Push-pull Portable Tube Amplifier

Inspired by my previous project, I decided to build a portable tube amplifier that uses normal sized tubes, which are easily found in guitar stores (12AX7 and 12AU7). The power stage is based on my bluetooth tube amplifier, which delivers about 1W of output power. For the preamp I choose a classic design, the OR120, famous for its characteristic tones. Coincidentally, this amplifier also uses a cathodyne phase inverter, which frees one extra triode for the preamp. Any other preamplifier with 3 triode gain stages would also work.

This instructables will be structures as:

1. Circuit and schematic
2. Working on the enclosure and hardware assembly
3. Layout and circuit soldering
4. Voltage adjustment
5. Final result and Soundcheck

Disclaimer:

Although this is a small tube amplifier that works with only 10mA, it still contain high voltages, up to 250 volts DC. Be careful! Do not touch the circuit while the amplifier is turned on.

This is not a project for beginners! If you want to build this amplifier, practice first with some lower voltage amplifier using the LM386, for example.

This amplifier, as in previous cases, is powered by a 12V 2A wall wart and uses a switched mode power supply (SMPS) to generate the 240V necessary for the tubes. To keep things simple, I decided to use a ready to use SMPS board available on ebay. Let's check the parts list:

Large parts

• 1x 12V to 250V, 20mA SMPS (15W SMPS)
• 1x 100V, 10W line transformer with 0.625W tap (I used this one)
• 2x 12AX7 (ECC83) tubes
• 1x 12AU7 (ECC82) tube
• 2x lug tag strips (7 or more lugs, use the smallest you can find with spacing matching the distance between tubes sockets: I used this and this)
• 3x 9 pin tube sockets with centre shield (this one or similar)
• 1x Hammond 1590N1 or 125B (roughly 120 mm x 66 mm x 40 mm, 4.78" x 2.61" x 1.4")
• 2x Knobs (Diameter smaller than 20 mm)
• 2x mini Knobs (Diameter smaller than 15 mm)

Hardware

• 1x Mono or Stereo 1/4" jack with metal ferrule for grounding (input)
• 1x Mono or Stereo 1/4" jack with plastic screw, to avoid a ground loop (output)
• 1x 2.1 mm mini DC jack
• 1x 1M Log (audio) (A1M) potentiometer (16 mm, metal shaft) - Gain
• 2x 1M Log (audio) (A1M) potentiometers (9 mm, metal shaft) - Treble, Bass
• 1x 1M Log (audio) (A1M) dual-gang potentiometer (16 mm, metal shaft) - Volume
• 1x 3 mm LED metal socket
• 1x mini SPDT (On On or On Off)
• 1x 5 pin header (like this one or this)

Circuit

• 1x 3 mm LED (any colour)
• 1x 4.7k resistor (LED resistor)
• 1x 1.4k 1/4W resistor (or close to this value) (R23)
• 1x 1.5k 1/4W resistor (R12)
• 1x 1.8k 1/4W resistor (R16)
• 2x 2.2k 1/4W resistors (R3, R8)
• 1x 6.2k 1/4W resistor (R26)
• 1x 15k 1/4W resistor (R24)
• 2x 22k 1/4W resistors (R6, R25)
• 3x 68k 1/4W resistors (R2, R17, R18)
• 8x 100k 1/4W resistors (R5, R7, R10, R13, R19, R20, R21, R22)
• 3x 220k 1/4W resistors (R4, R9, R11)
• 1x 470k 1/4W resistor (R14)
• 2x 1M 1/4W resistors (R1, R15)
• 1x 100 pF 50v capacitor (C7)
• 1x 330 pF 50v capacitor (C5)
• 1x 1 nF 50v capacitor (C6)
• 1x 1 nF 400v capacitor (the smallest you can find, 350v would also work) (C10)
• 1x 2.2 nF 50v capacitor (C3)
• 1x 4.7 nF 400v capacitor (the smallest you can find, 350v would also work) (C9)
• 1x 22 nF 50v capacitor (C4)
• 4x 64 nF 400v capacitors (the smallest you can find, 350v would also work) (C2, C11, C12, C13)
• 1x 100 nF 400v capacitor (C17)
• 3x 10 uF 400v capacitors (the smallest you can find, 350v would also work) (C15, C16, C18)
• 3x 47 uF 25v capacitors (C1, C8, C14)

Additional tools

• Soldering iron and solder
• Drill and drill bits (tube sockets are either 20 or 22 mm, Potentiometers are 6.5 mm, and so on)
• Sanding paper
• Clear coat

Additional parts needed for this amplifier to work:

• 12V DC wall wart
• Speaker cabinet with 8 ohms speaker
• Speaker cable

The most complex part of the circuit and less forgiving is the power amplifier. There you will have the highest voltages and currents, and we do not want to melt your tubes (or valves) after just some hours. Since I used the same power stage as in my previous instructables, we can go straight to the loadline, shown in the first picture. It is much easier to design this stage using an online loadline calculator.

With a 32k:8ohms transformer at 240 V bias is at -11.5 V, with a cathode current of 2x4 mA (2 triodes in push-pull), which gives the 11.5 V/ 8 mA = 1.4 k resistor and an output power of about 0.94 W.

Compared to the original design, which uses fixed bias at -47v (4 times our bias), the lower bias will overdrive much earlier. To counter that, I added the post phase inverter master volume, a grid-leak resistor of only 100k (R19, R20) and grid stoppers (R21, R22). I also reduced the gain of the phase-inverter by reducing the plate and cathode resistors to 68k (R17, R18). With the post-phase-inverter volume control I can achieve both clean and highly distorted levels (higher than in the original design). The 100k leak-resistors are also a safety measure, in case the potentiometer goes bad.

In case someone wants to make it closer to the original amplifier and skip the master volume control, adding a 300k resistor between C12 and R19 and C3 and R20 creates a voltage divider with the 100k grid-leak resistors. This results in the desired signal reduction (1/4 of the original signal).

This amplifier also uses a negative feedback network, as in the original amplifier. To achieve the 6 dB attenuation I measured the voltage amplitude of a 800 Hz sine wave at the output, with and without the feedback resistor. A value of 6.2k (R26) resulted in the desired attenuation. If you want to make things interesting, you can add a potentiometer and vary the resistance between 25k and 5k and hear how it changes the response of the amplifier.

The remaining part of the preamp was kept the same. The resulting schematic is shown in the second picture.

Before we start drilling and sanding, let's do a little design planning...

The pedal enclosure used here has to fit all parts, including tubes and transformer. The width of the front (side) limits the amount of controls one can use here, unless they are in two rows, as in my previous build. Here I wanted to keep some of the look of the original amplifier, and therefore I needed larger knobs. To fit both tone controls I still used 9 mm potentiometers. A complete tone stack (treble, middle, bass) would fit if at least the gain potentiometer and knob were also smaller.

Another aesthetic choice was the positioning of the three tubes in a row. Not only it gives more space in front of the tubes, it looks more what one would see in a large amplifier. To make it even closer to the original I wanted to add a large metal front plate that would also have the logo and hide the tubes. The tubes will still be exposed from the back, though.

A drill template with the position of the components is shown in the first picture of this section. I used real sized vectors of the parts to have an idea of the size and end result. Notice, that the screws that hold the tube sockets are not present. I always add them later, with the socket in position, so if I made any mistake during the drilling of the socket holes the screws will still be at the right position. The second image shows the box already done. For the input jack and the LED socket's nut to fit I had to remove some material from the inside of the enclosure (see red circle). A rotary tool will do the job quite nicely. Do not forget to drill the holes for the output jack and the power jack. They should be close to the side of the box where the transformer is located, while the output jack should be about 30 mm from the outer edge so that trimpot and capacitors of the SMPS board still fit.

Picture 4 shows the tube (valve) sockets in position. I also used the screws that hold these sockets to hold the lug tag strips. See if you can find one that matches the distance between the socket screws. Ideally it should be screwed at 2 positions. I drilled holes in one of them, so they would fit. Last picture shows the transformer in position. Notice that I cut a small part of the transformer, so that it is not longer than the width of the enclosure. The extra supports are for the front plate, but they are not necessary in case you prefer a design where the tubes are visible.

In my build I also added some etched symbols for the controls. If you are interested, check steps 5, 6 and 7 here.

The first picture shows the layout used for this build. It uses a point-to-point approach, without a PCB or turret board. Before installing potentiometers and jacks it is suggested that one installs the main circuitry first. The positioning of the wires (lead dress) and grounding scheme play an important role in keeping noises under control.

The steps are:

1. Solder heater wires (braided green and blue wires to pins 4 and 5 of all tubes).
2. Solder ground wires (black wires) between tube centre lugs and the lug of the terminal board.
3. Solder the resistors to the sockets. For lugs where there is more than one only solder when both are in position.
4. Solder the coupling capacitors (in red).
5. Solder the electrolytic capacitors and bend them, so that they are laying parallel to the back wall
6. Install the input jack. Add the 1M resistor directly to the jack. Add a wire or component leg between the plastic and the enclosure while you screw it in position. This will be the only connection of the ground to the enclosure. It should be right at the input to reduce noise.
7. Solder the components and wires to the gain, treble and bass potentiometer outside the box. To use the correct lengths and spacings insert the potentiometers from the outside of the enclosure and solder in position. In the end there should be 3 unconnected wires: ground (black), the input (100k and 330pF legs) and the output (blue wire to the tube socket). For the output use a shielded cable, and keep the connections to the 100k resistor and 330pF capacitor as short as possible.
8. Mount the three potentiometers and solder the unconnected wires. The ground wire should be connected to the input jack. This way it is closer to the preamp's ground bus.
9. Solder the 5 wires to the volume potentiometer.
10. Mount the volume potentiometer and solder the wires to the correct lugs. The ground wire should go to the central post at the 12AU7 tube socket, and not the input!. This way it is closer to the power amplifier ground star.
11. Add the LED socket.
12. Solder the LED to the resistor. If possible insulate any exposed part with heat-shrinking tubing.
13. Solder the LED and wires to the SPDT switch.
14. Add the SPDT and LED to the enclosure.
15. Solder the LED ground to the power star-ground.
16. Solder the wires to the transformer. 0 and 4 ohms on the secondary and 0W, 2.5W and 0.625W at the primary. If possible braid primary wires. Since this amplifier works in push-pull the 0W wire carries the opposite signal of the 0.625W wire, reducing any sort of noise.
17. Solder the primary wires to the tube socket and to the high voltage node.
18. Solder the secondary wires to the output jack.
19. Ground the sleeve of the output jack (power ground).
20. Add the feedback resistor directly to the output jack and a wire to the correct socket pin (purple wire).
21. Solder wires to the DC jack, according to the wall wart you are using (centre negative or positive). The layout shows the connection when using a 12v pedal power supply. I used a normal 12v power supply (centre positive)
22. Mount the DC jack to the enclosure and solder the wires to the SPDT and to the power ground.
23. Solder the wires to the 5 (or 6) pin connector (header) following the sequence shown on the SMPS board. In my case I left the EN pin unconnected and only used one of the GND connections, since both were already connected.
24. Connect the SMPS board. I soldered my connector, so that it is located close to the lateral of the enclosure, with trimpot and large capacitors above the DC jack.

The second and third images show the assembly with and without the SMPS board.

Before turning the amplifier on it is a good idea to check if parts of the circuit are working correctly.

1. Without the SMPS connect the 12v supply and turn the amplifier, with the tubes (valves) in position. The LED should light up and the filament of the tubes should glow.
2. To verify the SMPS and adjust the output voltage we need to add the 12v and GND connections to a DC jack. To ensure it will work at the desired load 240v/10mA= 24k, add a 24k resistor between HV and GND and connect the SMPS to the 12v wall wart. With a multimeter measure the voltage between GND and HV. Adjust the trimpot until the measured voltage is at 240v. If this adjustment is done without the load, the voltage will drop a little once there is a load. This way we can ensure the correct voltage when under load.
3. Unsolder or disconnect the DC jack and load resistor from the SMPS and connect the SMPS to the connector in the amplifier with the amplifier turned off.
4. Before turning the amplifier on, prepare your multimeter to measure the voltage at the high voltage node in the amplifier (transformer's centre tap or at the 15k resistor).
5. Connect the speaker to the output.
6. Turn the amplifier on and check the voltage. It should be around 240v and can drop a little while the tubes heat up.