So you want to know more about tube amps... [Archive] - Jam Session

Turbo7MN

December 2nd, 2006, 08:34 AM

Understanding the 5E3

The purpose of this guide is to get beginners a little more familiar with reading schematics, as well as what the components of an amp are and what they do. This guide assumes you have at least a basic knowledge of electronics. We’ll start with the schematic for Weber’s version of the Fender 5E3 Deluxe. This amp is considered by some to be the ultimate blues amp, and considering that it contains most commonly found amp components without being too complex makes it a nice place to start. This schematic is very close to the original 5E3 schematic, Weber’s version is what everyone should be building anyway (the only real difference is a safer power supply) and it’s easier to read than the original.

The original Fender schematic and layout can be found here:

http://www.schematicheaven.com/fenderamps/deluxe_5e3_schem.pdf

Weber’s kit schematic can be found here:

https://taweber.powweb.com/store/5e3_schem.jpg

I’ve taken Weber’s schematic and broken it down into sections that I will be referring to. You can get it from here:

http://i120.photobucket.com/albums/o186/Turbo7MN/5e3_schemlabel.gif

We’ll start with the basics. All components are usually marked with a letter and a number. The letter tells you what the component is (R for resistor, C for capacitor, etc.) and the number is for reference. The triangle-looking things are the symbol for ground. I like to use the line-style ground symbol (short parallel lines with the top being the longest and bottom being the shortest) like the ones used in the Fender schematic. The triangle ones can sometimes get confused with diodes. These are shown as a triangle with a line in front of it; one is shown in the additional bias circuit at the bottom of the page labeled D1. Okay, on to the circuit.

Let’s start with the power supply. This is marked as “1” on my schematic. It’s towards the bottom bordered in red. Look on the left at J7 (jack #7). Look familiar? This is your standard 120VAC power plug. The bottom pin is tied to ground; the other two go to the power transformer. F1 is the amp’s fuse. If it tries to pull more than two amperes (sometimes abbreviated to “amps”, it’s a measure of current), the fuse will blow and break the circuit. This protects the power transformer as well as the tubes, to a certain degree anyway. S1 is the main power switch; this closes the circuit and lets the power flow to the power transformer (T2). The symbol for a transformer is two parallel lines with the squiggly wires on both sides. Don’t get this confused with a choke or inductor, which only has the squiggles on one side. All the transformer does is change the 120VAC from the wall into different AC voltages that are useable by the amp. The rectifier is what converts it to DC, but we’ll get to that in a minute.

The “input” side of a transformer, in our case where the 120VAC is fed, is called the primary, and the other side is called the secondary. You’ll see there are three different secondary “outputs” with this transformer. The top output steps up the voltage considerably and goes to the rectifier. This schematic unfortunately does not give voltages, but this is usually between two and four times the input voltage. The next one down reduces the voltage to 6.3VAC and goes to the heater pins of every tube. This is what actually glows orange inside the tube and causes the cathode to emit electrons. The bottom one that isn’t used produces 5VAC and is wired to the heater/cathode of the rectifier tube if the amp has one. This amp is currently set up to use a Weber copper cap rectifier, which does not need this voltage applied. If you want to use a real 5Y3 rectifier tube in this amp you must connect these wires to the heater/cathode pins. Have a look at the original Fender schematic if you want to see how this is done.

Now we see the rectifier, V5. The “V” stands for valve. If you feed an AC voltage to the plates (anodes) of a twin diode and then make a tap off a heater/cathode pin, you end up DC voltage. For our purposes it’s not worth explaining how this works, all that really matters is that this occurs using either a tube rectifier like this or silicon diodes, and that it’s what provides the high voltage DC that tube amps have become notorious for.

This brings me to section 1a, the high voltage section of the power supply. This is called the B+ supply, or “B+ rail” as I like to call it. S2 is the standby switch. Using what you know about the power supply so far, take a second and guess at what the standby switch does. Got it? If you guessed that it allows you to power up the heaters and “warm up” the tubes before hitting them with the high voltage, you’d be right. As you can see, the B+ supply in this amp provides three different voltages, and is broken up into three different “stages” if it helps to think about it like that. R24 and R25 are “dropping” resistors that lower the voltage. C10, C11, and C12 are filter caps. These reduce the amount of power supply “hum”. The letters A, B, and C will pop up in other places on the schematic; these correspond to the taps off the B+ supply. The A (first) tap always goes to the plates of the output tubes, the second tap goes to the screen grid of the output tube, and the others go to the plates of preamp tubes. That should cover the power supply. As always, feel free to ask if you have any questions.

Now let’s get to the actual signal path, starting with section 2. This amp is interesting because all four inputs, even though this amp has two channels, are exactly the same. Some amps have a “bright” channel, but we can cover that later. The line with the bend in it is called the “tip” and is the part of the jack the signal passes through. The other part is the “sleeve” and is the signal ground. Note the arrow in the middle of the jack. This is a shorting jack, which means that when nothing is plugged in the input is shorted to ground. This keeps the amp quieter. R1 and R2 are there to drop the input signal. This is also the function of R3 through R6 to an extent, though these are more to keep the inputs isolated from each other, and there are a couple other minor things they do that aren’t really important for out purposes.

Now let’s look at how the preamp stages work. The symbol for a tube looks a little complex but it’s pretty simple. The 12AX7 and its variants actually have two sections, so you get labels like V1A and V1B. They’re separate gain stages, but they’re in the same tube. The line (sometimes shown as a block) at the top is the plate; this gets the high voltage and can be looked at as the “output” of the tube. See the letter C? Remember where that leads? The 100k resistors are the plate resistors. These drop the voltage to the plate even further. As a general rule, the larger the value of plate resistor, the more distortion you’re going to get.

The inputs feed into the grid of the tube. The grid is always shown on a schematic as a dotted or dashed line. The purpose of it is to regulate the flow of electrons from the cathode to plate. At idle (no signal) it will always have a negative voltage in relation to ground. This is what bias is.

The curved or “hooked” looking line is the cathode. This is always tied to ground one way or another. Looking at the cathode, notice that both of them are tied together and share a resistor and bypass cap. With the resistor there, a positive voltage on the cathode actually develops. If you measure between the grid and cathode, sure enough, you get a negative voltage. The value of this resistor is very critical; in fact changing the value of this is how you bias a cathode biased gain stage. The larger the value, the colder the tube is biased. The key is to find the value that puts the voltage halfway between cutoff and saturation. This is getting a little more advanced though. The cap increases the overall gain of the stage.

Section 2a is still pretty much part of the first stage. C2 and C3 are what we call coupling caps. These always go between the plate of one stage and the grid of the next one. The value of these will change the amp’s frequency response. R10 and R12 are volume pots. As you can see, these work by bleeding off a part of the signal to ground depending on where the knob’s at. R11 is the tone pot. All tone controls really consist of is one or more pots and several caps. This one happens to be very simple. I’m personally not a big fan of this style, the volume and tone controls affect each other too much.

Section 3 is the second preamp stage. The input to the grid comes from the pot network in section 2a. Nothing terribly complicated here. This is how a lot of 12AX7 stages are set up, with a 1.5k cathode resistor and 25uF bypass cap. The .022uF coupling cap is the most common value you’ll see in guitar amps.

Continued...

Turbo7MN

December 2nd, 2006, 08:34 AM

Here’s a fun part – the phase splitter. Some amps only have one power tube so you can run the signal from the preamp tube straight into it. Unfortunately with a push-pull amp with multiple output tubes you need something the break the signal into two “phases” that are 180 degrees apart. This style, with one output coming off the plate and the other coming off the cathode, is referred to as a “cathodyne” style. This setup is actually frowned upon in guitar amps, but there’s nothing really wrong with it. You actually just get a little extra distortion from the phase inverter stage. 5E series Fenders, vintage Orange amps, the Peavey Classic series, even my own 60 watt Bogen convert use this style. This is thought of as the “cheap” way to do it because it only uses half a tube, as opposed to the “long tailed pair” that a majority of amps use. This style has the output coming from the plates of both halves of a dual triode (12AX7, AT7, etc.).

R15 bleeds a small portion of the signal to ground. Replacing this with a pot is usually how a master volume is done. R16 is the cathode resistor, the 56k R17 is there to keep it “balanced” with the plate side output. Can you remember what we call C7 and C8? R21, 22, 19, and 20 are all there to reduce the level of the signal going into the output stage. None of these are terribly important; my 60 watter doesn’t have any of these.

Section 5, obviously, is the output stage. One thing that’s important to note here is the tubes have more than one grid. A pentode will have three of these and a tetrode, as shown here (it’s actually a beam tetrode, which is sometimes drawn as a pentode) will have two. The one closest to the cathode is the “control grid” and is pretty much equivalent to the single grid in a triode like a 12AX7. This is also where negative voltage would be applied if the tube was fixed-bias. The other grid shown here is the screen grid, which is fed from the B+ supply near (but never above) the same voltage as the plate. The other one not usually drawn is the suppressor grid which is always tied to ground, usually through an internal connection so it doesn’t have a pin of its own. The EL34 is an exception here though, as it requires pin 1 to be tied to ground. This is one of the reasons you can’t just stick an EL34 into an amp designed for a 6L6 or other octal power tube.

This one’s cathode biased, obviously. The tubes share a 250 ohm resistor and a 25uF bypass cap. The highest voltage in the amp is fed to the center tap of the output transformer’s primary winding, and then goes to the output tubes. The transformer converts the very high output impedance of the output tubes to 4, 8, or 16 ohms to match speakers.

And there you have it. Not that difficult now, is it? I’d like to do this for another amp or two if there’s interest. I’m open to any suggestions, including any of my own amps if you ask nicely .

Remember, high voltages kill! Just because you read this, don’t think you can just crack open an amp and start messing with stuff. I’ll get a beginner’s guide to actually working on amps done sometime too, if there’s interest.

-Darren