For those of you who want to have the warm sound of a vacuum tube amplifier, when playing guitar or just listening to music, but cannot afford to buy an expensive tube amplifier, this instructable is for you. This instructable will show you how to build a vacuum tube amplifier using parts salvaged out of old electronics and 2 purchased vacuum tubes. The tube amp sounds really nice and it amplifies music and guitar really loud. I estimate the output wattage to be about 10-15 watts. For an explanation and a demonstration of the tube amp, watch the video below. Lets get started!
Step 1: Gathering Materials and Tools
For this vacuum tube amplifier, you will need many different tools and materials.
- Soldering Iron
- Wire Strippers
- Wire Cutters
- Jig Saw
- Screw Driver
Materials: Most of the materials for this project can be salvaged from old electronic devices. The 1/4 watt resistors were bought in bulk on Ebay. The high wattage resistors were salvaged. The capacitors, chokes, power transformers, speakers, wood, and metal slabs were salvaged from old electronic devices. The tubes were found in a lot that I purchased off of Ebay.
- Various Resistors
- Various Electrolytic Capacitors
- Various Film Capacitors
- Inductive Choke
- Various Lengths of wire
- 12AX7 Vacuum Tube
- 6V6 Vacuum Tube
- Power transformers: 120:12, 120:9, 120:2, 0:120:277(Primary)
- Metal slab
- 4 Ohm and 8 Ohm Speakers
- Audio input/output jacks
- Volume potentiometer
- AC mains switch
Step 2: Setting Up Chassis
For this tube amp, I am using the breadboard technique. First, I take a wood slab, and set a metal casing down on the the wood. Then, I plan out where I am going to bolt down all the power supply transformers. After that, I add a back plate to the end of the chassis. In the middle of the metal cover, I glue down a piece of copper clad board and ground it to the metal part of the chassis. The chassis should be done now.
Step 3: Adding the Tube Sockets
The tube sockets were made by drilling holes in a wood block according to the octal formation of the tube pins. Then, wires are inserted and glues into the holes for easy soldering to the electronics. The tube sockets are then glued down to the copper clad board near the places they will be used. For example, the 12AX7 is placed near the audio input because of it's use as a preamp tube, and the 6V6 is placed near the audio transformer because it is used as the output tube.
Step 4: Wiring the Heaters
To wire the heaters of the two tubes, run the two wires for each tube under the ground plane(copper clad board) to the corresponding power supplies that have the recommended voltage for the tubes. Tturn on the power supply and you should see the tube heaters warm up and glow red.
Step 5: Drilling Faceplate
The face-plate can be made using an old piece of metal from a power supply. Then, holes should be drilled in the face-plate with the diameters of the switch, audio jacks, and volume potentiometer. When the different components are inserted into the face-plate, nuts can be used to secure them to the face-plate. The face-plate should then be grounded for safety reasons.
Step 6: Building the Power Supply
To build the power supply, you will need many transformers. For the b+ voltage power supply, you need to use a transformer with primary inputs for 0 volts, 120 volts, and 277 volts. The 0 volt and 120 volt pins are then wired in parallel with all the other power transformers, and the 0 volt and 277 volt pins are wired to the AC pins of the bridge rectifier. The collective 120v wire is then wired through a switch to the AC mains plug and the collective 0 volt wire is wired to the AC mains neutral.
The cooling fan transformer outputs 9 volts AC, and when put through a bridge rectifier, it powers the cooling fan.
The filament transformer outputs 12 volts AC to power the filament on the 12AX7 tube. In parallel with the tube, there is a rectifier and a buck converter module that step down the 12 volts into a usable 6.3 volts to power the 6V6 tube.
There are a total of 3 separate power supplies that are all turned on by one switch.
Step 7: Soldering the Audio Circuit
To wire the audio circuit, you mostly have to just follow the circuit diagram. Follow the lines in the diagram and connect the different shown components together by way of the the lines in the diagram. Wire all the ground connections to the the copper clad plate on the bottom and isolate any connections and solder joints from the ground using nails hammered into small wood cubes. When wiring, pay special attention to the voltage and wattage ratings of the capacitors and resistors. Make sure all the solder joints are secure and make sure that all the necessary components have been used. As long as the circuit diagram is closely followed, this tube amp should work.
This tube amplifier uses the dual triode 12AX7 tube to first amplify the small signal voltage that comes from audio in. The voltage then comes out of the tube through a interstage capacitor into the 6V6 tube that drives the audio transformer.
The volume potentiometer controls the grid bias voltage on the 6V6 tube, when the volume is all the way down, the grid is grounded.
Step 8: Audio Transformer
The audio transformer that I found was a power transformer from a radio. It has an input impedance of 1k ohms and an output impedance of 12 ohms. The input impedance is not the recommended impedance for the 6V6 tube, but it works just fine anyways. To find the impedances, measure the coil resistance. Resistance and impedance are not the same, but as resistance ratios match impedance ratios, this method should work fine. When adding speakers, remember to match the speaker impedance to the the output winding(secondary) impedance. The use of the output transformer is to step down the 300 volts at a low amperage that the tube uses into a more usable low voltage high current that drives the speakers.
Step 9: Cooling Fan
The cooling fan is just a squirrel cage type computer fan that cools the 6V6 tube when it gets too hot. It is quiet and blows straight on the 6V6.
Step 10: Building the Cabinet
The cabinet was built using plywood and particle board. It was built so the two speakers were in the front and the sides and back were open. On the back, two 2 inch wooden dowels support the top. This cabinet was built so the actual amplifier could sit on top. The speaker holes were cut using a jig saw.
The two speakers are connected in series to get the 12 ohms of impedance to match the audio transformer. They are then connected to the speaker jack that I added to the side of the amplifier.
Step 11: Troubleshooting
While building this amplifier, I ran into a big problem: When I turned on the amplifier, smoke started coming out of the power transformer. After problem solving, I realized that I had AC ground connected to chassis ground. Because of the autotransformer setup of the power supply the transformer was shorted out. This issue was fixed by disconnecting the AC ground from chassis ground.
Step 12: It Works
The vacuum tube amplifier works! When you plug it in, the tubes take about 45 seconds to warm up. When they do, you can plug in your audio source. There will be a slight 60hz hum until the sound actually starts playing. The tube amp also works well with my DIY electric banjo.
Good Luck Building!
Disclaimer: This project uses very high voltages. Do not build unless you are experienced with building electronics and familiar with electricity. I am not responsible for any harm caused to yourself or others during the building of this project.