UralTone JR - Build Guide

UralTone Jr DIY kit is the best-selling tube amplifier kit in Finland. And for a good reason — many current professionals have started their journey into the addictive world of tube amp building with this kit. In terms of sound, the Jr represents the American tradition of the 1950s–60s. It is not directly based on any existing amplifier but is entirely UralTone’s own design.

This article goes through the building process in fairly detailed steps. However, not every single detail is covered separately. The main purpose of this article is to familiarize you with the construction process before you start and to provide support during the build. Carefully study the pictures and instructions before starting — this will give you a good idea of the work involved.

UralTone Jr can be built with or without a spring reverb. In this guide, the JR is equipped with a reverb circuit. Building the non-reverb version is done in exactly the same way — you simply leave out the reverb components from the assembly.

Always follow the layout diagram and parts list provided with the kit during the build. We update the circuit designs of our kits from time to time, and over the years these may include improvements or changes, for example due to component availability. Therefore, deviations from the photos in this guide are possible. In case of changes, we aim to add notes about them to the text of this guide.

The layout diagram and parts list also serve as a construction log. Always mark each installed component or connected wire on the parts list or layout diagram. This ensures that all parts and solder joints have been completed.

Before starting the build, it’s a good idea to check the version numbers of the parts list and layout diagram from the links below. If the documentation included in your kit is from an older version, make sure to verify which guide you should follow before starting the build.

  1. UralTone Jr tube amp kit (version 3.0)
    UralTone Jr (3.5-20w) Tube Amplifier Kit (version 3.1)

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    All tube amplifier kit with vintage spirit. Double single ended output section. Learn More
    SKU: 950-UralToneJR3-KIT

UralTone JR - Layout

UralTone JR - Osalista - BOM - bill of materials

Mittausohjeet ja mittapisteet

Always start by checking that all parts are included in the delivery. If you notice any missing parts, please contact us by email or through the Contact us form on our website. Please note that the components may look slightly different from those shown in the photos. Also note that the voltage ratings of the components may be higher than those listed in the parts list. In addition, the component values may have been rounded up or down to the nearest E-series value (e.g. 250 pF = 220 pF). We aim to keep the component models consistent, but due to poor availability, we may sometimes need to change the manufacturer or model of a part. So if a component looks slightly different in colour than in the pictures, don’t worry — each component has the necessary value markings on its body. Read the notes in the parts list carefully.

The colours of the supplied wires may also vary. If a certain colour shown in the pictures is not included in your kit, simply replace it with another colour. The colour of the wire has no electrical significance, but the type of wire does. Use solid core wire only in the twisted heater wiring.

Building the amplifier consists of three stages: mounting the chassis components and wiring them together, assembling and soldering the turret board, and finally installing the turret board into the chassis and wiring it. The first two stages can be done in any order you prefer. However, we recommend following the assembly sequence shown in the pictures below. This way, the build will progress in a structured manner, guided by the flow of this article.

1 - Rear Panel Mounted Components

Cut approximately 1 mm off the guide pin of the potentiometers supplied in the kit. The guide pin is slightly too long, and after installation it will hit the front or rear panel because the enclosure wall is not thick enough.

Attach the components to be mounted on the rear panel: connectors, potentiometers, switch, and fuse holder. The rear panel is secured in place using the mounting nuts for these components. The IEC connector is installed from the front, and M3 countersunk screws are used for securing it.

If the amplifier is not being built with the reverb circuit, the jack and potentiometer shown on the left in the picture are not installed.

Same step seen inside of chassis.

2 - Front Panel Components

Attach the potentiometers, switches, indicator light, and jack to the front panel.

Same step seen inside of chassis.

3 - Small Components to be Attached to the Lid

Attach the components that will be mounted on the enclosure lid. Note the installation directions of the tube sockets:

  • For Noval sockets (small), the pinless gap should point to the 1 o’clock position in the picture.
  • For Octal sockets (large), the groove on the central pin should point to the 12 o’clock position.

In an amplifier being built without reverb, RCA connectors and the tube socket on the left edge of the image above are not used.

Attach the M3 spacers to the turret board. Note that the spacers on the left side in the picture are secured with countersunk screws. The dome-head screws will prevent the installation of the power transformer in this location.

4 - Transformers and Choke

Attach the power transformer, choke, and output transformer to the enclosure. Thread the wires from the transformers and choke through the designated grommets.

The reverb transformer is installed inside the enclosure.

The same step as seen from the inside of the enclosure. Note the installation of the reverb transformer.

5 - Wiring of the Mains Transformer, Power Wwitch, and IEC Connector.

Next, solder all connections that can be made between the components mounted in the enclosure. Some of the soldering involves transformer wires, while others are connections between connectors, sockets, potentiometers, etc. In some solder joints, multiple wires are soldered to a single solder tag. The instructions aim to specify when a wire brought to a particular point should be continued with one or more additional wires. In these cases, it is best not to solder the joint immediately, but to solder all the wires going from that point at once. Shared solder points can also be checked in the layout diagram.

Solder the following from the power transformer:

  • 230Vac wire pair to the power switch.
  • 5Vac wire pair to pins 2 and 8 of the rectifier tube socket.
  • 300Vac wire pair to pins 4 and 6 of the rectifier tube socket.
  • 330Vac wire pair to be shortened and the ends insulated with heat shrink tubing.
  • 6.3Vac wire pair to the indicator light.
    (NOTE: In the picture, the wires are soldered impractically, hindering the later installation of the turret board. Compare the picture below to see how the wire routing and soldering should be done.)
  • 6.3Vac center output to pin 8 of the power tube V2 (on the left).

Route the wires in the same way as the thinner heater wires, over the top of the power switch and around the corner of the enclosure edge.

Solder the mains wiring:

  • Solder the wire pair from the power switch to the IEC connector on the rear panel.
  • Solder the protective earth wire (green-yellow, GN/YL) from the IEC connector to the ring terminal, which is then attached to the amplifier chassis.
  • Solder the wire between pin 8 of the rectifier tube socket and the fuse holder.

NOTE: Leave the ground wire long enough. If the rear panel connector comes loose, the ground wire will be the last to disconnect from the connector. The ground wire is attached to the amplifier chassis in such a way that a star washer is placed between the ring terminal and the enclosure. Tighten the connection carefully.

Same step as previously.

6 - Wiring of the Output Transformer

Output transformer wiring and rear panel connections:

Bundle the wires from the secondary (speaker output) side and route the wire bundle to the impedance selection switch.

  • Solder the yellow and white wires to the outermost pins of the selection switch.
  • Shorten the orange and green wires to the same length as the previous ones and insulate the ends with heat shrink tubing.
  • Solder the wire from the middle pin of the switch to the speaker output jack. Leave the jack-side end unsoldered for now.
  • Insert a 1.5kΩ resistor into the same jack terminal as the speaker output jack and connect the other end to the Line-out potentiometer. Solder the connections.
  • Shorten the black wire from the output transformer to a suitable length and route it to the speaker jack terminal. Leave this end unsoldered for now.
  • Continue with the black wire from the same terminal to the ground of the Line-out jack, from there to the Line-out potentiometer, and finally to the 8-ohm power resistor.
  • Solder the wire between the center terminal of the speaker jack and the 8-ohm power resistor.

Output transformer input wiring:

  • Twist the blue and brown wires from the primary side of the output transformer and bring the twisted pair to the first power tube socket.
  • Shorten the brown wire and route its end to pin 3 of the V1 power tube socket.
  • Use the previously cut brown wire and solder its end to the same point as above.
  • Twist the wires together again and bring the wire pair to the V2 power tube socket. Cut the brown wire to the appropriate length and solder it to pin 3 of the V3 power tube socket.
    Note: In some parts of the image series, this wire is soldered incorrectly to pin 2 of the V2 socket.

7 - Wiring of the Reverb Transformer and Reverb Circuit

Reverb circuit components:

  • Twist the green and black wires from the reverb transformer output and solder the ends of the wires to the RCA connector for the reverb output.
  • Solder the red wire from the reverb transformer’s input to pin 1 of the reverb tube socket.
  • Solder a 220kΩ resistor to the RCA connector for the reverb tank input.
  • Solder the wire from the RCA connector to pin 7 of the reverb tube socket.
  • From there, route the wire to the footswitch jack.
  • Solder a 220kΩ resistor from the reverb potentiometer to pin 7 of the preamp tube socket.
  • Solder the ground wire between the footswitch and the reverb potentiometer.

8 - Wiring of the Front Panel Components

Front panel jack soldering:

  • Insert a 1MΩ resistor into the input jack’s solder terminals. Bend another wire so that it passes through both the middle and ground solder terminals. Solder the resistor only to the middle solder terminal for now.

Front panel potentiometer soldering:

  • Solder the wire and components between the Volume and Tone potentiometers. Leave the solder terminals unsoldered for the connections that will be made later.
  • Solder a shielded cable between the volume potentiometer and pin 7 of the preamp tube socket.
    Note: In the shielded wire, the ground is soldered only at the potentiometer side, and the shielding is cut off at the tube socket side.
  • Solder a 1MΩ resistor and two shielded cables to the master potentiometer. The other ends of the shielded wires will go to pin 6 of the power tube sockets. If there are two master potentiometers, each potentiometer will have its own resistor and a shielded cable.
  • Solder 1.5kΩ resistors between pins 5 and 6 of the power tube sockets.

The chassis soldering is now complete, and the amplifier build continues with the laying of the components on the circuit board.

9 - Assembling the Circuit Board

UralTone JR Circuit Board

The board’s part numbering is divided into three sections: Parts with numbers under 100 are installed in all JR versions. Numbers starting with 100 are for the Headroom adjustment components, and numbers starting with 200 are for the Reverb circuit. Install the necessary parts according to the version being built.
Note: The image series shows only the components for the basic version (not the Headroom or Reverb add-ons).

Each part corresponds to the part value given in the parts list. Different component types are marked with their own prefixes: R for resistors, C for capacitors, and D for diodes.

On the circuit board, some of the pads are shared between the component and the wire that will be soldered to them. Leave these pads unsoldered where wires will be soldered later. However, shorten the component leads for these parts as well.

Note: When assembling the circuit boards, it’s advisable to compare the previous and next image if the components are unfamiliar by name or appearance.

The assembly follows the familiar “low parts first” order from UralTone kits. This way, the parts stay neatly on the surface of the board when it is turned around for soldering. In the image above, the resistors for the amplifier section (R1-R27) have been soldered. Also, solder the resistors for the headroom and reverb circuits with the corresponding external dimensions if necessary.

Solder the smaller 25V axial electrolytic capacitors onto the board. Pay attention to their orientation. One end of the capacitor housing is grooved. This is the positive terminal of the capacitor. Align this with the positive terminal markings on the PCB.

Solder the film capacitors onto the board. These do not have a specified orientation.

Solder the 10W power resistors onto the board. These, like the other resistors, do not have a specified orientation.

Solder the power supply filter capacitors C9-C12 onto the board. These electrolytic capacitors must be installed in the correct orientation. Align the positive terminals according to the markings on the PCB.

The circuit board is now complete and can be installed into the amplifier chassis.

10 - Installation of the Circuit Board into the Chassis

Secure the circuit board to the chassis with four nuts. Solder the following:

  • The blue wire from the output transformer
  • The blue wire from the reverb transformer
  • The violet wire from the anode voltage output of the power transformer.
  • Leave the choke wires unsoldered for now, as shown in the image.

11 - Headroom Circuit

Attach the MOSFET to the chassis. An aluminum oxide insulator should be placed under the transistor.

Solder three short wires from the MOSFET pins to the circuit board. Solder the wire coming from the choke to the middle solder pad on the board, along with the third wire going to the Headroom potentiometer.

Solder three wires to the Headroom potentiometer.

NOTE: Ensure that the solder joints for the Headroom potentiometer are neat and the wires are cut short. Long wire leads may touch the chassis.

12 - Wiring Between the Circuit Board and Tube Sockets

Solder the wires between the circuit board and the tube sockets. Keep the wires apart from each other. If the wires cross, keep the angle between them close to a straight line and arrange them at different heights to avoid possible crosstalk. The wires for the grids (pins 2 and 7 on the preamp tubes) should be routed close to the surface of the chassis. The wiring will naturally stack into appropriate layers if the work starts with the preamp tubes, as shown with the white wires in the image (grids, pins 2 and 7). Next, solder the blue wires for the cathodes (pins 3 and 8 on the preamp tubes, and pins 1 and 8 on the power tubes), and lastly, the red wires for the anodes or screen grids.

Same step as previously

13 - Wiring Between Circuit Board and Front Panel Components

Solder the wires between the front panel components and the circuit board:

  • Input jack
  • Boost rotary switch. Note that some of the switch’s solder pads are bridged.
  • Volume potentiometer
  • Master potentiometer
  • The ground wire for the chassis and circuit. Solder the wire to the negative terminal of C11, and solder a ring terminal to the other end of the wire. This will be attached to the chassis next to the MOSFET (shown on the right in the image).

Rotary switch wiring in detail

14 -Twisted Pair of Feedback wiring etc.

Solder the following wires:

  • Twist the black and white wires together and solder the wire pair between the speaker jack and the circuit board.
  • Solder the (blue) wire between the reverb potentiometer and the circuit board.
  • Solder the (red) wire between the anode voltage fuse holder and the circuit board. At the same time, solder the other wire from the choke to the same point on the circuit board.

Same step - feedback and anode supply wiring in detail.

15 - Filament Wiring

Twist the single-core green wire supplied with the kit tightly together. The easiest way to do this is with a screwdriver, where one end of the wire pair is secured in a drill chuck and the other end is held with fingers. If you don’t have a drill, it’s best to do the wiring in sections, measuring the estimated distance between the tubes and twisting the wire together by hand.

Note: A tight twist is important. In a twisted pair cable, the magnetic field created by the current flowing through the wire is small compared to the opposing currents running in the nearby wires and the constant change in direction of the conductor. For this reason, twisting reduces potential hum. A careless or loose twist is practically as ineffective as leaving the wires untwisted.

Start wiring from the right of the reverb tube socket. Move through each socket one at a time. Route the horizontal wires about 1-2 cm above the socket, keeping them away from the other wires. The final connection from the output tube V2 socket to the indicator light should be routed along the side of the chassis, on the outer edge, leading to the indicator light.

Same step: filament wiring on pilot light end in detail.

Same step: details of the filament wiring on the preamp tubes.

The soldering work on the amplifier is now complete. As the final touch, attach the knobs to the potentiometers and rotary switch.

16 - Final Inspections

Before powering up, do another check of the work done since the previous inspection. Go through the circuit following the layout diagram and parts list as instructed in the power supply check. Ensure that parts which have a specific installation direction, such as electrolytic capacitors, are installed correctly. The inspection and measurement instructions provided for construction kits are also a useful tool for a systematic check:

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

After the visual inspection, measure the unpowered circuit using a multimeter in resistance mode.

  • Continuity / integrity of the ground: Place one probe on any part of the chassis and use the other probe to check the ground connection at the negative terminals of the capacitors C9-C12 on the circuit board, as well as the chassis of the input and output jacks and the footswitch jack.
  • Possible short circuits in the power supply: Place one probe on the chassis and use the other to check the positive terminals of capacitors C9-C12 in the power supply. A small resistance, zero, or just a few ohms might indicate a possible short circuit, which should be located before powering on. (Note: The meter might initially show a small reading that will increase. Allow the measurement to stabilize for 5-10 seconds.)

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

19 - Switching the Power

Turn all knobs to zero. If the amplifier has a Headroom control, turn it up to full.

Install the fuses into the fuse holder. Note that the fuse in the IEC connector’s fuse holder should be placed in the rearmost compartment. The compartment on the front panel side is reserved for the spare fuse.

Measure with a multimeter set to the AC voltage range (marked Vac or V~). If the meter does not select the range automatically, choose the highest range, 600V/1000V. Measure:

  • 5Vac from the rectifier tube socket pins 2 and 8
  • 6.3Vac from the preamp tube pins 4/5 and 9
  • 6.3Vac from the power tube pins 2 and 7
  • 6.3Vac from the indicator light. Ensure that the light turns on.
  • Check the anode voltage. Place one probe on the chassis and measure with the other probe between pins 4 and 6 of the rectifier tube socket for 300Vac.

Note: The voltages of an unloaded transformer are higher than specified. A 10-20% higher result is normal. The most important thing is that the voltages at the specified points are correct relative to each other. The purpose of measuring the transformer voltages is primarily to ensure that the transformer is wired correctly.

Install the rectifier tube in place. Measure the DC voltage with the multimeter set to the DC voltage range (marked Vdc or V⎓). If the meter does not select the range automatically, select the highest voltage range, 600V or 1000V. Measure:

  • 350Vdc from the positive terminals of capacitors C9-C12.

Note: The Headroom control reacts slowly to changes, especially when the tubes are not yet in the circuit and there is no current draw. In this configuration, a slow response from the control is normal.

If the amplifier behaves unexpectedly, turn off the power immediately. Before troubleshooting, make sure the amplifier is de-energized by measuring the voltage across C9-C12. A safe threshold is below ten volts.

Install the remaining tubes in place. Measure the voltages at the measurement points and check the expected voltages against the measurement guide table. The accuracy of the measurements is still within 20%. The most important thing is that the voltages are approximately aligned and follow some example reading pattern. There is variation in the operating points of the tubes, and voltages can fluctuate significantly. For example, if the power tubes draw more current, the entire power supply will be more loaded, causing the voltages across the board to be lower or higher accordingly.

Don’t focus too much on the numbers and measurements. It’s always a good idea to test the amplifier by playing and listening to it. A working amplifier – despite voltage deviations at measurement points – will always sound good.

If the voltages deviate significantly from the expected values, e.g., the voltage at the cathode is zero, turn off the amplifier. Before troubleshooting, disconnect the power cord and make sure by measuring that there are no voltages in the power supply capacitors (C9-C12). A safe limit is under 10V.

At the end of the article, there are troubleshooting tips collected for further reference.

20 - Audio Test

Once the amplifier has been confirmed to be working, connect the amplifier to the speaker and guitar. Open the Volume and/or Master potentiometers and listen. If everything sounds as it should, congratulations to the builder of the new amplifier for a job well done!

If the device is silent, makes unexpected noises, etc., turn off the power and check the wiring. Remember to unplug the power cord and ensure by measuring that there is no voltage in the amplifier, as previously instructed.

Before final casing, adjust the feedback trimmer R6 to the correct position by ear. Turn the Master/Headroom or Masters to full and play at all volume levels, including full! Adjust the trimmer and find the setting that sounds the best. It’s a good idea to start from the middle position.

21 - Wrapping up the Project

The amplifier is now complete according to this guide. However, it still requires a case, which can be made yourself or you can use the ready-made UralTone JR case.

For the JR with reverb, a single-button footswitch without a LED can be used. The reverb tank can be chosen based on the case being used. The 4AB3 series tanks are familiar from Fender amplifiers and are suitable if the amplifier is packed into a combo case. For a head-case amplifier, a shorter 8AB2 or 8AB3 series tank can be used, ensuring that the case doesn’t become wider than the chassis. In the marking system, the second and third digits indicate the tank’s input and output impedances. All tanks marked with AB are compatible with the JR. The fourth digit indicates the reverb length: 2 = medium, 3 = long.

22 - Instructions for Usage

UralTone JR is suitable for use with various types of preamp, power, and rectifier tubes. Different tube types can steer the amplifier’s power and distortion in the desired direction.

For the preamp tubes, suitable options include ECC83/12AX7, ECC82/12AU7, ECC81/12AT7, 5751, and 12AY7 tubes.

For the power tube socket V3, suitable tubes include 6V6, 6L6, 5881, 6P6, 6P3E, KT66, and similar tubes. For socket V4, in addition to the previously mentioned tubes (except 6V6), EL34, 6550, KT88, 6AC7, etc., are compatible. The amplifier can operate with either one or two power tubes, and removing one tube will affect the sound of the remaining tube, so it’s worth experimenting with tubes both individually and in pairs. UralTone Jr is a cathode-biased amplifier, so tube biasing is not required when changing tubes.

Speaker impedances relative to the power tube:

Speaker Connection / Impedance Switch Position1x 6V6 2x 6V6 1x 6L6 etc 2x 6L6 etcEL34, 6550,
KT88 and 6V6/6L6
4 ohmia 8ohm 4ohm 4ohm 2-4ohm 2ohm 
8 ohmia 16ohm 8ohm 8ohm 4-8ohm 4ohm 

Note: “6L6, etc.” refers to 6L6, KT66, 6P3C-E, 5881, and similar power tubes. KT77 is compatible with EL34.

The impedance recommendations in the table are guidelines, and a one-step jump in either direction (from 8 ohms to 4 or 16 ohms) will not damage the amplifier.

For the rectifier tube, options include 5Y3, GZ34, 5AR4, and similar tubes. Different tube types will result in slightly different B+ voltages, which also affects the sound. NOS rectifier tubes are superior to those made today. The socket is also compatible with a semiconductor rectifier socket, which provides the highest possible voltage, and as a result, the amplifier’s power and headroom are maximized.

23 - Troubleshooting Tips

No one really wants to end up at this chapter, but it’s good to know that all problems have been solved in the past, and all the construction kits eventually end up working just fine.

In almost all cases, the issue with a non-working amplifier is caused by a mistake made by the builder. Faulty components are rare, and assuming this as the cause can often steer troubleshooting in the wrong direction. The same applies to the schematic / design of the circuit. The DIY kits are thoroughly tested before being released for sale.

We’ve seen numerous non-working kits and solved their issues. Most of the time, the problems are due to wrongly installed components or poor solder joints. Applying the instructions, mods, etc., have also caused unnecessary challenges.

Use a multimeter for fast and easy diagnostics if the amplifier isn’t blowing fuses, spitting flames, etc. By comparing the results from the voltage chart to your own readings, you can quickly pinpoint which tube might be causing the problem. If there’s no cathode voltage (pin 3 or 8) on the preamp tube, and there’s a very high voltage on the anode (pin 1 or 6), it means no current is flowing through the tube. Check the wiring for the relevant pins and the heater wiring for that tube. Also, check the component values on the circuit board.

Swap similar type of tubes and see if the issue follows the tube. If the problem moves with the tube, it’s likely the tube itself is faulty.

Use an insulated tool, like a wooden chopstick, to gently poke around while the amplifier is connected to the speaker. Apply pressure to the circuit board and listen for any changes in sound. If something reacts to pressure, it’s likely a bad solder joint or loose connection.

Signal tracing is another helpful technique. Use either an oscilloscope or an audio probe to observe how the signal moves through the amplifier. The layout diagram has points marked as “test points for audio signal” (highlighted in red circles with numbers). By following these numbers, you can trace where the signal is getting interrupted. A common mistake is shorted shielded wires where the shield has melted through and shorted the signal wire. Check the resistance between the shield and the signal wire.

Once you’ve narrowed down the issue, solving it is easier. Inspect the components (values, orientation), wiring, and solder joints around the problematic area. You can use the circuit diagram to identify the components associated with a particular tube by looking at the tube’s part number and identifying the components connected to it. Alternatively, start from the tube socket and trace the connections to the components.

Tube technology is quite durable and can withstand a fair amount of faults, so don’t panic over small issues. The best approach is to measure voltages and find the problem area instead of repeatedly testing the entire circuit. “I was too scared to turn it on” is a common phrase we’ve heard from people. It’s important to overcome this fear. By measuring the voltages, you can much more easily isolate and check just part of the circuit instead of rechecking the entire circuit repeatedly.

If the amplifier can't powered, fuses blow immediately, or components are burnt, the fault must be identified with the power off. In these situations, check the solder joints and any possible shorts between adjacent solder points or solder pads (e.g., noval sockets) or wires underneath the circuit board that may have been left too long. A multimeter’s continuity or resistance mode can be very useful in detecting these issues. A visibly damaged part is naturally a good place to start in diagnosing the fault and its potential cause.

Once all components and wiring have been checked, ensure you inspect all solder joints. It’s surprising how a wire that appears soldered but isn’t, or a cold solder joint, can cause a complete lack of current flow, even if the joint looks connected visually.

Take breaks. The most important thing is not to try to fix everything at once. Long hours spent on the project combined with possibly even longer troubleshooting sessions often result in the builder becoming fatigued and unable to spot their own mistakes. If things are getting tough, take a short break and come back to the project with fresh eyes.