Mojotone Blackface Twin Reverb®
Style Amp Kit Instruction Manual
Tools needed for build:
• 40 W Soldering Iron
• Wire Cutters / Stripper
• Needle Nose Pliers
• Phillips Head Screwdriver
• Small Flat Head Screwdriver
• AC/DC Multimeter with Resistance Reading
• 44 Rosin Core Solder
Lay out parts for a quick inventory (Photo 1)
• Compare physical inventory to the included bill of materials. Packaging mistakes are rare for Mojo but
double check us before you start your build.
Install chassis mounted hardware
• Tube Sockets - Install so that they all face the same direction (noting pin #1 orientation)
• Power Transformer - Make sure to mount the PT so that the label faces toward tube sockets. This will be
helpful for anyone looking at the transformer model number without having to take the chassis out of the
amp.
• Output Transformer - Feed wires through grommets and note wire locations.
• Install Potentiometers.
• Install Bias Potentiometer.
• Install Input, Output Jacks, and Switches.
• Fuse Holder - Install fuse at same time so you won’t forget.
• Pilot Light Assembly (Lens and Bulb)
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 1
Photo 1 - Lay out parts for quick inventory
Photo 2 - Installing Chassis Mounted Hardware
Photo 3 - Filter Cap Fiberboard Assembly
Fiberboard Assembly
Tech Tip - Always use a clean tip on your soldering iron to make positive, clean solder joints, (“cold”
solder joints can be a major problem in hand-wired amps)
• Always try to rely more on the schematic than the wiring diagram in any circuit.
• Be sure of component placement and orientation, especially capacitors (positive, negative)
• Make sure that the white dot on the optoisolator is pointing in the direction of the tubes.
• It is recommended to place the components on the fi berboard and the wire leads before soldering.
This step can help prevent cold solder joints by eliminating the need to push wires through an already
established solder connection.
• Check and double check the traces on the back of the fi berboard. (this is one of the most common areas
that mistakes are made).
• If you plan on doing modifi cations such as “hot-rodding” (adding a bright switch etc.) and have never put
a kit together before, it is suggested that you assemble the basic amp and have it functioning properly, to
have an understanding of how the amp works before you start modifying it.
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 2
Photo 4 - Fiberboard Assembly (Top View)
Photo 5 - Fiberboard Assembly
D C B A
Photo 6 - Filter Cap Fiberboard Assembly
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 3
Photo 6 - Recti! er / Bias Fiberboard Assembly
Internal Wiring
• Follow the color coded wiring diagram closely, twist the fi lament wires (6.3v heater wires) together from
transformer and between tube sockets to reduce hum from 120ac voltage.
• Have enough of the power cord inside chassis to reach fuse, switch and ground
• Affi x the fl oating fi berboard, noting it is held in place by the wires, attach backing with rubber cement if
necessary (or mount directly to chassis)
• Place entire fi berboard inside the chassis noting the orientation (see schematic)
• Connect corresponding wires to hardware, try to keep wire short and runs as neat as possible. This will
help make future troubleshooting and repair easier.
• Solder (2) ½ W 100 Ohm resistors - solder from the pilot light to ground
• Solder (1) ½ W 1 Meg resistor from Input 1 to ground tied to Input 2. This grounds the input of the amp so
that no hum or noise picked up by the amp when the guitar cable is unplugged.
• On each power tube socket, solder (1) 1W 470 ohm resistor from pin 4 to 6. Also solder a ½ W 1.5K
resistor from pin 1 to pin 5.
• Try to avoid crossing wires over each other.
• Before you plug the amp in check and re-check your wiring. If you are not clear on any part of the
circuit, ask someone.
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 4
Photo 7 - Input Jack Wiring
Photo 8 - Recti• er / Bias Fiberboard
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 5
Photo 9 - Internal Wiring - Power Amp
Photo 10 - Internal Wiring - Preamp
Photo 11 - Internal Wiring - Power Tube Sockets
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 6
Photo 12 - Output Transformer Wiring
**WARNING**
The Voltages found inside your amp can cause serious harm or even death. Never
attempt to service your amp while a power source is connected.
Testing the Amp
• *WARNING* Never stick your hands inside amplifi er especially while power is on!
• Plug in speaker
• Turn amp on without tubes installed fi rst, checking transformers and components for shorting, excessive
heat, arcing, smoking, etc.
• Test tube socket voltage with a multimeter to ensure correct operation.
• Turn Amp off before putting tubes in proper location, and allow tubes to heat up for approx. 30 secs. Before
playing at volume.
• Listen. The best way to tell if there any problems with the circuit of the amp and/or tubes is just to listen....
excessive noise, popping/ crackling, hum, loud squeals, as well as ghost- echo sounds. These are all
symptoms of many common problems in tube amps.
• If you need to service the amp after having it on, you must “discharge” the capacitors. This is done by
unplugging the amp, turning the power and standby to the on position and letting it sit for 30 mins. or so.
Always use a multimeter to check the residual voltage in the cap to make sure it is fully discharged.
Feel free to call Mojo at 1-800-927-Mojo (6656) at any time if you any questions regarding your kit.
Thanks for choosing Mojo and don’t forget to crank it up!!
The Voltages found inside your amp can cause serious harm or even death. Never
attempt to service your amp while a power source is connected.
Online Resources
Grounding principles: www.aikenamps.com/StarGround.html & www.el34world.com/charts/grounds.htm
Signal Flow Diagram: www.el34world.com/charts/currentfl ow.htm
Printed Resources
“The Tube Amp Book” by Aspen Pittman
“A Desktop Reference of Hip Vintage Guitar Amps” By Gerald Weber
“All About Vacuum Tube Guitar Amplifi ers” By Gerald Weber
©Mojotone® Blackface Twin Reverb® Style Amp Kit Manual 2012 Page 7
**WARNING**
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