Designing A Guitar Tube Amp: Part 6

el84pp_acLongTailPhase Splitter Design

This was one of the more difficult procedures for me to figure out, and I think I might have given in a little to the “it looks like the book so I’ll roll with it” mentality. I understand that it is designed just like a regular preamp gain stage, but there seemed to be a little magic that I couldn’t quite grasp. Despite that, I know that this section is pivotal to the push-pull output stage, so I had to dive in eventually. I saved this section for last because I wasn’t ready to wrap my mind around it yet. Now’s that time. Using The Valve Wizard again for this, you can read up on it with me by using the link in this sentence.

el84pp_acLongTailDesignThe first thing to do is devote some voltage to the tail. I don’t have a high voltage power supply in my design, so I don’t have as much as I could give it. I figured that having 235 volts for the anodes would be sufficient, and since I have a 277 volt supply, this gives the tail about 42 volts. I picked the 82k and 100k resistors for the anodes based on many common designs. This gives me 2.8mA and 2.3mA respectively. Now I can draw my load lines from these point to 235V and find a bias point.

I chose my bias point to be around 1.4V. The only unfortunate result from this point is the awkward resistor values needed in the tail. The quiescent current is about 1.6mA so 42V/1.6mA gives me a total tail resistance of 26.2k ohms. In order to have my bias point at 1.4V, I need to divide this by the same quiescent current which gives me 875 ohms for the cathode resistor. Subtracting this from 26.2k ohms leaves 25.3k ohms for the remainder of the tail. (I’ll use a 820 ohm and 56 ohm resistor for 875 ohms, and a 22k ohm and 3.3k ohm resistor for the 25.3k ohm resistor.)

The grid resistors are 470k ohms, which since there are two this gives an input impedance of almost 1M ohms. The DC blocking caps at the input are chosen to be .01uF to satisfy the -3dB cutoff at 20Hz. You can see The Valve Wizard’s calculation and explanation of this.

 

Designing A Guitar Tube Amp: Part 5

EL84PP_toneStackPreamp and Tone Stack

I usually start with my preamp when designing an amp, but for this particular project I decided to go in reverse order. I am fairly familiar with the preamp design, so I decided to tackle the power amp first since I didn’t have much experience with a push-pull style tube amp. We’re in part 5 of this series where I outline the steps that I’ve taken to arrive at my final design, and now I’ll show you how I designed the preamp and tone stack.

Tone Stack

I used Duncan’s Tone Stack Calculator to design the tone stack. I’ve used a couple of different tone stacks over the last few years. My favorites, so far, are the Fender and Big Muff tone stacks. For this amp, I really want a versatile tone stack so I’m going with the Fender style tone stack. It uses three potentiometers and three capacitors, so there’s an added cost to this stack when compared to the Vox or even the Big Muff style tone stacks. I also have to take into account the real estate that the three pots will take up on the amp’s control panel.

Using Duncan’s TSC, much of the calculations and guesswork is cut to a short process. I used the stack as-is. In the image above, I show you what a flat response looks like with the bass and treble turned down and the mid cranked to ten. In this configuration, the treble acts as a gain knob, but only boosts frequencies above about 500Hz. The flat line gain (or loss) of this stage is -20dBv. I’m happy with this design.

EL84preampStage1LoadLinePreamp Stages

This is the labor intensive part of our design. There are too many choices to outline in this short blog article. Most of what you can do with preamps can be studied in the books and links mentioned in the first part of this series; research should always be your first stop when designing anything. The information that I’m sharing is the results of the calculations that I learned from Valve Wizard’s Common Gain Stage document. Go ahead and download this and read through it. The calculations in the images won’t make much sense without it.

I started my design by selecting a plate resistor. I’m keeping this design fairly run of the mill for this design. Sometimes I break rules in my designs to see what happens (like small plate resistors), but I’m not going that route this time. For the first stage I chose a 100k ohm resistor. There are countless other amps that do this very thing. Now I’ll use the load line to calculate the other part values; The load line will run between the 2.7mA and 271V segments on the graph. Now I need to find my operating point.

I’m choosing an operating point for my cathode voltage to be around 1.75V. When I draw a line from this operating point, I’ll find my quiescent operating current. Then I can calculate the cathode resistor needed to achieve this goal. The cathode resistor will be a standard 2200k ohm resistor. The bypass capacitor is calculated as 4.7uF.

  • Anode resistor is 100k
  • Cathode resistor is 2.2k
  • Cathode bypass capacitor is 4.7uF
  • Plate voltage is 271V
  • AC gain is 29.2dBv
  • Second order harmonic distortion is 16%
  • Voltage swing with 1.5Vp-p input is about 42V

EL84preampStage2LoadLineThe second stage is designed in the same way. I’ll choose the anode resistor first, then I’ll calculate the rest based on the results. I chose a 120k resistor which should give me a slight increase in gain and less harmonics. The second stage was calculated several times based on different operating points. I started with a 2V cathode voltage, but later changed it to 1.5V after finding that I didn’t like the AC load line at 2V. Choosing the cathode voltage to be at 1.5V gives me a little more room for a 3Vp-p signal to avoid clipping. this stage will clip, and it is meant to clip. The first stage when driven by a 1Vp-p signal will have a 28Vp-p signal from the output into the tone stack. Turning the tone close to max would allow most of this to stage 2. It doesn’t produce any extra volume at that point, but it does alter the waveform of the original signal substantially.

  • Anode resistor is 120k
  • Cathode resistor is 1.8k
  • Cathode bypass capacitor is 4.7uF
  • Plate voltage is 271V
  • AC gain is 31.4dBv
  • Second order harmonic distortion is 4%
  • Voltage swing with 2Vp-p input is about 75V

What’s Next?

The next stage to design for the EL84 push-pull amp is the phase splitter that will drive the two tubes in the power amp. I didn’t go to great lengths to design this section since there were many designs that fit the bill. After part 6, the amplifier circuit design will be complete. Part 7 will show how I do the chassis layout and turret board design. Part 8 will show how I designed the cabinet.

So far everything I’ve done has required no purchases! I’ve spent $0 at this point designing my amp, and it has provided many hours of fun (work). A design like this, including layout, usually takes me 40 to 60 hours. I spend a lot of time with the math, and then I spend a considerable amount of time using Google Sketchup and Inkscape drawing up inch-by-inch diagrams to make sure the layout is perfect. There’s nothing worse than rushing something just to find out that you can’t fit a chassis in the cabinet because a power switch is in the way.

Part 9 is where I’ll start spending money. Part 9 will probably be the cabinet build. Part 10 will show the chassis drilling and fabrication stage. Part 11 will show the turret board and chassis stuffing. Part 12 will show the initial power up; Part 12 will be interesting because if anything goes detrimentally wrong, this is where I’ll probably get hung up. Stay tuned. I’m attempting to meet a project completion date of Jan 15, 2015. This is due to the budget needed for this design and the time needed to complete the stages. I started this project around September 1, 2014. Stay tuned.

 

 

Designing A Guitar Tube Amp: Part 4

duncanAmpScreenshotDesigning the Power Supply

So far I have done my tube amp building research, defined what I want out of my amp, and I’ve designed my power amp. Before I go any further, I need to design a power supply to see if my amp design is possible. The steps involved aren’t difficult, but I’ll spend some time doing a little math, playing with Duncan’s Power Supply Design program, and doing a little shopping. I’ll be using power supplies from EdcorUSA. Their quality has always suited me well despite the lead time, and I haven’t had one fail. You can’t get things cheap, quick, and quality all at once.

First I need to find out what kind of power transformer that I’ll need. I want a 300V DC minimum 120mA output, so I’ll need a power transformer that has a 300V/1.414 = 212V AC at 140mA. I also know that the heater supply will need to support at least 760mA+300mA+300mA = 1.36A at 6.3V AC. After searching through the Edcor catalog, I arrived at the XPWR152 which is perfect for this. I’ll use a full wave bridge solid state rectifier.

I know from the load line that my power amp idles at 60mA. Under max usage, this will vary, but from my understanding the inductance of the output transformer can handle some of that without stressing the power supply. I also need to take into account that the 12AX7 tubes will also draw current, and so will the grids of the power tubes. The XPWR152 is rated at 200mA for the HT, so I believe it will be cool as a cucumber for this.

The rest of the design is done in Duncan’s Power Supply Design program. Check out the image to see how I set it. This shows me a pretty good assumption of how my power supply will react. To keep costs low, I used a combination of a C filter with two RC filters. The I1 current tap is B+1 for the power amp. I2 is B+2 for the phase splitter, and I3 is B+3 for the preamp. Since the push-pull power amp rejects common mode noise, a large ripple is acceptable for B+1. I have 8V ripple on B+1, and I could probably go with a smaller capacitor on B+1, but I am happy with the result. Just keep in mind that largecaps at huge voltage ratings equals big money.

The heater supply is the most simple part of the project. I will have a 100 ohm resistor as a false center tap. This gives it a ground reference.

It took quite some time for me to get familiar with how to work the power supply design program. I did quite a bit of research to figure it out.

Here’s where things get a little crazy as far a assumptions go. What size fuse do I use on the primary side of the power transformer? According to “The Art of Electronics”, this value can be assumed based on the current on the secondary. My understanding is this: secondary current is multiplied by the ratio of secondary voltage to primary voltage. These currents can then be multiplied by 4. This is your primary fuse rating.

  • 6.3V @ 1.36A [ (6.3/120) * 1.36A = 71mA ]
  • 225V @ 200mA [ (225/120) * 0.20A = 375mA ]
  • 375mA + 71mA = 446mA * 4 = 1.78A
  • A 2A 250V fuse will work just fine.

The next step of the way is to design my preamp. Part 5 will be dedicated to a simple way to design and build a preamp. It can be complicated, but I choose to ignore certain criteria. I’ll probably make an attempt to do some of the complicated math, but I’ll try not to bore you with it. Some things can be worked out in the testing phase when everything is fully assembled. Changing a plate or cathode resistor to dial in sound is fairly easy. Since this is a first run working prototype, I expect things to change.

Designing A Guitar Tube Amp: Part 3

loadLineEL84Designing the Power Amp

In order to design a tube power amp, you’ll need a load line chart using the plate characteristics of the tubes you are using. This is found on the datasheet.

I took my plate characteristics chart and laminated it so that I could draw multiple load lines on a sheet or start over without destroying a bunch of paper. You can follow my steps by reading this article at The Valve Wizard. The calculations below are a summary based on my amp design of what I learned at The Valve Wizard and from books on the subject.

I have decided to use an 8k primary output transformer as my starting point for this project. The first point on my load line will be the DC voltage, which is going to be about 300V. I put a dot at the bottom of the load line at 300V.

Next I find the anode current. These are drawn at 1/4Za-a (Class B) and 1/2Za-a (Class A). These correspond to Class A at 4k ohms and Class B at 2k ohms.

  • 300V/4000ohms = 75mA
  • 300V/2000ohms = 150mA

I will draw a dot at each of these points on the left side of my graph. Then, each segment will be connected with a line using a ruler. This isn’t the final operating point of the power amp. The next step is to find a comfortable bias point. This point will be about 75% of the plate dissipation point at 300V. I counted 6 squares up from the 300V/0mA point and placed a dot. I also placed a mark 6 squares up from the 75mA dot on the left side of the graph. I can calculate everything I need to know about my power amp from this line.

My bias point is at 30mA per tube (60mA total), and the cathode voltage will be at 9V. I now know that my cathode resistor will be 9V/.060A = 150 ohms. The power dissipated by this resistor is .060A*9V = .54W so I’ll use a 2 watt resistor for this. The cathode bypass capacitor will be a 100uF 100V capacitor.

The rest of the power amp design is pulled from most schematics that I’ve seen. Grid 2 is connected to the linear taps of the output transformer using 470 ohm 1W resistors. Grid 1 of each tube is connected to the phase splitter through a 4.7k ohm 2W resistors (grid stopper), and the grid reference resistors are 220k ohms.

In the next blog article I will design the power supply.

 

Designing A Guitar Tube Amp: Part 2

Defining Amp Specs

As stated in part one, reading up on the subject of tube amp building is a great place to start when you want to build something. If you use web resources, bookmark them in a folder in your browser because there will come a day when you’ll want to reference it. Sometimes I have an idea in my head, and I forget where I saw it. Having your stuff in order will help you locate something should you need it.

After you’ve become familiar with the process of building an amp, the next step is to figure out what kind of amp that you want to build. The easiest guitar amp to build is one with a single-ended output. I’ve build four of these in the last few years of which three still exist; I turned the parallel 6DG6GT into a single 6DG6GT in 2013, and I updated it in 2014.

The Specs of My Build

I’ve started a new project that will take me quite some time to complete due to the complexity of the build (in both the design and the cost). Here is where I started on this build. Every part of the project will be based on these findings. These specs may change over time given the availability of parts or feasibility of design, but this gives me a good starting point.

  • EL84 Push-Pull Class AB power amp
  • 15 to 20 watts power output
  • Fixed bias on power tubes
  • 8ohm output
  • 300V power supply
  • Single channel input
  • Single 12AX7 preamp
  • 12AX7 phase splitter
  • No effects (tremelo, reverb)

Based on this I have my starting point. Now I have to decide what I want to design first. In the next post, I’m going to start by using the EL84 datasheet to draw load lines, and find the operating point of my power amp section.