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┬áis 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.


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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.

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