METROAMP 100 WATT MASTER VOLUME STEP-BY-STEP INSTRUCTIONS V 1.
Introduction Welcome to the step-by-step instructions for the Metroamp 100 Watt Master Volume amp kit. We hope you will enjoy building your amp and will learn a thing or two about tube amps in the process. The purpose of this kit is to replicate the Marshall model # 2203 JMP amps produced from 1976 until 1980. The first Marshall amps to feature a master volume control, cascaded preamp channels, rocker switches and large 11” Marshall logo.
100 WATT MASTER VOLUME - BILL OF MATERIALS QTY 1 1 1 1 1 4 4 1 DESCRIPTION JMP STYLE LARGE HEADBOX METROAMP WHITE SCRIPT LOGO 100W MV STEEL CHASSIS BRUSHED ALUMINUM FRONT PANEL BRUSHED ALUMINUM REAR PANEL M6 METRIC CAGE NUT M6 - 30mm CHASSIS SCREW, WASHER COMPONENT BOARD – RED 11 5/8” x 3 1/8” 6 1 8 28 10 6 10 1/2" GROMMET 3/8" GROMMET #10- 3/8" SCREW, WASHER, NYLOC NUT #6- 3/8" SCREW, NYLOC NUT #4- 3/8" SCREW, NUT #4- 1 1/4" BOARD MOUNTING SCREW ASSEMBLY GROUND LUG - 2 HOLES 1 1 1 STOCK TRANSFORMERS M
100 WATT MASTER VOLUME RESISTORS QTY VALUE SIZE TYPE COLOR CODE 1 5 2 2 820 10K 56K 100K 2 WATT 2 WATT 2 WATT 2 WATT METAL OXIDE METAL OXIDE METAL OXIDE METAL OXIDE GRAY-RED-BROWN-GOLD BROWN-BLACK-ORANGE-GOLD GREEN-BLUE-ORANGE-GOLD BROWN-BLACK-YELLOW-GOLD 4 1.0Ω 2 WATT METAL FILM BROWN-BLACK-GOLD-GOLD 4 1K 5 WATT CERAMIC N/A 1 1 1 4 1 2 1 1 1 1 3 4 2 3 3 470 2.7K 4.7K 5.
STEP 1: Preparing to assemble the kit. When your kit arrives, we suggest unpacking it and examining the parts. Get familiar with what we have shipped to you. Compare the parts to the Bill of Materials included in these instructions and the packing slip included in your kit. When you’re ready to start the assembly, we suggest a clean, well-lit area with decent ventilation for the solder fumes. The more organized you are the better your build is likely to turn out.
STEP 2: Installing the power transformer (PT) When you are ready to start assembling, place the chassis on the bench with the front (pots, switches, input jacks) facing you, as seen in STEP 1. All steps in these instructions will reference this as the front of the chassis. In this orientation, the power transformer (abbreviated as “PT”) will mount on the LEFT end of the chassis. Before installing the PT, you need to first install (4) ½” RUBBER GROMMETS in the chassis holes through which wires will pass.
STEP 3: Installing the output transformer (OT) Locate the C1998 output transformer (abbreviated as “OT”), (2) ½” GROMMETS and (4) #10 –3/8” machine screws, flat washers and Nyloc nuts. NOTE: if you are using an optional Mercury Magnetics OT, it will replace the C1998. The appropriate part numbers are: • MMO-100 for EL34 output tubes • MMO-100M for 6550 output tubes (also compatible with KT88, KT77, 5881 and 6L6’s) Fortunately, each of these OT’s adheres to the original wire color scheme used by Dagnall.
STEP 4: Installing the choke and filter caps (part 1) Locate the Mercury Magnetics MMC-3H choke, 3/8” RUBBER GROMMET and (4) #6 – 3/8” machine screw and Nyloc nuts. Install the 3/8” grommet into the chassis (directly to the RIGHT of the OT). Feed the RED and BROWN choke wires through the hole until the choke sits flush on the chassis. Assure that the wires are routed directly through the chassis hole so they do not get pinched when you secure the choke.
STEP 5: Installing the filter caps (Part 2) Part 2 of the filter caps installation is mounting them on the chassis with the necessary ground lugs. Locate (12) #6 – 3/8” machine screws and Nyloc nuts, also (5) 2 HOLE GROUND LUGS. Install (6) filter caps in the positions shown using this hardware sequence: SCREW – CAN CLAMP – CHASSIS – (GROUND LUG) – NUT Reference the detailed pictures below for correct terminal orientation and which screws require a ground lug.
STEP 6: Installing the output tube sockets Locate (4) MICALEX OCTAL SOCKETS, (4) TUBE RETAINERS (EL34 OR 6550 size depending on your output tube type), (4) 2 HOLE GROUND LUG AND (8) # 6 – 3/8” machine screw and Nyloc nut. Install each of the four tube sockets and retainers referencing the pictures for orientation and location of ground lugs. The Micalex sockets install from the inside of the chassis and the tube retainers on the outside. Each of the socket assemblies is identical.
STEP 7: Installing the preamp tube sockets Locate (3) 9 PIN MINIATURE SOCKETS WITH SHEILD (NOTE: 4 if using an additional tube for a mod), (8) #4 – 3/8” MACHINE SCREWS and NUTS and the PREAMP SOCKET HOLE COVER. Install (3) 9 pin sockets as shown, taking note to orient the space between pin 1 and pin 9 so that it faces the front of the chassis. Consequently, pin 4 will point almost directly towards the rear. The socket sits flush on top of the chassis and is mounted using (2) #4 – 3/8” screws and nuts.
Step 8: Attaching the rear panel. Locate the ALUMINUM REAR PANEL, Super 77 or equivalent contact cement and (4) METRIC CAGE NUTS. Unlike the front panel, which is held firmly in place by pots, switches and input jacks, the rear panel needs to be secured to the chassis. We suggest contact cement for this. Here in the shop 3M brand Super 77 is used. But any high quality adhesive will work fine. Apply a thin later of adhesive to the back of the rear panel and align the panel on the chassis.
Step 9: Attaching the front panel, switches and pots. Locate the ALUMINUM FRONT PANEL, RED ROCKER SWITCH, BLACK ROCKER SWITCH and (6) ALPHA POTS (values: (2) 1Meg Audio, (1) 250k Linear, (2) 25k Linear). The brushed aluminum front panel included with your kit is held in place easily by the pots and switches on the front of the chassis. Installation could not be simpler. Put the panel in place on the chassis and press the RED and BLACK rocker switches in place. RED for POWER and BLACK for STANDBY.
Step 10: Installing the knobs. Locate (6) Marshall SET SCREW KNOBS. Install one on each pot assuring that the pot shaft is rotated fully in either direction. If fully clockwise, install the knob with the pointer aimed at the “10” text on the panel, as shown. Once in place, tighten the setscrew on the side of the knob with a 5/64” or 2mm Allen wrench. NOTE: occasionally these knobs will not fit on Alpha pot shafts due to excess plastic material in the hole.
Step 11: Wiring the PT (part 1) You can now start what I consider to be Section 2 of the assembly. Wiring inside the chassis. Fire up your soldering iron and we’ll start with the power transformer. NOTE: if you are using the optional Mercury Magnetics PT, refer to the appropriate diagram in the APPENDIX for wire colors. Connect the bias common wire (Blue) and heater center tap (Green with Yellow) to ground by routing as shown and soldering to the ground lug.
STEP 12: PT wiring (part 2). Connect the PT high voltage secondary wires (RED, RED and RED with YELLOW stripe). Route the RED with YELLOW to the negative terminal of the mains filter cap nearest the POWER switch and solder in place. Next, twist both RED wires together and route to the STANDBY switch, as shown. Cut to length and solder in place on the silver terminals. Note that the extra primary wires are bundled with the other wires now and a cable tie holds them temporarily in place.
STEP 13: Installing bias resistors, heater wiring (part 1). Locate (4) 1Ω 2W metal oxide resistors, color code BROWN-BLACK-GOLD-GOLD. These are referred to as “bias resistors” in Metroamp kits because they are utilized to simplify measuring and adjusting bias of the output tubes. In a stock 2203 amp, pins 1 and 8 (cathode and suppressor grid) of the output sockets are grounded. We replace this connection with a 1Ω resistor.
STEP 14: Wiring the heaters (part 2). Continue daisy-chaining sockets together until all of the output sockets are wired, as shown above. As you move to the left and wire the preamp tube socket heaters, wire routing and twisting become more important. Lower signal levels and closer proximity to tube grids increases the potential for hum caused by AC heater voltage. As such, try to get an extra twist of wire between each socket.
Step 15: Chassis wiring (part 1) The next several steps are dedicated to wiring inside the chassis in preparation for installing the component board. This may make it seem as though the instructions are jumping around a bit. Rest assured, the miscellaneous tasks are all aimed at the same destination. Locate the supplied 18 GAUGE BUSS WIRE (the larger of the two buss wires in your kit). It will be installed in three positions, two filter caps and on the back of the pots.
Step 16: Chassis wiring (part 2) Locate (1) 4.7kΩ 1-WATT CARBON FILM RESISTOR and (1) SOZO .1uf CAPACITOR. Solder the 4.7k in place between the buss wire and terminal one of the pot. Later, a VIOLET wire will also attach here. Next, bend the leads of the Sozo .1uf cap as shown and solder in place. One lead will attach directly to the pot. The other will attach to (and act as a jumper between) terminals 2 and 3. TECH NOTE: It is common to find a .68uf cap in place of the .1uf on many JMP Marshall amps.
Step 17: Wiring the OT primary wires, installing output jacks. Twist the RED and BROWN wires from the OT and route to V5 and V6. Make sure to complete a minimum of five complete twists, as shown. >> Attach the RED wire to the lower hole in the terminal at PIN 3 of V5. Solder in place. Route the BROWN wire to PIN 3 of V6 and attach likewise. Locate (4”) of each RED and BROWN 22g wire. Install these as jumpers between octal sockets as shown. RED jumpers V4, PIN 3 to V5, PIN 3.
Step 18: Wiring the output screen wires. Locate the 22g YELLOW wire and cut (3) 5” pieces and (1) 6” piece. These will be wired as jumpers for the output tube sockets (V4 through V7) PIN 6. Starting at V4 (far LEFT): strip the insulation from one 5” YELLOW wire and the 6” YELLOW wire. Insert these into the lower terminal hole of PIN 6. Solder in place, leaving the top terminal hole open. A large resistor will mount here.
Step 19: Wiring the OT secondary wires (part 1). Locate the MARSHALL IMPEDANCE SWITCH, (2) #4 – 3/8” SCREW AND NUT, 10” of 18g BUSS and a 10” piece of 22g VIOLET wire. Route the OT secondary COMMON wire (BROWN for the C1998) to the RIGHT Cliff jack as indicated. Solder in place in the lower part of the RIGHT terminal. Ensure a solid solder joint as this is a critical connection.
Step 20: Wiring the OT secondary wires (part 2). Your OT secondary wires should be routed as in the picture. The free end of the VIOLET negative feedback wire is left in the chassis, it will be attached to the component board in a later step. All that’s left to complete the secondary wiring is to add 18g buss wire to the speaker jacks and impedance switch. Feed a 6” piece of buss wire through four jack terminals, as shown, and route to the terminal in the center of the impedance switch.
Step 21: Installing output screen and grid resistors. Locate (4) 5.6kΩ 1W (GREEN-BLUE-RED-GOLD) resistors and (4) 1kΩ 5W resistors. These are the output tube grid and screen resistors, respectively. Cut one lead on each of four 5.6kΩ resistors to ¼” length and bend into a hook. Attach one resistor on each output tube socket PIN 5. Shown here in place on V4, prior to soldering. The other lead will be attached in a later step.
Step 22: Installing balancing resistors and input jacks (part 1). Locate (2) 56kΩ 2W metal oxide resistors (GREEN-BLUEORANGE-GOLD). These are installed, one each, on the screens filter caps. Bend the leads under on one 56k and route them through terminals on the cap nearest the chassis rear. Route the lead from the negative cap terminal on to the ground lug bottom hole. Solder in place at the lug and the negative terminal. Loop the other lead on the positive terminal and solder in place.
Step 23: Installing the input jacks (part 2). Install the prepped Cliff jack in the HIGH SENSITIVITY chassis hole. Tighten the nut with a 15mm socket or pliers. The hardware sequence is: JACK – FIBER WASHER – CHASSIS – NUT Next install the remaining Cliff jack on the LOW SENS. position. Before tightening the nut, route the bare wire from the first jack to the LOWER, RIGHT terminal on the second jack, as shown. >>> Assure the jacks are aligned and tighten the nut.
Step 24: Wiring the PRE-AMP VOLUME pot. Locate (8”) COAX wire, (1/2”) 1/8” SHRINK TUBE. Strip ½” of insulation and braided shielding from one end of coax wire. Strip 1/8” of clear insulation from the center conductor and solder in place at V1, PIN 7. Slide ½” of shrink tubing over the coax and shrink in place. Route the coax to the PRE-AMP VOLUME pot. Strip ¾” of insulation. Also strip 3/8” of braided shield. Ensure no stray strands are left hanging from the shield. Tin the shield to hold it in place.
Step 25: Final chassis wiring prior to board install. Locate (1) 1,000pf ceramic disk cap (102K). Cut the leads to ½” in length and solder in place on terminals 1 and 2 of the PRE-AMP pot. Locate (6) BOARD MOUNTING SCREWS and install in the chassis using the hardware sequence: SCREW – CHASSIS – LOCK WASHER – SPACER – NUT – NUT Install the screw, washer, spacer and one nut. Tighten with a ¼” nut driver, then install the second nut and tighten against the first.
Step 26: Installing buss wire on the board. Locate the COMPONENT BOARD, (3’) 22g BUSS wire, and (2’) 18g SLEEVING. Our point-to-point terminal board takes it’s cues from vintage Marshall component boards. It uses buss wire and insulation on top of the board to connect terminals. Refer to the photo at the bottom of this page or the BUSS WIRING DIAGRAM in the APPENDIX for buss wire locations.
Step 27: Wiring the component board. Locate each of the wire colors and prepare the component board for installation in the chassis by attaching the necessary wires. Another cue taken from Marshall component boards is running the wires through the board and then around the terminal. This isn’t mandatory, but does make for a great looking finished product. Reference the photos on this page and the BOARD WIRING DIAGRAM in the APPENDIX for correct wire color, length and connections.
Step 28: Attach chassis wires to component board. With your board wired for installation, you can now attach each of the chassis wires. These should be attached before installing the board on the mounting screws. Sit the board in the chassis with the bottom facing you. Begin attaching wires to the appropriate terminals according to the picture and WIRING DIAGRAM. Leave enough extra wire to allow the board to be flipped over, on to the mounting screws.
Step 29: Wiring the board into the chassis. With the under-board wires connected, flip the component board over and place it on the mounting screws. Before installing the mounting nuts, stretch each wire out and organize them into the general locations where they will be attached. Make sure none are tucked under the board. With all of the board wires accounted for, begin soldering in place in the chassis.
Step 30: Wiring the board into the chassis (part 2). To complete the wiring of the inputs jacks, cut a small section of PINK wire and install as shown. The purpose of this wire is to short the HIGH SENSITIVITY input to ground (to prevent noise) when the LOW SENSITIVITY input is used. Wire the YELLOW and RED wires to their corresponding filter cap terminals. Route the BLUE and RED wires to the HT FUSE HOLDER and the PHASE INVERTER FILTER CAP, as shown, and attach to the appropriate terminals.
Step 31: Wiring the output tube grid wires. The final board wires to be attached are the output tube grid wires (ORANGE, GREEN). These can be tricky to install since the grid resistors are flying off the output tube sockets. Prepare to install by cutting the lead on each resistor to ½” long and forming into a loop, as shown. >>>>>>>>>>>>>>>>> There are two ways to chain the grid resistor together, as shown below.
Step 32: Loading the component board! With the board fully wired into the chassis, the fun part can finally begin! Install the board securing nuts and tighten with a ¼” nut driver. Be sure no wires under the board are pinched between the board and the mounting screws. This is fairly common and is a pain to troubleshoot. Refer to the BOARD COMPONENTS diagram in the APPENDIX for component location and values, noting and changes specific to your build i.e. tweaked circuit values, or using 6550 tubes.
Step 33: Installing the IEC socket and mains fuse. Locate the IEC AC SOCKET, (1) FUSE HOLDER, (2) #6 3/8” screw and Nyloc nuts, (10”) ORANGE wire, (14”) BLACK wire and (6”) GREEN wire. Install the IEC socket on the chassis using #6 screws and Nyloc nuts. The hardware sequence is: SCREW – SOCKET – CHASSIS – NUT Tighten with Philips screwdriver and 3/8” nut driver. Install the MAINS fuse holder on the chassis and tighten in place with a 14mm wrench or pliers. Do not over-tighten.
Step 34: Cable ties and final inspection. Your chassis should now be fully assembled and ready for the testing and powering up procedures documented in the TESTING section. One final touch that makes for a clean amp build is to bundle wires together using cable ties. One cable tie every inch or so gives good results. Note: do not use a cable ties to bundle the output transformer primary and secondary wires as this can cause cross talk between them, resulting in oscillation.
TESTING SECTION: PART 1 WARNING!!! WARNING!!! WARNING!!! WARNING!!! WARNING!!! WARNING!!! IT CAN NOT BE STRESSED ENOUGH THAT TUBE AMPLIFERS CARRY HIGH VOLTAGES, HIGH ENOUGH TO CAUSE FATAL INJURY UNDER THE RIGHT CIRCUMSTANCES, EVEN AFTER AN AMP HAS BEEN OFF FOR SEVERAL HOURS. IT IS ABSOLUTELY IMPARITIVE THAT YOU FOLLOW THE SAFETY GUIDELINES DESCRIBED IN THE TESTING SECTION.
TESTING SECTION: PART 2 Locate and install (1) 1 AMP FUSE and (1) 4 AMP FUSE (3A for 220/240V operation). Also locate and install (3) 12AX7 preamp tubes in V1, V2 and V3. Locate IEC AC POWER CORD. Next, rotate the bias pot fully counter-clockwise (anti-clockwise) to ensure the bias voltage is at maximum at first power on. Make sure both switches, POWER and STANDBY are in the OFF position. Plug in the AC power cable, but don’t turn on the power switch.
TESTING SECTION: PART 3 Assuming there were no fireworks upon powering on, you can begin taking measurements. Set your meter to AC volts, with the negative lead grounded on the chassis, and test V1 pins 4, 5 and 9. You should measure approximately 3.15V AC. It is common for this to run slightly high, as seen here measured at 3.24V AC. Measure for the same reading on V2 and V3. Also V4 - V7, pins 2 and 8. Reference the voltage chart in the APPENDIX for an overview of tube and terminals that should have 3.
TESTING SECTION: PART 4 Assuming all has gone well to this point, with your meter still set for DC 500V measure both terminals of the HT fuse holder. Each should read the same high voltage you found at the positive terminal of the diode arrangement. If you do not, power off and check the 1 amp fuse. If it does, proceed testing for DC voltage on each of the tube sockets referencing the chart below, also included full size in the APPENDIX.
TESTING SECTION: PART 5 With all the tube sockets tested and receiving the correct voltage on each pin, you can install the output tubes. Once in place, turn the STANDBY switch to OFF and the POWER switch to ON. Check to see that the output tubes warm up and start to glow. They will be biased very cold at this point. But before we dial the bias in, we need to determine the correct idle current to set them at.
TESTING SECTION: PART 6 TECH NOTE: On the subject of biasing, we should discuss why we break from authenticity and install the 1Ω resistors. These were not found in original Marshall 2203 amps, but are a worthy addition as they allow simple bias adjustment using only a multi-meter. A task that otherwise requires a specialized bias tool or a signal generator, dummy load and oscilloscope.
APPENDIX • • • • • • • • • • • • • • • • • • • • • • • • • • • • FULL CHASSIS FULL CHASSIS LAYOUT AND WIRING DIAGRAM BOARD BUSS WIRE DIAGRAM BOARD WIRING DIAGRAM (COLORS AND LENGTHS) BOARD COMPONENTS DIAGRAM VOLTAGE CHART 6550 WIRING AND VALUE CHANGES DIAGRAM 220/240V AC WIRING DIAGRAM OPTIONAL FX LOOP WIRING DIAGRAM METROAMP 1203-80-MS AND C1998 DIAGRAMS MERCURY MAGNETICS MP100-460 DIAGRAM MERCURY MAGNETICS MP100-460 WIRING MERCURY MAGNETICS MMO-100 AND MMO-100M DIAGRAMS MARSHALL 2203 SCHEMATIC
100 WATT MASTER VOLUME – CHASSIS
100 WATT MASTER VOLUME – COMPLETE CHASSIS
100 WATT MASTER VOLUME – BOARD BUSS WIRES
100 WATT MASTER VOLUME – BOARD COMPONENTS
100 WATT MASTER VOLUME KIT - VOLTAGE CHART PIN 1 PIN 2 PIN 3 PIN 4 PIN 5 PIN 6 PIN 7 PIN 8 PIN 9 V1 12AX7 217 SIGNAL 2 3.15 AC 3.15 AC 260 SIGNAL 3 3.15 AC V2 12AX7 168 SIGNAL 1.1 3.15 AC 3.15 AC 302 168 170 3.15 AC V3 12AX7 218 28 42 3.15 AC 3.15 AC 214 28 42 3.15 AC V4 EL34/6550 0 3.15 AC 485 476 -45 480 3.15 AC 0 V5 EL34/6550 0 3.15 AC 485 476 -45 480 3.15 AC 0 V6 EL34/6550 0 3.15 AC 485 476 -45 480 3.15 AC 0 V7 EL34/6550 0 3.
100 WATT MASTER VOLUME – CHANGES FOR 6550 OUTPUT TUBES CHANGES NECESSARY FOR 6650 OUTPUT TUBES: • BIAS DROPPING RESISTOR CHANGE FROM 27K TO 15K • BIAS SPLITTER RESISTORS CHANGE FROM 220K TO 82K • NEGATIVE FEEDBACK WIRE ATTACHES TO 4Ω TAP
100 WATT MASTER VOLUME – 220, 240V AC WIRING
METROAMP TRANFORMER DIAGRAMS
MM MP100-460 POWER TRANSFORMER WIRING
OPTIONAL MERCURY MAGNETICS OUTPUT TRANSFORMERS
