UralTone Over Drive Special - Build Guide
UralTone Over Drive Special - Build Guide
The UralTone Overdrive Special is a version of the renowned Dumble ODS guitar amplifier adapted to a Princeton chassis. The circuit has been slightly scaled down from the original, with a 6V6 pair in the power stage, delivering a solid 22W of power.
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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UT Overdrive Special Tube Amp diy kit€695.00 €553.78Build Clone Kit from the Classic of Classics! The world's most expensive and hyped guitar amplifier is now possible to build on a reasonable budget and with a very interesting construction process. Learn MoreSKU: 950-ODS
UralTone ODS - BOM - Bill of Materials
Component identification
Soldering guide
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 - Preparing the Chassis
The UralTone ODS is built into a Princeton chassis. Before starting assembly, some modifications to the chassis are required. Depending on the power transformer supplied with the kit, the transformer cutout corners may need to be trimmed. First, test-fit the power transformer in place. It should pass through the chassis so that its mounting nuts rest against the chassis surface. If necessary, trim the corners of the transformer mounting hole as shown in the image.
2 - 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 10 o’clock as shown in the image.
- For Octal sockets (large tubes), the keying notch in the center post should point to 7:30.
- Install two grommets.
- Install seven 10mm M3 standoffs.
Noval sockets may be supplied with or without a pre-installed retaining clip. If the clip is not already attached, remove it from the shield and install it along with the socket. Sockets and standoffs are secured using M3x6 screws.
Same step viewed from the outside of the chassis.
3 - Rear Panel Components
Attach the components to the rear panel:
- Two toggle switches for footswitch bypass
- Plastic-bodied jack for the footswitch
- Mono jacks for the effects loop, one of which is equipped with a switch
- Speaker output jacks, one of which is equipped with a switch
- Presence control: B2.2kΩ potentiometer
- Power switch. During installation, place a nut on the switch threads inside the chassis and a large star washer against the chassis wall. On the outside, install a flat washer and a nut. The installation depth of the switch can be adjusted by the height of the nut inside the chassis.
- Two fuse holders
Same step viewed from the outside of the chassis.
4 - Front Panel Toggle Switches
Solder two 6cm brown wires and one 15cm white wire to the lugs of the front panel's two-position "Rock / Jazz" DPDT toggle switch. Note the bridged solder lugs for the white wire. You can cut a small short piece of hookup wire from any random component lead. For example, a small piece (less than a centimeter) can be cut from the leads of the 100Ω R32 or R33 resistor.
Solder a 10cm wire to the center solder lug of the front panel's two-position "Deep" SPDT switch. Insert a 390pF capacitor into the outermost solder lugs and solder one end in place. On the other end, insert a 10cm white wire and solder both the capacitor and the wire in place.
Note: Before soldering, ensure that the wire on the center solder lug does not touch the outermost solder lug when soldered.
Finally, solder a 6cm white wire to the center solder lug of the three-position "Bright" SP3T switch.
5 - Front Panel Components
Attach the three prepared switches to the front panel. Remember to install the front plate in place.
Attach the remaining front panel components:
- Indicator light
- Potentiometers
- Input jacks with switches
Same step viewed from the outside of the chassis.
6 - Transformers on the Chassis Top
The output transformer included in the kit has two red wires for the primary connection to the power tubes. To simplify installation, mark a stripe on the right-side wire in the image using a marker or similar. This will help identify the wire later.
Install the marked output transformer, choke, and power transformer in the chassis. The output transformer and choke are mounted using M4 screws and nuts. Pass their wires through the grommets. The power transformer is inserted through the chassis without removing or loosening its screws. Secure the transformer inside the chassis using M4 washers and nuts.
Same step viewed from inside the chassis.
7 - Remaining Chassis Components
Attach the turret board to the chassis using longer M3/8 screws and two nuts. One nut should be placed between the turret board mounting bracket and the chassis to slightly raise the installation height.
8 - Mains Cable
Install the mains cable on the rear panel. Strip about 30cm of the outer sheath from the supplied power cable. Instructions for shortening and installing the power cable into the panel can be found at:
https://en.uraltone.com/blog/strain-relief-installation/
Note: If the insulation of any of the three internal wires is damaged during stripping, exposing copper strands, the stripping has failed. Cut the cable and strip a new 30cm section.
9 - Soldering the Transformer Wires to the Installed Chassis Components
Solder the blue wire from the output (secondary) of the output transformer to the speaker jack. Continue the same wire to the auxiliary speaker jack. Do not solder the auxiliary speaker jack connection yet.
Solder the black wire from the output transformer to the ground of the jack. Strip a longer section of this wire so it can also be soldered to the middle lug of the jack. Continue with the same black wire to the extension speaker jack.
Finally, cut the yellow 4Ω output wire short and insulate the end with heat shrink tubing.
Slide heat shrink tubing over the brown and blue wires that will be soldered to the power switch. Solder the brown wire from the power cable to the fuse holder and continue from there to the lower right solder lug of the power switch. Solder the blue wire from the power cable to the lower left solder lug of the power switch. Heat the shrink tubing to secure it.
Detail of the same step. Note the heat shrink tubing installed over the switch solder joints for protection.
Crimp and solder an M4 ring terminal to the ground wire of the power cable (green-yellow). The length of this wire should be set so that it is the last to detach if the strain relief fails and the cable is pulled out. In the image, the wire has not been shortened yet. Attach the ring terminal with an M4 screw, star washer, and nut, ensuring the star washer is placed between the ring terminal and the chassis.
Solder the 5Vac and 325Vac wires from the power transformer to the rectifier tube socket. The 5Vac wires are soldered to pins 2 and 8, and the 325Vac wires to pins 4 and 6.
NOTE: The wire colors of transformers vary depending on the manufacturer and production batch. Always verify the correct wire colors and voltages using the layout guide provided with your kit or the markings on the transformer. Do not use the image above as a reference without confirming the correct wiring first.
Solder the primary (230Vac) wires of the power transformer to the middle pair of solder lugs on the power switch. Slide heat shrink tubing over the wires before soldering.
Detail of the same step.
Solder jumper wires from the ground of the footswitch jack to the center solder lugs of the rear panel switches. In the image, resistor leads have been used. Try to solder the wires to the jack so that its solder lug remains open. A wire will be soldered here later.
From the lower solder lugs, solder wires to the center contacts of the jack.
The chassis work is now complete, and the first phase of assembly is done. The chassis can now be set aside for loading and soldering the circuit board.
Note: The green wire shown in the image has been repurposed from a transformer wire. In the layout diagram, this wire is yellow. Do not use a single-stranded green wire here. This type of wire is only used for tube heater wiring.
10 - Circuit Board Assembly
Some pads on the circuit board are shared between a component and a wire. When soldering a component, always check if it shares a pad with a wire. If so, do not solder the pad yet. However, cut the wire short to avoid removing the board later during final assembly. Also, keep the clipped leads from components, as they will be useful later.
Solder diodes D1 and D2 (1N4148) and D4 (1N4007) to the board. Diodes have a specific installation direction. Align the printed stripe on the component with the marking on the PCB.
Solder the 0.6W metal film resistors onto the circuit board. Resistors do not have a required installation direction, but they can be aligned for easy reading, so the color bands are consistently oriented.
Solder the 1W carbon film resistors onto the circuit board. These components do not have a specific installation direction.
Solder the 2W metal film resistors onto the circuit board.
Solder the plastic film capacitors (polyester, polypropylene) and silver mica capacitors onto the board. These components do not have a specific installation direction.
Solder the electrolytic capacitors and the tantalum capacitor onto the circuit board. These components have a specified polarity:
- For horizontally mounted electrolytic capacitors, the positive lead is marked with a groove on the casing. Align the grooved end with the PCB markings.
- For vertically mounted electrolytic capacitors, the polarity is indicated by a stripe printed on the side of the casing. This stripe marks the negative lead. The PCB markings include a corresponding white semicircle.
- For the tantalum capacitor, the negative lead is shorter. The PCB markings follow the same pattern as for vertically mounted electrolytic capacitors: align the short lead with the white semicircle.
Solder the trimmers onto the board.
Solder relays K1 and K2 onto the board.
Solder the bridge rectifier and the JFET onto the board. More detailed images are shown below:
The circuit board is designed to accommodate different JFET transistors depending on availability. The kit includes a PN4416 JFET, which should be soldered onto the board as shown in the image above.
The metal-encased four-pin 2N4416A is installed onto the board as shown in the image above.
The bridge rectifier has a specified installation direction. Align the "+" marking on the component with the corresponding marking on the circuit board. It is recommended to mount the bridge rectifier slightly above the board for easier replacement.
The circuit board assembly and soldering are now complete, marking the end of the second phase of the build. Double-check the solder joints on the underside of the board to ensure good connections and trim all component leads short.
11 - Installing the Circuit Board into the Chassis
Secure the circuit board to the chassis using seven M3/6 screws.
12 - Soldering Wires Between the Circuit Board and Transformers
Solder the white center tap wire of the power transformer's high voltage winding (325V) to the circuit board.
Solder the blue wires from the choke and the center tap of the primary winding from the output transformer to the circuit board. Also, solder the red primary wires of the output transformer. The previously marked wire should be soldered to the socket of the leftmost power tube in the image.
13 - Initial Soldering for the Front Panel
Trim and solder the wires from the switches to both the front panel potentiometers and the circuit board. Note: Do not solder the wires for the Bass and Volume potentiometers yet, as additional components or wires will be soldered to them later.
The same step as above, showing the solder joints on the circuit board.
14 - Front Panel Grounding / Bus Bar
To simplify soldering, a dedicated grounding wire runs behind the front panel potentiometers. This wire should be installed approximately one centimeter away from the potentiometer solder lugs. One end of the wire is soldered to the ground lug of the FET input jack.
Bend a thick solid-core wire straight, then make a 90° bend at one end with a smaller additional bend for soldering, as shown in the image above.
Fit the wire into the chassis and measure or estimate the position of the bend at the jack end. Solder the wire at both ends.
Use clipped leads from components to solder the ground connections for the potentiometers. First, solder the wire to the potentiometer (or jack).
Wrap the wire around the ground bus bar and solder it in place. Four ground connections are soldered to the potentiometers and one to the normal jack.
The completed installation looks like this.
15 - Front Panel Wiring and Components - Switches
The ODS circuit uses shielded wires in some areas where it is sensitive to interference. Shielded cables are always grounded at only one end. The grounding is not meant to serve as the signal return, but solely as protection against external interference. Shielded wires can be installed as they are, but using heat shrink tubing results in a neater installation.
In this guide, shielded cable wiring is done by first preparing one end for grounding. Once this end is soldered in place, the wire is routed along the designated path and cut to length. The other end is then stripped and soldered.
Strip the outer shield of the shielded wire for 15mm at one end. Twist the shield together but do not twist it all the way to the wire base. A good technique is to twist the entire wire first and then turn it counterclockwise at the base, so the internal signal wire and its insulation are not under pressure. A tightly twisted wire can easily melt through the insulation of the signal wire.
Slide a piece of thin heat shrink tubing over the shield and solder the wire ends. The heat shrink will contract from the heat of soldering. It also absorbs some of the soldering heat, preventing excessive heating at the shielded connection.
Finally, tidy up the work with a piece of thicker heat shrink tubing.
Solder the prepared end of the shielded cable’s red wire to the middle lug of the volume potentiometer. The ground can be soldered to the bus bar.
Check the layout diagram for the routing of the wire to the tube socket and cut the wire to the appropriate length.
A resistor and capacitor are connected in parallel to the other end of this wire. Start by attaching the capacitor to the resistor as shown in the image.
Slide heat shrink tubing over the wire and solder the signal cable to the resistor lead. Ensure that no stray shield wires cause a short circuit at the solder joint. Heat the shrink tubing into place over the connection. Do not solder the other end to the tube socket yet.
Solder the capacitors of the Bright switch between the potentiometer and the switch. Start with the lower solder lug of the switch. Solder the capacitor at the very end of the wire. If space runs out, loosen the nut on the switch and rotate it slightly to create more space.
Then, solder the upper capacitor to the upper solder lug of the switch.
Depending on the length of the capacitor leads, both or at least the upper lead may reach the potentiometer lug. If both reach, insert them into the lug but do not solder yet. Ensure that the leads do not touch any wires running below them. When soldering later, place a temporary piece of cardboard or similar under them as protection. Although the soldering order may seem tricky, the insulation of the wires will later act as a safeguard against contact with the potentiometer housing.
If the lower capacitor lead does not reach, it can be soldered to the lead of the upper capacitor.
Measure a piece of white wire. Solder the right end to the potentiometer. Leave the left end unsoldered.
Solder a capacitor between the DPDT switch and the bus bar as shown in the image.
Solder a resistor between the DPDT switch and the center lug of the Treble potentiometer.
16 - Front Panel Wiring and Components - Potentiometers
Solder a wire between the center lug of the Bass potentiometer and the circuit board.
Solder a resistor between the bus bar and the left lug of the Bass potentiometer. Leave the soldering at the potentiometer side undone for now. Install a capacitor between the outermost lugs of the potentiometer. Solder the left lug, but leave the right lug unsoldered, as another wire will be soldered there later. (Note: In the image, this is already soldered).
Solder wires from the Middle, Bass, OD Volume, and Ratio potentiometers to the circuit board. The wire on the Middle potentiometer can be bridged across both lugs as shown in the image. The right lug of the Bass potentiometer may become crowded, as two wires and one capacitor lead will be soldered to the same point.
Same step as above - in this image, the soldering points on the circuit board are shown.
17 - Front Panel Wiring and Components - Input Jacks
Solder three wires from the input jacks: two white wires for the FET stage input and output, and one ground wire. The ground wire can be wrapped around the bus bar and soldered. Note the bridged solder lugs on the FET input jack (ground - middle lug).
Same step as above.
Solder the shielded cable to the ground and the outermost signal lug of the normal jack. The shielded signal wire is connected to the lower lug (as shown in the image), and the ground wire to the upper lug. A 1MΩ resistor is also soldered between these lugs. Additionally, solder the jack's ground to the bus bar.
Cut the wire to the appropriate length and solder a 22kΩ resistor to one end. This resistor is then soldered to pin 2 of tube socket V1. The shield of this wire is left unconnected at this end and insulated. Ensure that no strands from the shielded wire short-circuit the signal wire. Use heat shrink tubing to tidy up the connection between the wire and resistor.
18 - Front Panel Wiring and Components - Shielded Cables
Solder a shielded cable between the OD Level potentiometer and the circuit board. The potentiometer end is grounded, while only the signal wire is connected at the circuit board end.
Same step as above. It is easiest to solder the shielded cable by first soldering the signal wire to the potentiometer lug and then connecting the shield ground to the bus bar at the same point where the potentiometer's ground wire is soldered.
Same step as above. The soldering point on the circuit board.
Solder a shielded cable to the center lug of the Ratio potentiometer. Solder the ground to the bus bar.
Solder a 100kΩ resistor to the other end of the wire, and solder it to pin 7 of tube socket V2.
Solder the signal wire of the shielded cable to the rightmost lug of the Master potentiometer. Leave the shield ground unconnected for now.
Solder the other end of this wire to the circuit board.
Solder another shielded cable to the center lug of the Master potentiometer. Now solder both ground wires to the bus bar.
19 - Rear Panel Wiring - Shielded Cables
This wire is routed under the circuit board and soldered to the send and return lugs of the effects loop. Align these two lugs next to each other first. It is convenient to solder a piece of clipped component lead between them before soldering the shielded cable to one of the lugs.
Solder a shielded cable between the Return jack's signal and ground lugs. Strip the shield back at least 2 cm to ensure the signal wire reaches its connection point as shown in the image. Route this wire under the circuit board.
Solder the wire coming from the Return jack to the circuit board.
20 - Wiring to Tube Sockets and Installed Components
Solder the shielded cable to the circuit board. This is the only shielded wire where the shield ground is soldered to the board. Route the wire under the circuit board before soldering.
Solder a 68kΩ resistor to the other end of the wire and connect the resistor's lead to pin 2 of tube socket V2.
Solder the wires between tube socket V1 and the circuit board. The wire from pin 3 runs under the board and is soldered to the front panel side. Leave the wire for pin 6 unsoldered for now.
Same step as above, shown from a different angle.
Solder the components between the socket and the solder lug. Ensure that the lead of the resistor on the left side in the image does not touch the effects loop jack's contact. You can test this by inserting a plug into the jack.
Same step viewed from a different angle.
Solder the wires for tube socket V2. Note the 22pF ceramic capacitors, which should be soldered between pins 1 and 3, as well as between pins 6 and 8.
Solder the wires for tube socket V3. The wire for pins 3 and 8 should be stripped over a longer section so it can be soldered to both pins simultaneously.
Solder the wires for the power tube sockets. These sockets have two overlapping solder lugs. Solder the wires in the image to the lower holes.
Same step as above.
Solder resistors to the sockets. These are soldered into the upper holes of the solder lugs. The 1.5kΩ resistors in the center can be installed by trimming their leads sufficiently, allowing them to fit securely.
21 - Wiring the Rear Panel Components
Solder three wires from the footswitch jack.
Solder the two wires between the Presence potentiometer and the circuit board.
Solder a wire from pin 8 of the rectifier socket to the fuse holder, and from the other fuse holder lug to the circuit board.
Solder the bias power supply wire from rectifier socket pin 6 to the circuit board.
22 - Twisted Heater Wiring
Tightly twisted heater wiring reduces the magnetic fields caused by current flow, helping to achieve a quiet and hum-free amplifier. A loosely twisted wire is about as effective as leaving them untwisted. UralTone amplifier kits use solid-core wire for heater wiring since it holds its shape well.
The work can be done in two different ways. The easiest way to twist the wire is by using a power drill. The other method is to twist the wires by hand. Both methods have their advantages and challenges. A pre-twisted wire will remain curly at the ends when untwisted, while a hand-twisted wire may end up uneven. It is best to choose the method that feels most suitable for your working style.
Using the drill method, attach one end of the wire to the drill chuck. Secure the other end in a vise, or if your hands have enough reach, hold the end firmly with your fingers. Twist the wire until it forms a tight braid. Avoid over-twisting, as this can cause the wire to buckle at different points. Holding it with your fingers acts as a good limit for twisting.
Another method is to pre-measure the heater wires between the tube sockets and solder them in place. If the wire is already twisted, measure a suitable length, untwist the ends slightly, and solder them to the tube socket.
Preamp tube wiring is done by soldering wires to pin 9 and to the bridged pins 4 and 5.
In the layout diagram, the heater wires are shown in two shades of green. The polarity of the wiring does not need to be followed strictly. The most important thing is that the heater wires are soldered in series from one socket to the next.
Twist the wires together until the twisted section extends past the next tube socket.
Trim and strip the wires coming from the previous socket and solder the next pair of wires. The bridge between pins 4 and 5 can be made by bending both the incoming and outgoing wires towards each other and soldering them together.
Proceed with the same technique for all three preamp tube sockets.
For the power tubes, the heater wires from the previous socket are soldered to pins 2 and 7. Insert the wires through the lower holes and solder them in place. Solder a new pair of heater wires into the upper holes of the same pins and continue twisting or using pre-twisted wire to the next power tube.
Proceed to the second power tube socket and continue towards the pilot light.
To make soldering the pilot light easier, strip the wire ends slightly longer and bend the tips.
Bend the pilot light terminals sideways and wrap the wire ends around them.
Estimate the wire length needed from the transformer to the pilot light and cut the wires. Strip the ends of both the transformer wires and the newly cut wires, then twist them together as shown in the image.
Wrap the wires around the pilot light terminals and solder both sides. Finally, solder the remaining two wires to the circuit board as shown.
23 - Constructing the Solid-State Rectifier and Finalizing Assembly
Lightly attach the solid-state rectifier housing to the amplifier chassis.
First, bend the leads of one diode and insert it between pins 6 and 8. Solder the diode in place from the top or bottom. Ensure proper orientation—the striped end should face pin 8.
Bend the lead of the second diode towards pin 4 and trim the excess on pin 8. Solder pin 4 similarly as before. The striped cathode end should connect to the lead of the first diode.
Finally, glue the rectifier cover in place.
Attach the knobs on the front and rear panels. Turn the amplifier with the open side facing down and shake off any loose debris.
The amplifier is now ready for inspection.
24 - Final Checks
Before powering on, go through the entire circuit. Check each installed component, its value, and any required orientation. Mark each verified component on both the bill of materials and the layout diagram.
Also, carefully check all wiring. Compare the amplifier to the layout diagram and mark each verified wire at both ends. Ensure that the transformer wiring and voltages are correct.
Take your time with this inspection. It is far easier to catch and fix an error before powering on than to replace a damaged component that failed due to an incorrect installation. Ordering a replacement part will only delay completion.
Remember that success in building is not about skill or experience; it comes from attention to detail and focus.
25 - Powering On and Measurements - Testing Without Power
A carefully inspected and correctly assembled amplifier is theoretically ready for use. However, it's best to proceed step by step and test the newly built kit incrementally.
The first measurements are done without power. Set your multimeter to the resistance measurement mode, usually marked with the Ω symbol. If the meter has multiple resistance ranges (200Ω, 2kΩ, 20kΩ, etc.), select the lowest, such as 200Ω. Attach the black multimeter probe to the amplifier chassis. For now, leave the tubes uninstalled and remove the solid-state rectifier from its socket if it is already in place. Install fuses in their respective fuse holders.
Measure continuity / near-zero resistance between the following points and the chassis:
- Negative terminals of power supply electrolytic capacitors C29, C30, C31, C32, and C33
- Ground terminals of input and speaker output jacks
- Bus bar and its connected potentiometers
Note: The measured value will always be slightly above zero. If the resistance is tens of ohms or higher, check for possible errors.
Resistance measurement between C32’s negative terminal and chassis. The black multimeter probe is connected to the potentiometer bus bar using an alligator clip.
Next, measure the resistance at the positive terminals of these capacitors. The resistance should increase to tens or hundreds of kilo-ohms, depending on the meter. A low resistance in the hundreds of ohms is not normal.
26 - Powering On and Measurements - Testing Without Tubes
Ensure that the tubes and solid-state rectifier are not installed. Set the multimeter to voltage measurement mode and select AC voltage. Depending on the meter, AC and DC voltage may have separate positions, or there may be a single voltage mode with an additional AC/DC selection. AC voltage is marked as V~ or Vac, while DC voltage is marked as V⎓ or Vdc.
If your meter has multiple voltage ranges, select the highest available. Also, ensure the meter can handle the voltages used in the circuit.
NOTE: Voltage readings will vary based on the local mains voltage, component tolerances, etc. Don't worry if your readings are slightly higher or lower. The key is that the voltages are in the correct range and proportionate to each other. A 20% variation in readings is acceptable at this stage.
Ensure the black multimeter probe is connected to the chassis. Turn on the power and take the following measurements:
- Measure the AC voltage at rectifier tube pins 4 and 6. It should be around 330Vac.
- Disconnect the black probe and measure the heater voltage at the pilot light terminals or any preamp/power tube socket. The reading should be 6.3Vac.
- Measure 5Vac between rectifier tube pins 2 and 8.
- Reconnect the black probe to the chassis and switch the multimeter to DC voltage mode (V⎓). Measure the bias voltage at power tube socket pin 5. Adjust the bias trimmer until the meter reads -44V.
Voltage measurement at rectifier tube pins 4 and 6.
Heater voltage measurement at tube V2 socket.
If the amplifier blows fuses, smokes, or behaves unexpectedly, turn off the power immediately. Ensure the circuit is discharged by measuring the voltage across capacitor C35. The safety threshold is below 10V.
27 - Powering On and Measurements - Testing with Only the Rectifier Installed
Turn off the power and install the solid-state rectifier. Reconnect the black multimeter probe to the chassis. Turn on the power and measure voltages at the following points:
- C33 positive terminal: 470V
- C32 positive terminal: 470V
- C31 positive terminal: 445V
- C30 positive terminal: 415V
- C29 positive terminal: 410V
- C23 positive terminal: 17V
A 20% variation is acceptable.
If any voltage is missing or significantly lower, turn off the amplifier. Before troubleshooting, ensure that the capacitors are discharged. The safety threshold is below 10V.
28 - Powering On and Measurements - Voltage Measurements with Tubes Installed
Turn off the power and install the tubes. Set all potentiometers to zero.
Turn on the power and allow the tubes to warm up and the voltages to stabilize. Measure the voltages at the following points:
- C33 positive terminal 440V
- C32 positive terminal 439V
- C31 positive terminal 372V
- C32 positive terminal 322V
- C31 positive terminal 315V
- C23 positive terminal 12V
Measurement accuracy is within 20%.
If any voltage is missing or significantly lower, turn off the amplifier. Before troubleshooting, ensure that the above-mentioned capacitors are discharged. The safe voltage limit is below 10V.
29 - Bias Adjustment and Voltage Checking at Tube Sockets
Measure the voltage across the 1Ω resistor R49. Adjust the bias trimmer until the meter reads 0.027-0.038V. If the multimeter does not automatically adjust its range, set it to either 200mV or 2V.
The linked measurement guide includes the voltage test points for the ODS circuit. Check the voltage at these points. The accuracy margin remains at 20%. The important thing is that voltages are in the correct range, not necessarily exact. Tube operating points vary, and voltages may fluctuate significantly. Even though the measurement points are marked on the tube sockets, it may be easier to trace a wire from the socket to the circuit board and measure from there.
If voltages deviate significantly from expected values, turn off the amplifier. Before troubleshooting, ensure that the mentioned capacitors are discharged. The safe voltage level is below 10V.
Some troubleshooting tips are provided at the end of this guide.
29 - Initial Sound Test
The tested ODS is now ready for playing. Connect a speaker and guitar, and turn up the volume. Note that the volume control is distributed across multiple points, and the overdrive might be engaged, meaning there are several gain stages affecting the signal. The FET input and overdrive trimmers on the board may be set to zero, muting the amplifier. The easiest starting point is to use the normal input, bypass the overdrive from the rear panel switch, and adjust the trimmers once the amp produces sound.
If the amplifier produces no sound when toggling the footswitch settings (rear panel override switches), turn off the power and inspect the circuit. Again, as a reminder: measure and wait for the voltage to drop below 10V before working on the amp.
The bias adjustment can be fine-tuned during the initial play session. The recommended range in this guide is 50-70% (0.027-0.038V measured across R49) of the maximum 6V6 tube rating. A higher value reduces crossover distortion but increases tube wear. You can adjust the bias current within this range while listening for when the tone starts to degrade. The optimal setting is where the sound remains clean while keeping tube current as low as possible. Recheck the bias after a few hours of use, as tube characteristics may shift slightly over time.
The FET input and overdrive trimmers can be adjusted to your preference. The FET input can boost or attenuate the signal depending on your instrument’s output. The overdrive trimmer controls the gain of the first stage. The best way to adjust these is to take time and find what sounds and feels best to you.
30 - Wrapping Up the Project
The amplifier is now complete according to this guide. However, it still needs a cabinet, which you can build yourself or use a ready-made Princeton-style enclosure.
The ODS can be used with a two-button footswitch equipped with a stereo plug. A suitable footswitch can be built. The footswitch controls the overdrive channel and the Pre-Amp Boost (PAB). When connected, it overrides the rear panel switches.
31 - Usage Notes
The ODS may differ from conventional amplifiers, especially if you are used to traditional guitar amp circuits. Some functions might seem unusual or broken. The Pre-Amp Boost (PAB) bypasses part of the tone circuit, meaning front panel tone controls may have little to no effect. The tone controls also respond differently based on the Rock/Jazz switch position— in Rock mode, the controls have a more pronounced effect.
32 - Troubleshooting Tips
No one wants to end up in this section, but the good news is that every problem has a solution. All kits have eventually been brought to life.
In nearly all cases, non-functioning amplifiers are due to builder mistakes. Faulty components are rare, and assuming this as the cause often leads troubleshooting in the wrong direction. The same applies to circuit design— all kits are thoroughly tested before release. We've seen many troubleshooting cases, and most issues stem from incorrect component placement or poor soldering. Deviating from instructions or attempting modifications can also cause unnecessary challenges.
The fastest and easiest troubleshooting method is using a multimeter, provided the amplifier does not blow fuses, spark, or emit smoke. Comparing your voltage readings to the expected values quickly reveals which tube stage has an issue. If a preamp tube has no cathode voltage (pin 3 or 8) while its anode voltage (pin 1 or 6) is too high, the tube is not conducting. Check the wiring for these pins, as well as the heater wiring. Inspect the circuit board to ensure all components are correct.
You can also swap tubes between sockets to see if the problem moves with the tube. If it does, the tube may be defective.
Another troubleshooting technique is using an insulated tool, such as a wooden chopstick. Connect the amplifier to a speaker and gently tap or move wires and components while listening for any changes. This often reveals poor solder joints.
Besides voltage checks, you can also trace the signal. This requires either an oscilloscope or an audio probe to monitor how the signal passes through the circuit. The layout diagram marks “test points for audio signal” in red. Following these in sequence helps identify where the signal is lost. A common mistake is shielding wire shorts— where the shield has melted through the signal wire insulation during soldering. These should be checked by measuring resistance between the shield and signal conductor.
Once the faulty section is identified, resolving the issue becomes much easier. Inspect components (values, polarity), wiring, and solder joints around the problem area.
If you are not familiar with reading schematics, you can narrow down the problem area to the tube, its socket, wiring, and the components connected at the ends of the wires.
Tube technology is quite durable and can tolerate faults relatively well. Don’t panic over minor issues—take measurements and determine where the problem is. A common concern we hear is, “I was afraid to turn it on.” It’s important to move past this mindset. It is much easier to check a specific part of the circuit rather than repeatedly inspecting the entire circuit.
If the amplifier cannot be powered on, blows fuses immediately, or has burned components, the fault must be diagnosed without power.
Once all components and wiring have been checked, inspect all solder joints. It is surprising how a single unsoldered wire or a cold solder joint can completely interrupt electrical continuity.
Spending a long time on a project, especially combined with extended troubleshooting, can lead to fatigue and make it harder to notice mistakes. If troubleshooting becomes difficult, take a short break and return to the project with fresh eyes.