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.



About robbie

I am an electronics enthusiest and a ham radio operator (W1RCP). I like to play with electronics. It's fun and educational. I looked forward to working in the engineering field in the future. I have a BS in Electronics Engineering Technology from DeVry University. I also have an Associate's degree in Marketing Management from Moultrie Tech, and a diploma in Electronics from MTC.