COVID 19 Update & Project “Princeton” – Part 1

COVID 19 Update & Project “Princeton” – Part 1

The Coronavirus is truly one of those things that none of the world was prepared for, and it’s tragic that it’s claimed as many lives as it has. As a society, I think we all need to do our part to keep ourselves and each other safe. Wash your hands often, wear a mask when in situations involving interaction, and only go out if you truly need to. That’s what I’ve been doing. As of uploading this update, I’m healthy, and actually enjoying the time I’ve been spending at home. Many of you know that staying home for me, means working on another project. So without further ado, let me tell you about my most recent project!

Project “Princeton”

Original schematic for a Fender Princeton Amp, Model 6G2, circa 1961.

Yearning to expand my electronics knowledge, and because I have developed a fascination for them, I decided I wanted to build a tube amplifier, specifically something from the vintage Fender lineup. I decided on the Model 6G2 Princeton, because it’s a Push-Pull Class AB amplifier that isn’t terribly complex. I’ll explain just what that means later in this project. First, I’d like to give a brief history of the Princeton 6G2. It came out in 1961, and offers about 12 Watts of reasonably clean tube tone, with a little bit of overdrive as it gets into higher volumes. Due to it’s tonal profile and lower wattage, it is popular for recording. I just think they sounded marvelous.

One of the early 60s Fender Princeton Amps, Model 6G2. Also known as a “Brownface” for the brown color of the control panel.
Original layout for the Fender Princeton Amp, Model 6G2, circa 1961.
My layout I created from the original circuit, with a few modifications for both improvements in performance, and safety.

What goes into an amp?

When building an amp, it’s important to understand what electrical components are needed, and how they will operate and interact with your circuit. To give you a crash course, let’s talk about the primary components in an amp, resistors, capacitors, diodes, tubes, transformers, and potentiometers. Since we’re discussing just the amp portion, we can talk about the speaker at another time. Today, we’ll discuss resistors, capacitors, and diodes.

Resistors

Resistors are an electrical component that adds electrical resistance. The electrical resistance of a resistor reduces the amount of current able to pass through it. In layman’s terms, it limits the amount of electricity allowed to pass. In amplifiers, they are basically relied on for insuring the proper amounts of electricity are going where they need to go. Electrical resistance is measured in a unit called Ohms. The higher the Ohm value, the greater the amount of electrical resistance that is being introduced into the circuit.

Resistors are labeled and identified using colored bands, typically 4, but sometimes 5. These bands tell you the Ohm value of a resistor. The chart below shows you what the colors mean. Assuming you have a 4-band resistor, the first 2 bands give you a 2 digit value from 01 to 99. The third band give you the multiplier, and the 4th band give you tolerance. On a 5-band resistor, there is simply 3 bands to give you the value instead of 2. If you don’t work with resistors often enough to warrant memorizing this chart, you can get a little (cheater) color coder device that will let you plug in the colors on some wheels, and it provides you the value (see second picture below).

Resistor code chart.
Resistor color coder.

One thing to be cautious of in amplifiers, particularly in areas where accurate resistance is needed, is the tolerance of the resistors being used. Many of the inexpensive resistors you’ll find come with a tolerance of +-10%, which is signified by the final band being silver. This means the value of your resistor will be somewhere in the range of +-10% of the proper value. For example, a 220k Ohm resistor with a +-10% tolerance, could have any value between 198k Ohms and 242k Ohms. Below, you can see I checked my 220K Ohm resistor and it has a value of 228K Ohms, which is roughly 3.6% higher than the proper value. Not bad for a +-10% resistor.

A 220K Ohm resistor connected to my multi-meter.

Capacitors:

A pile of capacitors I’ll use for this amp.

Capacitors are electrical components that store electrical energy. Think of them as similar to batteries, with a few key differences. Batteries store energy in chemicals and discharge is slowly and constantly over long periods of time. Capacitors store their energy in a dielectric, and discharge it much quicker, often in seconds or less. In amps they serve a couple of useful purposes. Firstly, amps use AC and DC power in the circuit, however, only AC comes from the wall. Once it’s been “converted” by a rectifier (will touch on this in the tube section), the voltage often isn’t super smooth. Filter capacitors smooth out the signal by not letting it fluctuate as much. For example, if I have rectified AC power from an outlet, it will look like a sine wave, however, all of the negative portions of the wave will be positive, making the voltage go from 0 to 120 and back to 0, 60 times ever second. Having a filter capacitor would make the voltage smoother, because the capacitor would then discharge when the voltage starts to drop, making the resulting voltage drop significantly less before the next peak in voltage arrives. See the diagram below for a visual of this:

Another important use of capacitors in tube amps is as bypass and signal capacitors, which can be a little tricky to understand. In a tube amp, there are many instances where a portion of the circuit will see both AC and DC at the same time. Capacitors will block DC, but allow AC to pass freely, so bypass and filter capacitors are used to control where the AC and DC signals can and cannot go. The most common place in an amp is on the screens of the tubes. The screen is where the signal for amplification is fed (AC), but it’s also where a positive charge has to exist (DC) to insure the tube functions properly (again, more on this later). If you’re interested in learning more about this, I recommend reading into it yourself, because it’s an interesting topic. Also, I’m trying to write a blog post, not a textbook.

Small and interesting note about vintage style capacitors:

Capacitors built using older methods implement a ‘foil end,’ simply meaning one of the ends of the capacitor has an insulator that is sandwiched between foil. This end should be installed towards ground in your circuit to cut down on noise. This is less relevant in modern times, because equipment has gotten much better.

Checking which end of your capacitor has the foil is rather fun. If you connect your capacitor to an oscilloscope, and hold it between your fingers, you’re functionally acting as an antenna that picks up the residual 60 Hz hum that’s all around us from our AC power. One direction, the waves will be much smaller, and the end towards ground will your foil end.

SUPER IMPORTANT NOTE ABOUT CAPACITORS!!!!!!

Capacitors continue to store energy even when the power is disconnected. In many instances, this is enough stored energy to potentially kill you if discharged into your body. Be extremely cautious when working with capacitors, and always verify they’re discharged before touching them with your hands.

Diodes:

I won’t talk extensively about diodes, because they’re quite simple, but I’ll share the basics. Diodes are a component that only allow current to move one direction. Think of it as a one way valve for electrical current. More diodes can be found if you’re amp is not using a rectifier tube, because one of the most common uses of diodes is in rectification, or the process of converting AC to DC. If interested in learning more about diodes, look into rectifiers, as they’re a common use of them. I’d be lying if I said I think full bridge rectifiers weren’t sexy…

To Be Continued in Part 2…

Thanks for checking in. When I have some more time, I’ll continue sharing this project, and finish explaining the remaining components. Feel free to comment on my post with questions or comments. I love hearing from you. Stop back soon for Part 2!

Some of the components for the “Princeton” Project.

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