UralTone Over Drive Special - Build guide

UralTone Over Drive Special - Build guide

The UralTone Overdrive Special is an adapted version of the renowned Dumble ODS guitar amplifier, fitted into a Princeton chassis. The circuit has been scaled down slightly from the original, featuring a pair of 6V6 tubes in the power stage, which deliver a robust 22W output.

This article walks through the building steps in considerable detail. Not all specifics are covered individually, as the article aims primarily to familiarize you with the construction process before starting and to assist you during the work. Carefully review the images and instructions before beginning the build. This will give you a good understanding of the task at hand.

Always follow the layout diagram and parts list provided with the kit. We periodically update the circuit kits, incorporating improvements or changes over time due to parts availability, for instance. Thus, there might be deviations from the image series in this guide. We strive to include notes on any changes within the text of this guide.

The layout diagram and parts list also serve as your construction log. Mark each installed part or connected wire on the parts list or layout diagram. This helps ensure that all parts are installed and all solder joints are completed.

Before starting the build, check the links below for the version numbers of the parts list and layout diagram. If the documentation included in the kit is from an older version, confirm which guide to follow before beginning the construction.

UralTone ODS - Layout

UralTone ODS - BOM - bill of materials

UralTone ODS - Schematics

Testing instructions

Always first check that all parts are included in the shipment. If you find any missing items, contact us via email or through the "Contact Us" form on our website. Note that the components may look slightly different than in the photos. Also, note that the voltage ratings of the parts may be higher than those listed. We strive to keep the models of the components the same, but due to poor availability, we sometimes have to change the manufacturer or model of a part. So if a part is slightly different in color, don't worry, each component has the necessary values marked on its side. Read the notes in the parts list carefully.

Building the amplifier involves three stages: installing the parts to be attached to the chassis and wiring them together, assembling and soldering the circuit board, and installing the circuit board into the chassis and wiring it. The first two stages can be done in any order that suits you. However, we recommend following the assembly sequence shown in the images below. This way, the construction proceeds in a structured manner as outlined in the article.

The UralTone ODS is built into a Princeton chassis. Before starting the assembly, additional holes need to be drilled into the case, particularly for the potentiometers to be installed on the front panel. The front and rear panels of the kit are used as guides for drilling these holes.

Align the front panel in place and, if necessary, secure it with painter's tape or similar. The alignment hole on the panel has a diameter of 3mm. Both the panel and the case are drilled to the correct size at the same time. The hole dimensions are:

• Master volume: 8mm
• Bright and Rock/Jazz switch: 6mm

The completed front panel looks like this:

Two holes are drilled into the rear panel for the footswitch bypass. Both holes have a diameter of 6mm.

A 3mm hole is drilled into the lid for securing the choke. Use the choke as a guide for positioning the hole.

Depending on the mains transformer included in the kit, the corners of the transformer cutout may need to be trimmed. First, fit the mains transformer into place. The nuts of the transformer stack should touch the surface of the case.

Attach the components to be installed on the rear panel.

The same view from the inside of the case

Install the components that go on the lid. Pay attention to the orientation of the tube sockets:

For noval sockets (small), the gap without a pin should point to the 10 o'clock position in the picture.

For octal sockets (large), the slot in the middle pin should point to the 7:30 position.

Noval sockets are provided either with a pre-attached shield base or without it. If the shield base is not pre-attached to the socket, remove it from the shield and install it stacked with the socket.

The same step viewed from inside the case.

Use seven M3-10mm standoffs to install the circuit board. These standoffs can be left slightly loose for now and tightened once the circuit board is secured in place.

Install the components that go on the front panel. The Bright switch is a three-position switch, and the Rock-Jazz switch has two positions.

The same step viewed from inside the case.

The output transformer included in the kit has primary wires for the output tubes that are the same color. To facilitate installation, mark the wire on the right side in the picture with a stripe. This way, the wire can be easily identified later.

The power transformer is installed by pushing it through the case. Do not remove the mounting screws; the attachment is made with the M4 washers and the new M4 nut set provided with the kit. If the transformer does not fit into the holes, you can slightly loosen the screws. Tighten the stack back before securing it.

The choke is installed with two wide-head M3 screws.

The output transformer is installed with two wide-head M4 screws.

Thread the primary wires (output tube side) of the output transformer and the choke wires through the grommet above the choke in the picture. Thread the secondary wires (speaker side) of the output transformer through the grommet above the output transformer.

The same step viewed from inside the case. Note the large washers used for securing the power transformer.

For the components of the preamp tube's feedback circuit, install a small soldering terminal strip. Use two nuts to raise the strip to the height of one nut above the bottom. This solution allows the wires to be routed along the surface of the case.

Strip 30cm of insulation from the power cord included in the kit. Be careful not to damage the insulation of the wires inside. If you expose any copper, cut the wire and strip a new section. When using cutters, a knife, or similar tools, bend the wire into a tight curve at the cutting point and make a small cut on the surface. The bend ensures that the insulation tears open even if the tool doesn't go through the insulation.

The crimped strain relief is somewhat challenging to install. You can find installation instructions here.

Don't skimp on the stripping length—once the strain relief is in place, it is difficult to remove.

Solder a ring terminal to the protective ground wire. Do not shorten this wire; leave it at its full length. The idea is that if the strain relief fails, the ground wire will be the last wire to come loose. Coil the protective ground wire neatly and attach the ring terminal to the chassis of the amplifier. Use a star washer between the ring terminal and the chassis.

Once the components attached to the case are in place, it's time to get the soldering iron out. Solder the following connections:

• Speaker jacks to output transformer wires.
• Power cord to fuse holder to power switch to the input wires of the power transformer.
• Output transformer to output tube socket wires.
• Power transformer to rectifier tube socket wires.
• Power transformer to pilot light wires. Bend the legs of the pilot light 90° to the side (see picture below for reference).
• Resistor R50 between the Presence potentiometer and the speaker jack.

The same step from different view.

Solder the ground bus bar to the back of the potentiometers. Make approximately 20mm bends at the ends.

Solder R7, R10, C3, C6 C7, C8 and C9 and the white wires that go between the potentiometers and switches.

At this stage, it's also a good idea to solder all the wires that will be connected to switches SW1, SW2, and SW3 (even though these wires are not shown in the image). It is easier to solder these wires with the switches removed.

The work on the chassis is now temporarily complete, and it can be set aside for the time being. Next, proceed with the assembly and soldering of the circuit board.

Components and wires share a common pad, thus the implementation is similar to the eyelet-board idea used in old Fender amplifiers.

The assembly order generally follows the height of the components, starting with the lower ones. This way, the components remain neatly on the board's surface when it is flipped for soldering. Components can be soldered from either side: from the top to avoid flipping the board or from the bottom to keep the board cleaner by confining flux splashes to the underside.

Leave the pads unsoldered where wires will be soldered later (refer to the layout diagram). This makes soldering the wires later much easier.

Place the resistors on the board according to the part numbers and the parts list.

Each part has its own part number in the list (resistors are prefixed with R) and value. You can either measure the values with a multimeter or use a color code interpreter (Google "resistor color code calculator") to help identify the values.

Solder the diodes, diode bridge, JFET transistor, and trimmers to the board. The diodes, bridge, and transistor have a specific orientation for installation. Align the diodes with the striped end on the component and the marking on the circuit board. The bridge's positive terminal is indicated on the board by a square pad; on the component, this terminal is marked with a "+" sign and is typically connected to a longer lead. The JFET has a small metal tab on its casing.

Solder the ceramic and film capacitors to the board. These components do not have a specific orientation for installation. Capacitor part numbers are prefixed with C. The labeling of capacitors can vary: some have their value given in microfarads (uF), while others use a numerical code where the third digit indicates the number of zeros to add to the value (e.g., 471 = 470 = 470pF).

Solder the electrolytic capacitors to the board. These components have a specific polarity: For horizontally mounted capacitors, the positive lead is marked with a groove on the casing. Align this groove with the markings on the board. For vertically mounted capacitors, the negative lead is marked with a stripe, which corresponds to the white half-circle marking on the board.

NOTE: The C35 (47uf) is placed reverse on these pictures. Mount the capacitor as it's printed on the pcb.

Attach the circuit board to the case. Solder the following:

• Power transformer wires
• Center tap of the output transformer primary
• Choke wires

Solder the following:

• Screen grid wires (2 brown) for the output tube sockets, and screen grid resistors R51 and R52.
• Cathode wires (2 black) for the output tube sockets.
• Anode voltage wire from the rectifier tube socket to the fuse holder, and from the fuse holder to the circuit board (red).
• Bias supply wire from the rectifier tube socket to the board (red).

Solder the tathode wires (blue) for the preamp tubes (5 pieces). For the third preamp tube, which is the phase inverter, bridge the cathode across the socket with a small wire.

Solder the preamp tube (phase inverter) grid wires 4 pcs. Solder grid wires for the output tubes and the grid-stopper resistors R47 and R48.

Same as above, with a close-up of the installation of resistors R47, R48, R51, and R52.

Solder the anode wires (brown) for the preamp tubes (6 pieces). For the distortion channel tube, solder two small 22pF capacitors, C15 and C18.

Solder the footswitch and presence potentiometer wires. It is recommended to remove the footswitch jack when soldering the switch wires to gain some additional working space. First, solder a couple of wires from the center pins and small wire pieces from the lower pins. Install the jack and thread the wires from the switches through the lugs of the jack. Finally, solder the wires between the board and the jack.

Solder the shielded cables. In all connections, the shield is only connected at one end. We recommend using heat shrink tubing at the ends where the shield is cut off, to prevent the shield strands from touching places they shouldn't. At the grounded ends, twist the shield together and, if desired, tidy it with heat shrink tubing. Do not twist the heat shrink all the way to the base of the wire. When pulled tightly, it can melt through the insulation of the signal wire during soldering. is stage, solder the following connections:

• Input jack to the first preamp tube. Solder resistor R2 to the end of the preamp tube.
• Volume potentiometer to the first preamp tube. Solder resistor R11 and capacitor C10 in parallel at the end of the preamp tube.
• OD Ratio potentiometer to the second preamp tube. Solder resistor R21 to the end of the preamp tube.
• Circuit board to the second preamp tube. Solder resistor R19 to the end of the preamp tube.
• OD Level potentiometer to the circuit board.
• Master volume to the circuit board.
• Master volume to the loop send jack.
• Loop return to the circuit board.

Same as on previous picture, front panel side wiring.

Same as above, solder the connections on the rear panel and preamp tubes. Note: The end of the resistor and the wire are protected with heat shrink tubing.

Solder R14, R15, and C13 to the soldering strip.

Solder the wires on the front panel side, 12 in total, between the potentiometers/switches and the circuit board.

Same step as previously: The input side of the front panel is cramped. The layout diagram provides a better representation of the wiring than photographs can.

Solder the twisted filament wiring. Start from the first preamp tube. Cut two lengths of single-strand wire long enough to reach between the tube sockets. Solder the ends to the first tube socket and start twisting the wires together. Note that pins 4 and 5 are bridged. Proceed to the next tube socket and thread the wire ends through the solder lugs. Cut another two lengths of wire, solder the previous ends and the new ends to the second socket. Continue this way from socket to socket, and finally to the pilot light.

Alternatively, you can twist the entire length of wire with a drill and cut appropriate lengths from it. The most important thing is that the twist is tight, as this helps in preventing interference.

Finally, solder the wire pair from the pilot light to the circuit board.

Same step as previously: detail of pilot light soldering.

The amplifier is now almost complete! Finally, attach the knobs to the potentiometers.

The UralTone ODS is designed for solid-state rectification, and for this, a separate solid-state rectifier bridge needs to be constructed for the amplifier. Solder two diodes as shown in the picture: the striped end of the diode is soldered to pin 8. The anodes of the diodes are soldered to pins 4 and 6.

The amplifier can also use a 5AR4 tube rectifier, which will slightly reduce the maximum power output.

The installation work on the amplifier is complete. Next, follow the layout diagram to check that all wiring is correct and all components are properly installed.

The updated inspection and measurement instructions can be found here:

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

First, go through the front page instructions and find the detailed measurement instructions on the amplifier-specific tab. Turn all the potentiometers and the bias voltage trimmer to zero. (Bias voltage zero is the most negative voltage.)

Go through the circuit wire by wire, part by part, and mark each checked part on the layout diagram. Once everything is confirmed to be correct, use a multimeter to check the integrity of the grounding and rule out power supply shorts:

Measure with the multimeter on the resistance setting:

• Place one probe on any part of the chassis and with the other probe, check for 0 ohm resistance between the chassis and the negative terminals of the electrolytic capacitors C29, C30, C31, C32, C33, and C34.
• Measure with the multimeter on the resistance setting: Measure the resistance between the + and - terminals of the filter cap. Initially, the resistance will be low but should settle at several hundred kilo-ohms. Be patient and allow the multimeter to settle. A small resistance, zero, or just a few ohms might indicate a possible short that should be found before powering on.

Once the measurements and checks have been successfully completed, you can power on the device. Install the fuses and the diode rectifier, leaving the other tubes uninstalled. Measure the voltage from the ground to the capacitors on the circuit board. The unloaded voltage on all positive terminals should be approximately the same and significantly higher than the results indicated at the measurement points on the layout diagram. If the capacitors do not have voltage at this stage, or if the device smokes, sparks, or behaves unexpectedly, turn off the power immediately.

Before troubleshooting, ensure by measuring that the power supply capacitors C29 - C34 are discharged and the device is unplugged. The layout diagram also includes AC voltage measurement points. These can be measured with a multimeter set to 1000Vac if the device has issues. Points marked with a single arrow are measured relative to the ground, while those with double arrows are measured between the arrows.

Install the remaining tubes and connect a speaker or dummy load to the output. Turn on the power. Let the amplifier stabilize for a moment and measure the voltages at the test points. Ensure the amplifier does not behave unexpectedly. Adjust the bias as per the instructions.

The layout diagram and the inspection instructions specify test points 400V, 325V, etc. These test point values are measured relative to the ground, so attach the black probe of the multimeter to the chassis and use the red probe for the measurements. You can sit on your other hand for safety. Use the 1000Vdc setting on the meter and allow for about a 10% variation from the given readings.

The voltages at the test points are always affected by the variance in tube parameters and differences in current consumption. Another factor affecting the readings is the mains voltage, which can vary between 220-240Vac, and this will be reflected in the measurements. It’s advisable to consider the voltages more holistically and allow for some leeway. Significant deviations at a single test point, while others are correct, may indicate an incorrectly placed resistor in that part of the circuit. Also, trust your ears: if the device sounds bad, something is wrong.

Before casing, adjust both the FET preamp and Overdrive channel trimmers to your desired level. There is no correct way to set these; the adjustments can be made according to your personal preferences by listening. You can choose to set the FET channel so that the FET input sensitivity matches that of the Normal input, or alternatively, increase the sensitivity. The Overdrive adjustment affects the distortion of the channel's first tube stage.

Note: If you measure the voltages without the tubes, the voltages from the power transformer will always be higher when unloaded and will not match the voltage chart values. Except for the AC voltage measurements of the power transformer, always measure the voltages with the tubes installed.

Once the measurements have been satisfactorily completed, the amplifier is ready to be enclosed.

Good luck and enjoy playing your new amplifier!