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┬álargecaps 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.

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.