UralTone REDD.47 - Build Guide

UralTone REDD.47 - Build Guide

The UralTone REDD.47 RACK is a 1- or 2-channel microphone preamplifier based on the famous REDD.47 preamplifier of the REDD.51 mixing desks used in Abbey Road studios. The preamp sound can be heard in all recordings made at the studio between January 1964 and the summer of 1968. The most famous, of course, are The Beatles recordings.

The idea of this article is to make the builder familiar before starting work and to help during work. Read the pictures and instructions carefully before starting assembly of the kit. This way you get a good idea of what kind of task the assembling the kit is. The build process is not written on level of fitting individual parts, so do notify all part numbers etc. found on area being introduced in the pictures.

Tässä artikkelissa käydään rakentamisen vaiheet läpi melko tarkasti. Kaikkia yksityiskohtia ei kuitenkaan käsitellä erikseen. Varsinainen rakentaminen tehdään layout-kuvan ja osalistan kanssa. Kuvasarja toimiikin apuna lopulliseen toteutukseen ja opastaa oikean rakennusjärjestyksen. Tutustu huolella kuviin ja ohjeisiin ennen rakentamisen aloittamista — näin saat hyvän käsityksen siitä, millaisesta työstä on kyse.

In this article, the stages of construction are outlined. Links for these are found below. Note that not all details are reviewed, and the architects' idea is mainly to familiarise the builder with its main features. The assembly is done by using the layout picture and BOM. The pictorial guide works as additional help to understand the assembly process and as well the correct order of it.

REDD.47 Rack - BOM - Bill of Materials

REDD.47 Rack - Layout

REDD.47 Rack - Schematics

First, always check that all parts are included in the delivery. If you find any deficiencies, please get in touch with us by e-mail or via the "contact us" form on the pages. Please note that the components may look slightly different from the photos. Also, note that the voltages of the parts may be higher than what is written in the list. We try to keep the models of the components the same, but sometimes we have to change the manufacturer or model of a part due to poor availability. So, if a part is a slightly different color, don't panic; each component has the necessary values ​​marked on the side of the component. Read the notes on the part list carefully.

On this article the preamp is built in Adam Hall rack enclosure. This option does have less of metal work to do: Front and rear panel are available as pre-made and silk screened and these will replace the corresponding panels on rack enclosure kit. For needed stiffness and durability, two of the front panel boards are stacked. The wall thickness is only 1mm. It's also possible to use single front panel and stack this up with blank panel supplied with the rack enclosure by using the ready made panel as drilling template. This full panel sized panel is only available on stereo configuration.

As alternative option the preamp can be built into Hammond rack enclosure. By quality starndards, this one is better. This however requires more metal work to perform, for example making large diameter holes on back panel for XLR connectors and a square one for IEC power intake.

List of the needed tools for each option is listed on the web shop product description.

Note: read all text below of four following pictures before starting.

Start by marking the starting points of the holes for fastening the preamp boards. On vertical (by picture) the boards are located in the middle. This allows generous space for wiring. The horizontal placement can be done by using the front panel as guide. The boards are set on the center of the individual set of dials of a channel marked to panel. Boards are attached with M3 screws and M3x10 spacers.

Ready made holes for the boards.

On this quite the bottom board is installed as intended, the face of the bottom is flush with the sidewalls. To keep the bottom flat and prevent scratching the devices under it' recommended to work the holes for countersunk screws.

Alternatively the bottom board could be turned upside down, the lips pointing down raising the "floor" by 5mm. By this the screw heads wouldn't need to use countersunk screws.

Note: The top and bottom plates are unsymmetrical by the positions of the screws. Confirm the right orientations with the aid of side wall panel. On this build this was not noticed and the bottom plate needed new set of holes for the screws.

Take the another side panel and mark the spots for drilling. Use the PSU board and transformer as a help to find the positions. The power supply as whole is located somewhat in the middle, maybe bit closer to front panel end. The wires on transformer may be short so confirm the parts placement is ok to wires to reach the power switch.

The power supply regulator and mosfet transistor are attached to metal bottom for improved cooling. Bend the pins of these as in picture to find the right distance for the mounting hole. The pins should drop in the pads of psi board when fitted. Both of these can be fitted to wait the assembly later on. Note the regulator on front. It has transparent mica sheet under as well as the plastic insulator washer under the nut. The metal tab is live so this needs to be insulated from the grounded chassis. Before fitting the parts clean off burrs and chips of metal. Having some debris under lifting the parts won't transfer the heat from component to chassis.

Set the metal parts aside for a while and lay and solder all resistors of the power supply board. The values of individual parts are marked with colour rings. These can be solved by searching online tools (Resistor color code) or using multimeter. Resistors can be fitted on both directions. We recommend to set these on uniform direction, so the reading the values are bit more easier.

Solder all diodes (D1-D5), the bridge rectifier (BR1) and trimmer potentiometers. Diodes and the bridge need to be set on right orientation: The diode casing has ring and the board has corresponding marking. The bridge rectifier has positive output marked as + with longer pin on it. On the board the positive pin has + marking. This pin on top right corner.

Trimmers have their resistances marked as two value numbers plus number of zero -format. 504 is 500k and 502 is 5k. (Last number states the amount of zeros added after two first numbers).

Solder the fuse holders

Solder the capacitors on the board. C6 is polyester capacitor and can be soldered either way. The rest are electrolytic capacitors with polarity. The casing has stipe on the negative pin side, as well the pin itself is shorter. On the pcb the marking has white area to mark the negative pin.

Same as above, from different angle.

Attach the pcb to M3x10 spacers and solder the regulator and mosfet.

Attach the toroidal transformer and solder the secondary (output) side wires. Use the supplied rubber disks between transformer and metal plate as well between transformer and the metal disk on top. The wire colors may vary on transformers by manufacturer and patch. Make sure you connect right output to right pads. On the picture two wires on top are 6.3V output where the two on bottom are 235V.

Note: From layout v1.4 / parts delivery from 03/2025 mains transformer is fitted with two 115V primary coils for 115V/230V usage. Both wiring versions are found on the link of layout on top of the page. Refer correct wiring per location of usage. On both versions, correct phase of primary wiring is important. Notify the black heat shrink sleeve markings on the transformer wires. On serial / 230Vac operation the marked red and unmarked blue are soldered together and the connection is isolated. The remaining unmarked red and marked blue are soldered into mains switch.
On parallel / 115V both marked blue and red wires are soldered to one tab and non marked blue and red on another tab of the mains switch.

Finally solder a jumper wire between right most AC and leftmost F pad. Use shielded wire to prevent short circuits on pads below the wire.

Set the power supply now aside.

Continue with the main preamp circuit. Solder all resistors on the pcb. These can be installed both ways. Start with smaller 0.6W resistors. By this method the parts are pushed next to board when its flipped around for soldering.

Note the wrong values marked on pcb. Follow the layout and bom.

Solder:

Phantom supply diode, note the alignment of the ring end.

Phantom supply regulator, align this by the marking on the pcb.

Solder the ceramic, polyester and paper in oil capacitors. These does not have polarity to follow. 400pF ceramic capacitor is made out of two 180pF and 220pF parallel connected capacitors. The large 470nF capacitor on output is delivered without plastic sleeve. Use the clear shrink sleeve to cover the capacitor to prevent the metal shell to cause any shorts. Due the size this capacitor can be secured with two zip ties preventing the part to rip off from board during transportation etc.

Set the excess wires clipped of from the oil capacitors for later usage.

Finally solder the trimmer of the phantom power supply.

Solder the axial electrolytic capacitors. These have polarity, the positive terminal is marked with groove on the casing. The pcb has corresponding marking on it.

Finally solder the transformers on the board.

Build the second channel as shown previously and install the circuit boards to bottom plate using the M3x10 spacers.

As a last subassembly the front panel is built. The ready made panel has thickness of 1mm so as is it won't carry the weight of the preamplifier when installed in rack. The panel can be beefed up by layering two boards stacked or using the blank panel and ready panel in same manner.

If you're using the blank panel on bottom, use the ready made as template to mark the holes.

When done, glue or use double sided tape to tie up the stacked panels. This improves the ability to resist the panel to bend and prevent the corners to separate.

Lay the resistors and solder the front panel board

Note: The pcb has error on printing of Rph1, Pph2. These designators are Rph4 and Pph5 and value is 6.8k.

Component marked RN1 is resistor network. The kit may have either such part which is aligned with the marked ends to match. This component may be replacer with set of resistors. Start by soldering one resistor as in picture by living the wire as in square shape leaving space for four resistors more.

Solder rest of four resistors in the between and solder the ends to wire above and under the board.

Solder the ceramic and plastic film capacitors.

Solder the electrolytic capacitor on right bottom corner. Note the polarity.

Prepare all rotary switches by bending the tab straight as show on left.

Snap off the guide pin of the potentiometer with pliers. Cast aluminum will break relatively easy. Use the wires set aside on the solder lugs of the potentiometer as shown.

Fasten the instrument in jack(s) on the front panel. If you use the original blank front panel board, the threads of the jack are too short to use the insulator kit on both sides. Use tape to insulate the chassis - jack ground connection. Confirm the isolation with multimeter.

Set the rotary switches, potentiometer and the toggle switches on the board. Fit the two types of washers on the switch threads for right mounting height. The flat washer is fitter upside down, tab pointing towards the pcb.

Insert the panel with switches and potentiometers into the holes on the front panel. Tighten all nuts.

The installation height of the toggle switches remains a little short, where the solder lugs are at same level as the board on the solder side. Try to keep the pcb and front panel on same plane. If needed, remove the pcb and do additional soldering for the toggle switches on the component side.

Use again the wires set aside and solder jumper wires of the jack. Wires can be soldered through the hole.

Prepare the second channel board by the same way. Finally attach the power switch and the pilot light. Front panel is ready.

Install the back panel parts.

Same as previously. Note the type of the XLR (input / output).

Finally, the preamp is wired. You should not assemble the case yet, because there is significantly more working space when it is not assembled. However, the panels can always be temporarily lifted up, so that the lengths of the wires can be estimated more precisely.

First, solder the wiring between the front panel and the preamp boards. The shielded microphone cable ends are ended by using heat shrink sleeve. The braid is twisted into a bundle and protected with a shrink sleeve.

Note the front panel boards have interconnecting wires of the phantom supply.
Note: Do not solder the black cable between the boards seen on picture.

Note: PCB v1.1.1 has missing ground on insert wire pads:

Solder a jumper wire between INSG - OPT- pads or solder the ground wire directly to OPT- pad.

Solder the shielded microphone cables between the front and the rear panel.

By now the rear panel, left side and front can be attached.

Do the wires between the power supply and preamp boards and chassis ground.

Attach the power supply side and solder the mains input wiring. Note the dual primary wiring used on v1.4 layout / mentioned before.

Cover the solder tabs of the mains switch and the IEC input on back with shrink sleeve. Remember to set the sleeve in prior soldering.

Same phase as previously in overall view from top.

Attach the knobs to rotary switch and potentiometer axles. Preamp is almost ready for initial powering up.

Before connecting the power, check the entire circuit. In the final inspection, following the layout image, check that all the wiring has been run correctly to point to point and that the parts have been laid out correctly. Allow plenty of time for this. It is often easier to fix an error found in before the power is switched on than after connecting.

Go through the connection wire by wire, part by part, and mark the checked part on the layout picture. When everything has been found to be correct, measure with a multimeter as well as the integrity of the grounding and rule out short circuits in the power source:

• Measure in the resistance range of the multimeter: One test lead to any frame, e.g. the banana connector on the rear panel, and with the other test lead, check the 0 ohm resistance between the capacitors (C1) of the front amplifier boards, pin 1 on XLR connectors (note the ground lift switch) and ground on front panel jack.

• Measure in the resistance range of the multimeter: Resistance em between capacitors + and - poles. In this measurement, the resistance is small at first, but settles to several hundred kilo-ohms. Malta is waiting for the meter to settle. Small resistance, zero or only a few kilo-ohms can be a sign of a possible short circuit, which should be looked for before connecting the currents.

After everything is checked and any mistakes are corrected, put all three fuses in the fuse holders and turn on the power.

First, adjust the Phantom voltage to 48V from the pre amplifier board. After that, adjust the power source to 6.3V and the anode voltage to 305V. When the voltages are correct, turn off the power and push the tubes into the sockets.

In case of possible problems, measure the test point voltages marked in the layout picture. Large deviations, no voltage or max supply voltage usually shows the problem area. Measure the circuit always with tubes installed. If the problem persists please contact us by e-mail or via the feedback form on the website. We are unable to provide technical support over the phone or in the store.

We wish good recording sessions to the builder of the new microphone preamp!