Black Box Model and Power Supply Simulation

So it's been close to a month since the beginning of this project. Part of me feels like I have gotten no where in the last three weeks. But I have to remember I have had to familiarize myself with amplifier models and jargon. I have learned about guitar amplifier classes, common guitar amp vacuum tubes, rectifiers, smoothing capacitors, theory of distortion, theory of filter design, and much more. My professor recommended I begin with power supply design, but as I began my research I realized I had to go through guitar amp boot camp before even begin that.

So! The purpose of this post is to discuss a black box model of what goes on inside a guitar amp. The following scan is a quick sketch I made of the essentials of an amp. By no means is it exhaustive, but I feel it is a good assessment of the basics.

I plan to build a Class A, single ended 10 watts (ish) amp. Probably will use a couple of 12Ax7's and an EL84, or at least cheaper copies of these vacuum tubes. I've read about Class AB Push-Pull designs, but feel it is a bit complex for my first build. An acquaintance and fellow U of M student also encouraged me to use a simple power supply design. No tubes, just a rectifier bridge and a few filter stages. This will save on money and time.

I decided to test a simple rectifying circuit/power supply in PSPICE. Took a while to get the output I wanted, but it seems to be doing what I need it to. I plan to try and implement this circuit sometime this week to see if I can get it to work in the lab.

I have also been studying the schematic for my own tube amp in my current guitar rig. The Epiphone Valve Junior seems to be the kind of amp I would like to build. Only a few stages and a power supply, nothing fancy. I still plan to integrate an arduino into my design, just not sure how yet.

That's all for now. More to come soon.
-Mark

Tube Selection and Power Supplies

I had my first meeting with my professor this Monday. It seems the best place to start in amplifier design is power supply and vacuum tube selection. Considering vacuum tubes have special needs (high voltages, max inputs before overheating, other stuff I don't know about yet, etc.), deciding which tubes I will use is essential in design.

But honestly, I've been wandering a bit. Even with a starting point, I still find many of the resources I have collected overwhelming. There is a lot of jargon in tube circuits and I'm still catching up.

I've read the first three chapters of the ancient "Vacuum Tubes" by Karl Spangenberg from 1948, which was helpful, but very theoretical. (http://www.tubebooks.org/Books/Spangenberg_vacuum_tubes.pdf) I'm attempting to find the happy medium between theory and practical application. As my professor said, engineers aren't necessarily good at getting stuff to work, we are more interested in the scientific method. Prediction, implementation, measurement, analysis. When do I apply this? After I've actually got something to predict!

Power supplies seem less mysterious than tubes. I've studied half wave and full wave rectifiers in EECE 3211 (Electronics 1) and transformers in EECE 3201 (Circuit Analysis). I get the basics of those and the power supplies I've been looking at make sense.

The basic function of a power supply seems to be the following:

120 V 60 Hz "wall" input signal ==> transformer to step down/up voltage ==> rectifier to change AC signal to DC signal with lots of ripple ==> some kind of filtering capacitor to smooth out ripple ==> DC signal to power circuit ==> Face-melting guitar solo

I guess that ripple reduction stuff my prof was talking about during filter design in EECE 3201 last semester was important... Not that I didn't pay attention or anything...

I'm thinking my best option is going to be choosing a common, simple tube amp design that can get me comfortable with the basics. Dave Funk's Tube Amp Workbook, which I've read most of this week, mentioned the Vox AC15 being the simplest of simple designs:

What do you guys think?

I'm thinking: Get Behind Me Satan! If this is simple I've got a LOOOOONG way to go.

About lunch time. I'll see you folk later!
-Mark