25 June 2012

Guitar Amp: How it works? - Part 4

Putting all together

During the three previous parts, we discussed a while about the different parts of the amp, what they do individually and what role do they play in the amp as a whole set.
Now, we will use some very easy amp design to identify the parts that we were describing.
I am not going to discuss it in depth, since I have no electronics skills but, I will try to let you know the basics whithout having to deal with highly tech stuff.

Sample: Fender's '57 Champ

This little amp has been included in a waste amount of record tracks. Even being one of the simple designs, this amps delivers a really nice sound so, let me guide you in its design.
First of all, let see the complete diagram. Please, remember to double-click over the images to see them at real size.

There is some information that can be known at a single glance, just reviewing a bit the schema, by example:

  • It has a single preamp tube, one of its triodes should be the Driver or PI so, it has a single preamp amplifying stage (the first triode).
  • It has a single power tube.
  • It has a tube rectifier
  • It's single ended
  • Probably, this is a Class A amp.
But, let see the schema by blocks.

The Power Transformer (PT)

Look at this picture:

The block highlighted with that red rectangle, corresponds to the PT and everything related to get the source power from our wall's mains and transform such a power to different levels to satisfy the specific power needs of every part of the amp.
Let zoom in:

At left hand, you can see the mains plug, followed with the power switch of the amp and some protective components (fuse, etc) until we reach the Primary coil of the PT (inside that red rectangle).
On the right side of the PT, we can see three coils that support the different tappers of the Secondary.
The lowest coil seems to be providing very few power (3.3VAC = 3.3 Volts of Alternate Current) and, it seems just lighting the ON light of the amp, with some protection around (see Fuse F3, by example).
Also, It seems to be providing the power needed for the filaments (cathodes) of the two tubes of this amp, see V1-C and V2-B 6V6GT labels at right hand, before the bulbe symbol.

The two other coils above, are delivering different levels of power to be used by the rest of the amp.
Special interest has the middle coil, which extreme conductors go to a tube rectifier (inside that green circle).
I think we didn't talk about rectifiers before. The rectifier transforms the input power delivered as AC (Alternate Current) to DC power (Direct Current). Most of the components inside the amp will work with DC power, instead of the typical AC power that we can find in our mains sources.
This amp has a tube rectifier but, nowadays is usually more common to find a solid state rectifier, instead.
Tube rectifiers doesn't work in a very lineal way, so, they produce some kind of compression and a behavior that is called rectifier' sag.
Solid state rectifiers are way more linear and quicker in their response. While a tube rectifier can have a bit of delay in the responsiveness of the amp, solid state rectifiers react instantenely so, tube rectifiers are usually found in Vintage and Vintage-voiced amps.

This tube rectifier has an octal socket (see max pin number is 8) and it's a double triode (see the two plates inside the blue shape). As per the schema, it goes on position V3 (the third tube in a row).

We don't need to go further. The PT is transforming the power of the input into several levels of power, depending on the needs of the rest of components of the amp. The Rectifier is transforming some of that power delivered by the PT in one tap, from AC to DC and then, the rest of amp is being powered.

First amplification stage

Let me show you the block we are going to zoom now:

And, let zoom in, then:

We can see that this amp has two imput jacks (input J1 and J2). The resistor R1 of 1M is there to avoid noise in the circuit if nothing is plugged to none of those inputs. On the picture, they are enclosed in that green rectangle.

The first real component that our guitar signal crosses is just the resistor (R3 for J2 and R2 for J1) named grid biasing resistor. In this picture, those two resistors are enclosed in a red rectangle.
This resistor has as main goal to avoid some inestability within the tube, and filters a range of frequencies that are way lower than the lower frequency that a guitar signal generates so, it's not specifically affecting the "tone" ... or does it?. Well there are large discussions about which components affect to the tone and, which are the best materials or models to use for each position but, I will not enter in such a discussion.

You see that after crossing that grid biasing resistor, the signal path is directly entering in the grid (dashed horizontal line) of the first triode of the first tube (V1-A = triode A of tube 1). This first triode of the tube is enclosed in a yellow rectangle in this picture. The upper 'dash' within the tube corresponds to the anode or plate. The lower 'bracket' corresponds to the cathode or filament.
The three numbers close to those three elements are the pin number. So, pin #1 is the plate (output of the triode), pin #2 is the grid (that regulates the flow of the electrons beam released by the cathode) and pin #3 is the cathode (generator of electrons).

Time to remember how this triode works and first surprise for you.
Remember, the cathode, highly resistive goes hot when receiving energy (from the PT), while its hot, his surface starts to release electrons that create a beam that travels to the plate (polarized positive).
The grid goes negative, until being more negative or equal negative as the plate to interrupt the flow of the beam so, delivering no output on cathode.
Do you know that key-copying machines that you can see in some supermarkets?. You go with the key of your home and you want them to make you an exact copy of that key (and we know that, sometimes, the copy isn't 100% perfect).
Well, that's exactly what is happening at every single triode within the amp!.
The triode is some kind of signal-copying machine but, with the difference that the copied signal is being scalated way up than the original (more powerful).
But, what does it really mean?.
Surprise, surprise: your guitar signal deaths just in the grid of the first triode!.
The signal that will be propagated to the rest of amp at this time comes from the beam of electrons released by the cathode but, the shape of that signal will be modulated with the signal characteristics that come in the grid.
In fact, when the output of this triode enters in the following triode (cascaded gain), same thing happens.
The new triode is the one creating the signal that will be propagated to the rest of amp and, the output of the first triode is just "modulating" or "sculpting" the "shape" of that signal.

We already said that every triode (tube or solid state) should be biased but, a bias should be applied to every of its three components.
The resistor R5 in that purple rectange is biasing the anode or plate. This resistor will fix the range of voltage that the plate can variate when the beam from the cathode is being regulated with the help of the grid.
The resistor R4 in that blue rectangle, is biasing the cathode and, determines the amount of gain for this triode. Also, we can see a capacitor C1 bypassing the resistor. This capacitor has an effect basically on the feeling of low end frequencies. Depending on its value, the sound can have more or less presence on basses.

So, this is what we will typically see around a single gain stage triode.

I surounded one more componente inside that grey rectangle, capacitor C2 is usually named a coupling capacitor. It's called like this because it couples (links) two amplification stages. The function of that capacitor is basically to avoid the output of triode 1 to come back from next triode 2, it's some kind of door that opens in a single way (more or less, you get the idea).

Second amplification stange and Driver

Let see the block we are going to face now:

Let zoom in:

It seems quite well the same as the first stage, right?.
We can recognize quite well the same components with minor differences.
Firstly, the grid biasing resistor (that was a fixed value resistor) has been substitued here with a potentiomer (variable resistor). The input for this pot is just the output of the first stage, after crossing the coupling cap.
The output of that pot is the input for the grid of the second triode of the first (and only) preamp tube (V1-B).
You can see that this grid biasing variable resistor is called VOLUME and, it corresponds to the knob named Volume that you will see in the front of the amp and, yes, its function is to regulate the amount of volume of the signal that will be delivered to the power amp. When rolled off, the amp will shut up.

Is in this position, just before the grid of the second triode, where we usually find the so called Tone Stack. In this case, this amp has no more controls than just a single volume knob but, most of amps can stack here controls for basses, mids and trebles and, on amps with more than two stages, we can even found a Gain pot instead of Volume, by example. It's all about amp's design, at the end.

You can see that the bypass cap that was present in the cathode biasing resistor (R7 in this stage) is missing here. That cap is usually present just in the first amplification stage. Some amps, could have a switch to enhance basses, this switch is usually adding such a capacitor to the circuit or removing it (if no enhancement of basses is needed).

Like in the case of the first stage we already discussed, here R7 is the cathode biasing resistor, R8 is the plate biasing resistor and C3 is the coupling cap that will link this amplification stage with the following one (that, in this case is just the power tube), as we will see below.

Since this triode is the last stage between the preamp and the power sections, this is the Driver.

Power Tube

Let see the block we are going to work now.

Let zoom in:

The tube is clearly a pentode. See those three grids (dashed lines) between the cathode (pin 8) and anode (pin 3). Our well known grid is in pin 5 (the lowest dashed line). The upper grid is linked to the cathode so, same resistor that biases cathode is biasing that grid. The middle grid (pin 4) has it's own biasing resistor (R11).
So, the output of the driver (seen previously) goes thru the coupling cap C3 and enters in the grid (pin 5).
Resistance R9 is biasing the grid. We see here a difference. Instead of having the resitor in the middle of signal's path (as we have seen in the two pre-amp stages), resistor is connected to ground, in the same way as the biasing resistor for cathode is. This is known as a cathode-biased arrangement and, what at the end means to us is that this tube doesn't needs to be re-biased after substitution. This is also supported because we cannot see any variable resistor around to re-bias the tube.

Resistor R10 is biasing the cathode (setting the gain factor) and, we see coming back this bypass cap (C4) that will affect the amount of basses in the signal, making the amp to sound thicker and warmer.

The output of the tube is the plate or anode, in pin 3, which signal is being directly sent to the OT.
So, that single Power tube, together with the fact that is cathode-biased points to a Class A cathode-biased amp topology but, let take a look to the OT, first.

Output Transformer

Let first identify the block:

And, zoom in:

The output of the power tube is linked to one of the extrems of the Primary of the OT, while the other extreme of the OT is linked to the power supply so, this is a single-ended topology.
In this case, we see a single tap in the Secondary of the OT. The OT is adjusting the power output for an impedance of 4 Ohms so, an speaker with an impedance of 4 Ohms is expected there.
As I said, we can hung there speakers with higher impedances (8, 16, 32 Ohms) but, in that case, we are lowering the output power and overall volume.
Usually, the OT will have more than one tap and, at least 8 Ohms and 16 Ohms taps are present in most of amps.

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