture again.


See that line pointing twords the right? Thats the signal after it leaves the first half of the first tube. The signal goes in pin 2, out pin 1. Now, this is where a lot of the majic happens in the tube. And I'm not going to cover it becuase its out of the scope of this page. For your reference, you can read HERE and learn about plate resistance, dissipation , Mu, etc.

So, at this point, we have our inputs, and a full 1/2 of a 12ax7. See the diagram below.



To summerize, the signal was been put into the amp in a 'stronger' (or 'weaker') input jack, the signal passes through our 68k resistor to filter out RFI and enters the tube, the tube amplifies the signal and at the same time, due to the tube & the cathode bypass cap/resistor, the 'tone' has been colored (i.e. frequencies got filtered). The amplified & colored tone leaves this half of the tube.. onward and upward!




Kinda-HOW-To - The Tone stack!!!

There is a LOT of things to say about the tone stack. What is the tone stack? This is, from the users perspective, the bass, mid, treble, & volume controls - or variations of this theme. These are the vast majority of the time in our guitar amps passive circuits - meaning - by turning up the treble (for example) you do not increase the higher frequencies, but you reduce the lower frequencies. On the same token, by turning up the bass knob, all you are doing is bleeding off higher frequencies to ground so they don't get past this part of the circuit. Some amps do have 'active' EQ's, which actually add to the signal, but again, the majority of guitar amps have passive EQ's. The particular frequencies controlled by each potentiometer (the component that IS the knob) is dictated primarily by the capacitors within the circuit.

Here is a potentiometer, or pot for short.


The following picture is a continuation of our circuit.... (one of these days I should redraw them, and make them look better!)



Here we have a very basic tone stack from a Fender amp. You could add a 'mid' control in three (and a half) steps:
1) Replacing R6/6k8 with a 250k potentiometer
2) Connecting C4 to the middle log of the new mid pot
3) Connecting lug #3 from the bass pot to lug #1 on the mid pot
And our half step - lug #3 on the mid pot would contiune to ground as in the original schematic.

There are a few schools of design for tone circuits like Baxandall/James design, the Fender or Bassman style, & the Marshall style, which is a rather modified bassman design. For our simple needs, we're going to stick the the classic Fender style. And I guess I should now mention the volume control. A potentiometer is jsut a variable resistor. turned all the way down (in our case) it provieds full attenuation, or resistance) to the signal - meaning, in the case of the volume, zero volume. Turned up all the way, it allows every bit (in theory) of the amplified signal to pass through.. simple as that!

So.... here is our amp so far. Its starting to take shape. We have our input, our first bit of amplification, our coloration of the tone, we can now adjust the quantity of frequencies which pass through the tone stack circuit, and control the volume.



Next stop, the PI!!!

Till I get more time, have fun.
-tw