UralTone DR103 Guitar Amplifier Kit - Build Guide

The Hiwatt DR103 belongs to the same group of classic British amplifiers as Marshall and VOX, which have defined the term "British sound." UralTone's kit version is based on the DR103, originally developed from the Sound City amplifier, and has influenced the sound of many bands and albums.

The UralTone DR103 stays true to its inspiration – it offers plenty of power and dynamics. The circuitry remains faithful to the original, with the addition of toggle switches for fine-tuning the tone. These allow for subtle adjustments and experimentation with different DR103 circuit variations.

This article covers the building steps with a comprehensive series of images. While not every single detail is shown separately, the process is walked through almost step by step. If you have not yet purchased the kit, carefully review these images and instructions before starting the build. This will give you a good understanding of the work involved. When assembling, always read each section of the guide in its entirety before proceeding. Some steps include multiple images.

Always follow the layout diagram and the bill of materials (BOM) provided with the kit. Do not modify the circuit unless you know what you are doing. We cannot provide support if the kit does not match the instructions.

We occasionally update the circuit of our kits, and over time, improvements or changes may be made, for example, due to component availability. Therefore, some deviations from the images in this guide are possible. When changes occur, we aim to add relevant notes in the guide's text.

The layout diagram and BOM also serve as a build log. Always mark each installed component or connected wire on the BOM and/or layout diagram. This helps ensure that all parts and solder joints are completed.

Before starting the build, check the version numbers of the BOM and layout diagram via the links below. If the documentation in your kit is from an older version, verify which guide should be used before beginning the build.

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UralTone DR103 - Layout

UralTone DR103 - BOM - bill of materials

UralTone DR103 - Schematics

Instructions for measuring and startup

Always check first that all parts are included in the package. If you find any missing components, contact us via email or through the "Contact Us" form on our website. Note that some components may look slightly different from the ones in the photos. Also, voltage ratings of components may be higher than listed in the BOM. We aim to keep component models consistent, but due to availability issues, we may need to switch manufacturers or models. So if a part looks slightly different in color, don't worry—each component has its required values marked on it. Read the BOM notes carefully.

The amplifier assembly consists of three main phases: installing and wiring the chassis-mounted components, assembling and soldering the circuit board, and installing and wiring the circuit board to the chassis. We recommend following the assembly order shown in the images below. This makes the build process easier and avoids soldering in difficult or tight spaces.

1 - Small Components on the Chassis Top

Install the components that mount to the chassis top. Pay attention to the orientation of the tube sockets:

  • For Noval sockets (small tubes), the pin gap without a contact should point to 2 o’clock as shown in the image.
  • For Octal sockets (large tubes), the keying notch in the center post should point to 10:30.
  • Install four grommets.
  • Install eleven 10mm M3 standoffs.

Sockets and standoffs are secured using M3x6 screws.

Same step viewed from the outside of the chassis.

2 - Front Panel Components

Install the front panel components, excluding the potentiometers. The front plate is secured using the components mounted on it. Not all switches need to be fully tightened—installing them ensures that the front plate does not remain raised in the middle when the side components are tightened in place. The switches will be removed later for soldering.

Same step viewed from the outside of the chassis. Note the orientation of the input jacks.

3 - Rear Panel Components

Attach the components mounted on the rear panel: connectors and the fuse holder. The Speakon and IEC connectors are installed from the front and secured using M3 countersunk screws.

Same step viewed from the outside of the chassis. Note the orientation of the speaker jacks, ground soldering tab points upwards.

4 - Transformers on the Chassis Top

Attach the power transformer (on the right) and the output transformer to the chassis. Thread the transformer wire bundles through the cover’s cable pass-throughs.

Use M6 screws for installation. The output transformer is mounted with shorter screws. It is easier to leave the screw head inside the chassis and install the nut on the outside, on the transformer side. This makes tightening the screw easier.

The power transformer is secured with longer M6 screws, but in this case, the installation direction is reversed. The screw head remains on the outside, while an M6 washer and nut are used on the inside. The longer screw threads will later be used to mount the power supply circuit board.

Same step viewed from the inside of the chassis. Note the mounting of the M6 screws described previously.

Same step: detail of the mains transformer wires.

Same step: detail of the mains transformer wires.

5 - Transformer wires

Solder the wires from the output transformer to the chassis-mounted components.

Note: Check the wire color coding provided with the transformer in the kit, as it may vary.

  • Solder a silver-plated copper wire between the ground lugs of the speaker jacks and the negative terminal of the Speakon jack. Leave the 4-ohm output jack unsoldered for now, as the black grounding wire from the transformer will be soldered there.
  • Solder the 4-ohm, 8-ohm, and 16-ohm outputs to the jacks. In the picture, the 16-ohm output is on the left.
  • Solder the black grounding wire from the output to the common ground of the jacks.
  • Twist the primary wires of the output transformer together as shown in the picture. Solder the wires to the tube sockets.
  • Extend the blue and brown wires to the other socket. You can use an extra piece of wire left over from the previous soldering step.

Tip for assembly: Always twist and tin the ends of the wires before soldering them. This may seem like extra work, but it often makes soldering easier. A pre-tinned wire end also prevents the strands from spreading when inserted into a solder lug or pad.

Same step detailed

Solder short wires to the solder lugs of the indicator light. Wiring the indicator light is easier if the switch terminals are bridged. This way, the indicator light and power transformer wires each get their own solder lugs.

Use clipped component leads as jumper wires. At this stage of assembly, components have not yet been trimmed, so these pieces must be cut from an uninstalled part. For example, the leads of the power stage coupling capacitors (C101 and C102) are long and can be trimmed into the necessary ~2 cm pieces.

Solder the power transformer input wires to the center solder lugs of the power switch. Twist the transformer wire bundle as shown in the picture, so that the wires pass underneath the power supply circuit board from the perspective of the image.

Note: Check the wire color coding provided with the transformer in the kit, as it may vary. The power transformer has multiple input voltage options. Amplifier built in this guide is wired for mains voltage of 230V. Do  the wiring accordingly local voltage provided. Unused wires should be insulated with heat shrink tubing.

Same step: detail of mains switch wiring.

6 - Heater wiring

Tightly twist the single-strand green wire included in the kit. The easiest way to do this is with a power drill: secure one end of the wire pair in the drill chuck and hold the other end with your fingers while twisting. If a drill is not available, it's best to work in sections—measure the estimated length needed between the tubes and twist the wires together manually.

Note: A tight twist is important. The magnetic field of twisted pair cables is small, which helps eliminate potential hum. A careless or loose twist is ineffective.

Solder twisted pair wires between output tube sockets. Start from right hand side socket and proceed one socket gap at the time towards left.

Same step on closeup: Be careful when soldering the socket solder lug shown higher in the picture (pin 7) and ensure that the wires going to pin 2 do not come into contact with pin 7. When soldering, hot metal can melt through the wire insulation and create a short circuit.

Continue the wiring for the preamp tubes and solder the interconnecting wiring between the preamp tubes. For the power tube socket, it is recommended to solder the wire going to the preamp tube into the same lower hole as the wire coming from the other power tube. The hole in the lug is narrow. Cut the wire at an angle so that the sharp ends can be pushed through the solder lug with some force. Leave the upper lug free, as this will be used later for soldering the wire from the power transformer.

The same step: a detail on the preamp tube soldering. The right-side solder lugs can be bridged by stripping the wire a bit longer and bending the wire ends to the other lug.

7 - Chassis mounted parts wiring and soldering

Twist the green 6.3V wire pair coming from the power transformer together and solder these wires to the power tube socket.

The center tap of the heater is routed to the fuse holder. Leave this wire un-soldered for now. If desired, the wires from the power transformer can be bundled at the location shown in the picture.

Solder 1mm² thick PVC wires to the solder lugs of the IEC connector as shown in the picture. A ring terminal is crimped onto the end of the wire for protection. The connection is then secured by soldering. Attach the ring terminal to the amplifier chassis using an M3 screw. An M3 star washer should be placed between the ring terminal and the chassis. Leave the wire long. The protective wire should be the last wire connected to the terminal in case it gets torn off from the chassis.

In the picture, the solder joints of the IEC connector are protected with heat shrink tubing.

Solder the brown and blue wires coming from the IEC connector to the top solder lug pair of the power switch.

The work on the chassis is now complete for the time being, and it can be set aside for a moment while you build the power supply circuit board.

8 - Power Supply Board

Power supply circuit board. This circuit board is designed to be versatile for different amplifier circuits. For this reason, some components are left uninstalled. The part number (e.g., R101) is printed on the board for each component. The value of each component corresponds to the part number listed in the Bill of Materials (BOM).

Note: When assembling the circuit boards, it's helpful to always compare the previous and next images, especially if the components are not familiar by name or appearance.

We recommend building the circuit boards of the kits using the "low parts first" method. This way, the components stay on the surface of the board when it's flipped over for soldering. Start the assembly by soldering the diodes (identified by the letter D in the part number) onto the board. Diodes have a specific installation orientation. The stripe printed on the casing should be aligned with the square-shaped solder pad on the board.

Solder the resistors (identified by the letter R in the part number) onto the board. These do not have a specified installation orientation. In this image series and layout, the resistors are installed in the same direction as the part numbers printed on the board.

Solder the capacitors onto the board. Only electrolytic capacitors are used on the power supply circuit board. These have a specified installation orientation. A stripe is printed on the casing of the capacitor, indicating the negative terminal. This terminal should be aligned with the round pad on the board. The positive terminal pad is square-shaped, and there is a "+" symbol next to it.

It is recommended to place the capacitors on the board in order of height, with the shortest ones first.

Same step from another angle.

The largest capacitors on the board are snap-in type. Although their leads are short, trim any excess off of them as well. (The part on the right shows the area left untrimmed.)

Finally, solder the 1k / 5W resistor to the board in a vertical position by bending one of its leads underneath its casing.

9 - Power Supply Board wiring and mounting

It's easier to install the wires to the power supply board when the board is not yet mounted in the chassis. Solder:

  • Black and gray wire from the left side of the board to the fuse holder on the rear panel.
  • Wires for the power transformer's anode voltage (350Vac) to the left side of the board. Note: Bridge the switch solder lugs as shown in the layout diagram.
  • Stand-by switch wires from the underside of the board.
  • Power transformer wire to the underside of the board.

Neat the solder joints by cutting any excess wires short on the solder side of the board.

Same step - wires soldered to fuse holder

Same step: wiring of the stand-by switch.

Attach the PCB into threads of the mains transformer screws by using another set of foot M6 nuts (see layout picture guide). Tidy up the wiring with a few tie wraps if wanted.

10 - Power Supply Board Testing

The power supply is now ready for testing. The power transformer's bias voltage wires are temporarily loose. Secure them in a way that prevents the ends from touching the chassis, other components, or yourself.

Install both fuses.

Note: The fuse in the IEC connector’s fuse holder is placed in the rear compartment. The front space is reserved for a spare fuse.

BEFORE TURNING ON THE POWER, CAREFULLY CHECK THE WIRING.

Verify all solder joints, component placement, installation directions, and the power transformer's wiring. Use the layout diagram and parts list as a reference. Mark the checked components or wiring on these lists. In the parts list, the power supply components are numbered starting from two hundred.

Check that the wire colors and voltages match the transformer’s accompanying datasheet and are connected correctly as defined in the layout. Do not compare this with the picture.

Once everything has been double-checked and confirmed to be correct, the device is ready to be connected to power. Remember to also turn on the stand-by switch.

Wait a moment before taking measurements and observe to ensure the device is not doing anything unexpected.

Note: The amplifier remains electrically live for quite a while after power is turned off. The voltages are potentially lethal. Always measure the voltages before working with the wiring (see pictures below). A few volts should be considered a safety margin.

Set the multimeter to DC voltage measurement. If the meter does not automatically select the measurement range, choose the 1000V DC range. An auto-range meter will select the range automatically.

Measure the B/2 voltage from the points indicated in the picture. The expected voltage should be around 500V when unloaded.

Measure the B/3 voltage from the points indicated in the picture. The expected voltage should be around 500V when this unfinished circuit is unloaded.

Measure the B/4 voltage from the points indicated in the picture. The expected voltage should be around 450V

Measure the B/5 voltage from the points indicated in the picture. The expected voltage should be around 430V

Set the multimeter to AC voltage measurement. If the meter does not automatically select the measurement range, choose the 200V AC range. An auto-range meter will select the range automatically.

Measure the heater voltage from the points indicated in the picture, as well as from the other tube sockets. The expected voltage is 6.3V.

Move the amplifier aside during the assembly of the preamp circuit board.

11 - Preamp Circuit Board

Preamp circuit board. The markings and practices are the same as those presented earlier.

Solder the resistors to the board.

Solder the ceramic and plastic film capacitors to the board. These components do not have a specified installation direction, but you can use the same direction of markings on the package and circuitboard if desired.

Solder the electrolytic capacitors to the board. The square pad on the board is the positive terminal, and the round pad is the negative terminal. Both horizontal and vertical mounted capacitors can be installed on the board. Horizontal-mounted capacitors are rarer nowadays, so an additional negative pad has been added to the board. This allows a capacitor intended for vertical mounting to be installed horizontally.

Same step, detail of electrolytic capacitor mounting.

12 - Preamp Circuit Board - Potentiometers and Switches

Cut approximately half of the control shaft of the potentiometer. This shaft is slightly too long, and if left at its original length, it will push the front panel outward.

Cut 7 brown and white, as well as 5 yellow, pieces of hookup wire, approximately 4cm in length. Twist the ends of the wires together and tin them.

Solder the wires prepared previously into solder lugs of the potentiometers.

Note: On VR1 and VR2 (A500k) solder only the outermost white and brown wires.

Solder the potentiometers prepared to circuit board.

Same step as previously.

Cut four approx 4cm pieces of blue wire. Strip the other end on 1cm length.

Remove the slide switches from the front panel if they have been installed. Solder the slide switches to the ends of the blue wires. Bridge the solder pads as shown in the picture.

Note: The switch is for on-off use, so two wires are sufficient, even though there is a third pad on the switch on the board.

Same step as previously

12 - Preamp Circuit Board - Wiring and Mounting

Mount the slide switches to chassis.

Mount the potentiometers to chassis.

Solder wires between the preamp board and power supply board on preamp board side. Check and cut the excess wire ends on solder side of the board.

Route the three wires mentioned above to pass between the raised pads on the right side of the preamp circuit board as shown in the picture, and secure the board to the chassis.

Shorten the wires to the appropriate length and solder them to the power supply board.

Solder the ground wire between the preamp and power supply. (In the picture, the wire comes from the top right corner of the preamp board.)

Same step in detail

Solder resistors R12 and R11 between the preamp and the potentiometers VR1 and VR2. Do not solder the pad on the circuit board side yet.

13 - Input jacks

The soldering of the input jacks is done as follows: (the other side can be done simultaneously in parallel).

  • Insert a small jumper wire between the ground of the upper jack and the center solder lug. Do not solder yet.
  • Cut a short wire and solder it to the solder lug of the upper jack.
  • Insert a 1MΩ resistor through the opposing solder lugs of the upper and lower jacks. Solder the upper solder lug.
  • The other end of the resistor is inserted into the ground of the lower jack. Solder this connection.

Twist the two 68kΩ resistors together and insert the other ends into the solder lugs of the lower jack. Solder both connections.

  • Cut a piece of black wire and solder it to the input stage ground pad. The other end is inserted into the middle solder lug of the upper jack. Do not solder this yet.
  • Cut a piece of shielded cable and solder its shield to the ground point mentioned above. Solder the signal wire to the 68kΩ resistors' lead.
  • Solder the other end of the shielded cable to the V1 tube socket.

The shield in the shielded wire is soldered only at one end. This prevents ground loops in the circuit. In the example image, the outer braid of the shielded wire is inserted into heat shrink tubing. A second piece of shrink tubing is placed at the end of the wire insulation to clean up the final result. A piece of heat shrink tubing is also used at the tube-side end.

Solder the second input following the same instructions, or if you did it in parallel, complete the wiring.

Cut a suitable length of shielded cable to be soldered from the front-stage board to the V2 tube. This cable should only be grounded on the circuit board side.

Solder the wires between the front-stage tubes and the circuit board. Keep the wires relatively short and spaced apart. Leave the red wires from the V4 tube unsoldered on the socket side.

Same step, detail of wiring.

Move the amplifier aside again for the assembly of the power stage circuit board.

14 - Power Amp Circuit Board

Power amp circuit board

Solder the lower parts first. In the picture, the smaller resistors, the bias adjustment trimmer, and the bias power supply diode and capacitors are soldered. Pay attention to the diode's and capacitors orientation.

Pay attention to the orientation of the C103 and C104. The disadvantage of vertical mounting is the difficulty in replacing them. When mounted horizontally, the part can be replaced without removing the board.

Solder the PIO (paper in oil) coupling capacitors to the board. Leave C103 and C104 unmounted for now.

15 - Power Amp Circuit Board - Turret and Jumper Wire

Solder the jumper. It is located above resistor R101 / next to the left turret pin.

Press the turret pins of resistor R119 onto the board.

Solder the turret but avoid of filling the center of it.

Install resistor R119 onto the pins. Solder it in place from the top. Shorten the resistor leads so that they only go a few millimeters deep into the pins.

16 - Power Amp Circuit Board - Mounting and Wiring

Mount the board to chassis.

Solder the resistors and cathode wire between the power tube sockets and the circuit board. Repeat this step for all four tubes.

Solder the bias voltage wires coming from the transformer, as well as the two wires between the power supply and the power stage.

Twist the wire between the front stage and the power stage into a tight twist and solder it to the anode solder pads of the V4 tube. Pay attention to the wire colors and polarity.

Same step seen on V4 socket.

Measure an appropriate length of shielded cable between the 16-ohm output and the front stage board. Solder the shield only at the jack side.

Same step from wide angle.

The soldering work for the amplifier is complete. Finish the work by attaching the knobs to the front panel.

17 - Final Checks

Before connecting the power, perform another check on the work done since the previous check. Review the connections according to the layout diagram and the parts list as instructed in the power supply check. The inspection and measurement instructions for the kits are also a helpful tool for a systematic inspection:

https://docs.google.com/spreadsheets/d/1CRP6kfBlP8VWHthzhgFmR605qaoRH0X6vjR967P3Yy4/edit?usp=sharing

After visually inspecting the connections and components, measure the unpowered circuit using a multimeter's resistance measurement function.

  • Continuity/integrity of the ground: Place one probe on any chassis ground and the other probe to check the ground connections from the power supply, power stage, and preamp circuit boards at the GND points. In the layout diagram, a black wire is soldered to these points. Also, check the grounding of the jacks' housings.
  • Possible short circuits in the power supply: Place one lead on the chassis and use the other to check the B+/1, B+/2 ... B+/5 points on the power supply. A small resistance, zero, or just a few ohms may indicate a possible short circuit that should be located before powering up. (Note: The meter may initially show a small reading that increases. Allow the measurement to stabilize for 5-10 seconds.)

Once these measurements and checks have been successfully completed, you can power on the device.

18 - Switching on Power

Measure the bias voltage from any of the power tube pins 5. From a checking perspective, it's recommended to perform the measurement from all four sockets. Adjust the power stage's trimmer on the circuit board so that you achieve the highest negative voltage, which should be around -50V. If at this stage the device smokes, sparks, or does anything unexpected, turn off the power.

Turn the front panel controls to zero, install the tubes, and connect a speaker or dummy load to the output. Power on the device. Let the amplifier settle for a moment and adjust the bias to the correct value following the layout diagram.

Measure the voltages at the measurement points. At the same time, make sure the amplifier does nothing unexpected. The layout diagram and the checking instructions indicate the measurement points (400V, 325V, etc.). These voltage values are measured relative to ground, meaning the black probe of the multimeter should be attached to the chassis, and the red probe is used for the measurements. You can even sit with the other hand on the device. Use a 1000Vdc setting on the meter and allow around a 10% deviation from the given readings.

Note: The voltages in a tube amplifier can vary quite a bit from the provided values:

The voltages at the measurement points are always affected by the variance in tube parameters and power consumption differences. Another factor that alters the results is the AC line voltage. This can vary between 220-240Vac, which can easily be seen in the measurements. It's better to assess the voltages as a whole and allow some margin for fluctuation in either direction. A 20% deviation is not an unexpectedly large discrepancy.

A significant deviation at a single measurement point while the others are correct could reveal, for example, an incorrectly sized resistor in that part of the circuit. It’s also a good idea to trust your hearing: if the device sounds bad, something is wrong.

Note: If you're measuring voltages without the tubes, the voltages on the transformer without a load will always be higher. With the exception of measuring the AC voltage of the transformer, always measure the voltages with the tubes in place.

Once the measurements have been successfully completed, the amplifier is ready to be tested with guitar and then housed. This guide does not go further into the enclosure assembly.

Good luck and enjoyable playing moments with your new amplifier!

19 - Usage

The DR103 is straightforward to use. The amplifier has two parallel inputs, with the Brilliant input equipped with a high-pass filter that cuts out lower frequencies.

The circuit also includes Deep, Low Cut, and Boost switches. These three functions allow different versions of Hiwatt preamps to be used within the same circuit. The Boost increases preamp gain, the Low Cut reduces low frequencies, and the Deep enhances bass. The latter is particularly useful when the amplifier is used as a bass amp.

20 - Troubleshooting Tips

Generic trouble shooting guide can be found at end of ODS build guide