UralTone SVT Preamp – illustrated building instructions
UralTone SVT Preamp - Illustrated Building Instructions
SVT Preamp is a renowned bass amplifier kit adaptation in a compact preamp format. The preamp circuit is based on the 1972 version, followed by a tube-implemented transformer-isolated balanced output.
This article goes through the construction phases in detail with both images and text. Carefully review the images and instructions before starting the construction — this will give you a good understanding of the work involved. By familiarizing yourself in advance, you will also gain a better understanding of what will be done later and why certain instructions are given at specific stages.
Follow the parts list made for the series during construction. We periodically update the connections of the series, and over time there will be improvements or changes, for example, according to the availability of parts. Therefore, deviations from the image series in this guide are possible. In the case of changes, we strive to add notes to the text of this guide.
Before starting construction, it is advisable to check the version number of the parts list from the links below. If the provided series contains documentation of an older version, ensure which instructions to follow before beginning the construction.
UralTone SVT preamp - BOM - bill of materials
Always first check that all parts are included in the delivery. If you find any deficiencies, contact us via email or through the "contact us" form on the 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 what is listed, and the values of the parts may have been rounded down/up to the E-series value (e.g., 250pF = 220pF). We strive to keep the models of the components the same, so sometimes due to poor availability, we have to change the manufacturer or model of a part. So if a part is slightly different in color, don't be alarmed, each component has the necessary values marked on its side. Read any notes on the parts list carefully.
In construction, connection plates are temporarily installed in the enclosure so that the parts on the board that come through the panels are initially unsoldered. When the board is installed in the enclosure, the unsoldered parts find their correct position. This method ensures that any misaligned drilled holes or incorrectly positioned parts are not damaged when they are attached to the enclosure. Soldering done at the end ensures an installation that does not pinch or twist the part when installed.
1 Preparing the aluminum enclosure - cover
Attach the front panel of the kit to the cover of the case. Ensure that the panel is centered, i.e., the space left on each side is the same. It is advisable to secure the panel with tape before making markings. The panel can easily slide out of place, and the markings may not stay at the desired distances from each other.
Above is the perforated cover. The dimensions of the holes are:
The size of the holes can be slightly larger, and a misdrilled hole can be filed towards the correct position as the front panel covers the misplaced spots.
NOTE: There is a through-hole drilled at the bottom of the case for the boost trimmer adjustment screw. If you want to reserve the adjustment option through the case, measure the hole in place using the circuit board. A hole must also be drilled in the panel.
2 Preparation of the Value Aluminum Case - Back Panel
Here is an image of the drilling model
The drilling of the back panel is done in the same way as the opening of the cover.
Attach the back panel to the enclosure so that there is a gap of approximately 2-3mm between the top edge of the panel and the open side of the enclosure, as shown in the picture. The panel is installed in the center laterally.
Note: The back panel can be left uninstalled, the cover has markings for connections, etc.
Above is the perforated back panel. The diameters of the holes are
- DC jack 9-10mm (depends on the diameter of the connector body to be installed in the jack)
- Jakit 11mm
- Toggle switch 5mm
- Potentiometer 7mm
NOTE: If the series is constructed without a balanced output (series acquired without a small signal transformer), the third and fourth holes in the picture should not be drilled.
The drilled enclosure looks like this.
Note: An XLR connector can also be installed in the SVT enclosure for output, and the necessary hole should be made now if needed. See the instructions at the end of the guide on how to do this.
3 PCB Assembly
SVT preamplifier circuit board. Each part's part number (e.g., R4) is printed on the board. Each part number corresponds to the component value given in the parts list (BOM). In this series of images, the parts are placed in the order of the lowest first. This technique facilitates work as the components automatically press against the board's surface when the board is turned for soldering.
When assembling circuit boards, it is advisable to always compare the previous and next image if the components are not familiar by name and appearance.
Note: Minor changes have been made to the series disk and the values of some parts have been altered. Follow the part values given in the parts list and stack all the parts listed there.
4 Circuit Board Assembly - Resistors
Solder the resistors onto the board. The SVT uses two different types of resistors: The brown resistors in the image are carbon film resistors and the blue ones are metal film (or metal oxide) resistors. Place these according to the parts list in their correct positions.
The value of resistors is indicated by color bands. You can confirm the correct component list value either by measuring or by reading the resistor color codes. There are also online tools available for reading color bands. Resistors do not have a specified installation direction. However, in the image, the resistors are aligned in the same direction so that the reading direction of the color bands matches the print on the circuit board.
Cut the resistor leads short on the solder side of the board. Save a few leads for later use.
Note. The image is missing resistor R16 and its installation location. Place and solder this part onto the board as well.
5 Circuit Board Layout - Diodes
Solder the diodes onto the board. These have a specific installation direction: the diode casing has a printed ring that should be aligned according to the circuit board's printing. D101 is installed with the flat side facing down.
Note. The image is missing diodes D1 and D2 and their installation locations. Place and solder these onto the board as well.
6 Breadboard Layout - IC Socket and Jumper Wire
The IC socket U101 is oriented so that the semicircular notch at its end is aligned according to the board's printing.
Solder a jumper wire to location R106 using a piece of wire that was previously removed.
7 Circuit Board Layout - Trimmer Potentiometers
Solder the trimmers RV3 and RV101 onto the board. RV3 adjusts the amount of boost controlled by the footswitch. RV101 is the power supply adjustment. Set both to zero at this stage by turning the adjustment screw counterclockwise.
8 Circuit Board Assembly - Transistors
Solder transistors Q101 and Q102 onto the board. These components have a specific installation direction. Q102 is installed so that the shape of the casing matches the print on the board. Q101 is installed on the board with the flat side down.
9 Circuit Board Assembly - Ceramic and Plastic Film Capacitors
Solder the ceramic and plastic film capacitors onto the board. These components do not have a specified installation direction.
Note. C107 is not installed / used in the connection.
10 Circuit Board Assembly - Electrolytic Capacitors
Solder the electrolytic capacitors onto the board. These components have a specific installation direction. Align the lead marked on the capacitor and the shorter lead with the end marked with a white semicircle on the circuit board.
Note: The image is missing capacitor C23 and its installation location in the middle of the board. Place and solder this part as well.
11 Circuit Board Assembly - Coil
Solder coil L101 onto the board (center bottom). This part does not have a specified installation direction.
12 Circuit Board Layout - HI Z Output Mode Jumper
Solder a two-contact length piece of pin header onto the board at location JP1 (bottom right). Installing and holding the part in place while soldering is more convenient by placing the jumper in position before soldering.
This jumper controls the HI Z output. The output footswitch either toggles the output (jumper open) or keeps the HI Z output always on regardless of the footswitch position (jumper closed). This way, the output selection can be used, for example, as a switch between an amplifier (Balanced / LO Z) and a tuner (HI Z), or when closed, as an attenuation for the balanced output only (LO Z for mixer, HI Z for stage amplifier/monitoring).
If the preamplifier is made without a balanced output, a jumper does not need to be installed.
13 Circuit Board Assembly - Footswitches
Solder the LED's front resistor to the footswitch circuit board. Two of these boards are made.
On the V2.3 board, a third pad is reserved for the LED. The output footswitch's bi-color LED is installed on these three pads so that the longer lead of the LED goes to the pad on the right side of the image. A single-color LED is installed according to the board's markings.
Note: There are two versions of 3PDT circuit boards, v2.2 and v2.3. The latter is used with the output footswitch. The boost switch can use either the v2.3 board or older versions.
Solder long-pin headers to the footswitch board pads. The two and three segments on the side are made from individual pins. The following images provide detailed instructions for soldering the pins.
Soldering the pins is significantly easier if the pins are placed on the actual circuit board on which the footswitch board is placed for soldering. This way, the pins are aligned straight, provided there is enough free space reserved under the actual board.
The footswitch or LED is not soldered at this stage.
14 Circuit Board Assembly - Potentiometers
Cut or remove the small guide pin on the body of the potentiometers.
15 Circuit Board Assembly - Components to be Mounted on the Solder Side
In this stage, several things happen simultaneously. We recommend reviewing the entire photo series of stage 15 before starting the work.- Attach two 10mm spacers to the connection board.
- Insert the potentiometers, toggle switches, and rotary switch into the board pads. Do not solder these parts either.
The footswitch plates are installed from below, meaning their pins go through the main board pads to the solder side of the board.
- Unscrew the outer nut from the foot switch and turn the inner one all the way down. Leave the star washer on the switch threads. This will remain between the housing and the nut screwed onto the aforementioned base.
- Insert the footswitches into their circuit boards. Do not solder them in place
- Insert the LED into the footswitch pads. Pay attention to the LED's installation direction. Do not solder the LED yet. The output footswitch has a bi-color LED. The shorter pin on the side of this LED should go into the square-shaped pad on the board.
- Insert the footswitch PCBs into the preamp circuit board from the solder side.
Note: Ensure that the toggle switches are in the correct position, on the left Ultra High is a two-position switch, on the right for Ultra Lo selection there is a three-position toggle switch.
Place the enclosure over the circuit board assembled in the previous step. Do not drop the circuit into the enclosure, but instead place the enclosure from above, guiding the component shafts, threads, etc., through the enclosure cover.
When the board is in place, attach the circuit board to the case using the M3 screws in the center (missing from the picture). This attachment ensures that the board is installed at the correct height.
Attach the potentiometers, toggle and rotary switches, and footswitches with nuts. This way, the parts will rise to the correct installation height when tightened with nuts. The four-position rotary switch will rise slightly from its pads - this is intentional.
Same phase from inside the case. Guide and push the LEDs into the installation holes of the cover.
Once you are completely sure that all parts are correct and the board is in place, solder the potentiometers and switches in place. It is advisable to cut the thick legs of the potentiometers only after the board is removed from the enclosure. In this task, sturdier side cutters are appropriate.
Guide the LEDs through the front panel openings and solder them in place.
Solder the footswitch boards in place at a few points. Not all pins can be soldered yet - the most important thing is that the footswitches stay in place when the board is removed from the enclosure.
When the board is removed from the enclosure, solder the remaining pins of the footswitches. Cut off the excess pins.
16 Circuit Board Assembly - Auxiliary Tube Circuit Board.
To prevent the preamplifier from becoming too tall, the tubes are mounted on the board with two small auxiliary circuit boards at a 90° angle.
Cut off one pin pair and solder the 90° angle pin headers onto the board. Solder one part of the header to just one pad first and ensure that the connector is perpendicular / the pins are aligned with the board. If necessary, adjust the position by heating the joint. Once the position is correct, solder all the pins.
Solder the tube sockets to the reverse side of the board.
17 Circuit Board Layout - Installation of Auxiliary Boards on the Main Circuit Board
Attach an M3/70mm spacer between the boards. The spacer aligns the boards and reinforces the auxiliary circuit boards when the tubes are installed in the sockets. Insert the boards connected with the spacer into the pads of the connection board.
Ensure that the auxiliary circuit boards are attached to the surface of the main circuit board. Solder the pins and cut the leads short.
The actual circuit board for the SVT connection is now ready.
18 Jack Plate Assembly
All parts for the rear panel are installed on the SVT jack board, including the middle control coil used in the connection and the small signal transformer for the balanced output.
Note: The new version of the board includes an added fuse holder and an option to select the input function as Bright / Normal, like the original, or High / Low, where both inputs are of normal type with two different sensitivities. The front panel of the series has markings depending on which way you want to implement the connection.
Lay the components on the board. Ensure that the parts to be installed through the panel are flush with the board's surface so that they pass through the panel.Pay attention to the installation directions of the transformer and coil. One side of the coil's auxiliary circuit board is beveled. Align this side according to the markings on the circuit board. The small signal transformer is installed as shown in the image, with the red dot facing up to the right. The potentiometer and toggle switch are placed in their pads but left unsoldered until the board is mounted in the enclosure. This ensures that any measurement discrepancies are corrected when the board is attached to the panel.
19 Introduction
Wire the boards as shown in the picture. The two wires on the left are thicker, as they carry the device's power. It is advisable to measure the wires so that the connection can be opened on the table if necessary, as shown in the picture above, with the two boards in sequence. When the preamplifier is enclosed, the four wires in the middle are sufficient to loop over the edge of the jack board. (see picture below). However, the wires should not be left too long to avoid picking up interference or touching hot tubes.
At this stage, it is advisable to go through the connections, component placement, soldering, etc. The work is easier when not enclosed than when enclosed.
20 Final Assembly
Attach the preamplifier back into the enclosure. Ensure that the input and output wires running on the right are not tangled with each other, i.e., separate the input wire bundle from the output wire bundle.
Insert the tubes into the sockets. Unlike the others, the ECC82 / 12AU7 type tube goes to the top left.
Attach the knobs to the potentiometers and rotary switches. The preamplifier is now assembled.
21 Settings
Before turning on the power (12V, 1.5A, Center positive), ensure that trimmer U101 is turned fully counterclockwise. Attach the negative probe of the multimeter to the chassis of the preamplifier and measure the left leg of resistor R104 with the other probe. Turn on the power. Adjust the voltage to 310V.
The amount of boost is adjusted according to personal preference, and this adjustment is made by calling.
The preamplifier is now ready for use!
XLR connector for output
The SVT Preamp LO Z output is implemented with a TRS jack. The output can be converted to an XLR connection with a suitable cable where the tip of the stereo plug T is connected to pin 2 of the XLR connector, R to pin 3, and the sleeve S to pin 1.
Alternatively, the XLR connector can be built into the preamplifier's enclosure. The jack board has ready-made soldering points for the XLR connector. It is advisable to drill the hole for the XLR connector at the same time as creating the necessary openings in the enclosure. The connector is installed on the right side of the enclosure. The center point of the connector from the right side is 51mm and from the bottom of the enclosure is 18mm. (Measurements according to the layout in the image above) During the final assembly, when the rest of the wiring is installed in the enclosure, attach the XLR connector to the enclosure and solder the wires from it to the jack board. The XLR is normalized in relation to the LO Z jack. If a cable is connected to the jack, the XLR connector disconnects from the circuit.