SOHA II Instructions


Before you start

The SOHA II is designed to fit either the Hammond 1455T2202 or 1455T1602 aluminum cases. The 1455T2202 has enough room at the back of the case to house the toroid transformer and even a DAC. The 1455T1602 is shorter and can be used if you want to enclose the transformer in a wallwart box. You can print silkscreen images of the Amplifier board from these links:

SOHA II Board Silkscreen (150dpi PNG) SOHA II Board Silkscreen (600dpi PNG)
SOHA II Board Silkscreen (300dpi PNG) SOHA II Board Silkscreen (PDF)

It's a good idea to study the board and components. Also, look at the Amplifier Schematic and PS Schematic diagrams to locate each part on the layout. Measure each resistor before you install it to make sure that you're reading the code properly.

Board Assembly Instructions

The board is equipped with three sets of jumper pads. They are labeled J1, J2, and J3. Find these pads using the Test Point and Jumper Finder. The jumpers disconnect the PS from the amplifier as follows:

Jumper Rail
J1 +15V
J2 -15V
J3 +105V

Determine which Transistor/Regulator mounting you are going to use.

You can populate the entire board but don't strap any of the jumpers because leaving them out will allow you to test different sections of the amplifier in sequence. See Initial Setup for this sequence.

Have the components specific to your build in the parts box.

Solder the components to the board from lowest profile to highest, roughly in this sequence:

  1. Resistors and diodes.
  2. Mount vertical diodes with cathodes up and anodes down, like this:
    Vertical Diode Mounting
  3. Small signal transistors.
  4. Film capacitors, trimpots, opamp socket, and connectors.
  5. If you plan to try zeners for R4, install sockets in the R4 pads
  6. Tube sockets and large electrolytics.
  7. Output transistors and their heatsinks.
  8. PS regulators and their heatsinks.
  9. LED wiring if you will be lighting the tubes.
  10. Install the Opamps. Be sure to get the correct orientation.
  11. Jumper the heater block header for the tubes you are using.

Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush.

Inspect all solder connections carefully, using a magnifying glass, to make sure there are no solder bridges or cold solder joints. Use a multimeter in ohms scale to check for short circuits. Correct any mistakes before proceeding.

 

Power Device Mounting - Top and Bottom

Mounting the Power Transistors

The normal position for the BD139 ECB devices is on the front side of the heatsinks. The backside of the heatsinks are used for the BCE devices. Both types of BJTs can be mounted on the bottom of the board.

  Top Side Bottom Side
Standard
ECB
Normal Mounting (PNG) Bottom Mounting (PNG)
Alternate
BCE
Normal Mounting (PNG) Bottom Mounting (PNG)

Mounting the Regulators

The normal position for the regulator devices is on the front side of the heatsinks. The backside of the heatsinks are not used. However, the regulators can be mounted on the bottom of the board along with their heatsinks. Refer to the diagrams.

Top Side Bottom Side
Top Regulator Mounting (PNG) Bottom Regulator Mounting (PNG)

 

Casing Assembly

Use these dimension diagrams to locate and drill holes.

Board Dimension Drawing (PNG) Board Dimension Drawing (PDF)
Top Panel Dimension Drawing (PNG) Top Panel Dimension Drawing (PDF)
Front Panel Dimension Drawing (PNG) Front Panel Dimension Drawing (PDF)

Board Dimensions

The PDF version of the file should be printable as an accurate template to work from.

  1. In the Hammond cases the only required hole is the star ground standoff which is located at the center between the O/P transistor heatsinks. This point should be firmly attached to the bottom of the case.
  2. If you will be tube rolling, you should also fasten the board using the three mounting holes along the centerline of the tubes, one in the center of the board and one just to the outside of each tube. These attachment points will minimize board flexing when tubes are inserted and removed.

Top and Front Panel Dimensions

The PDF version of the file should be printable as an accurate template to work from.

Warning: The SOHA II dissipates a lot of heat. The top plate must be opened so that there is a high amount of ventilation. More than 50% of the area of the top plate over the entire board must be opened for ventilation.

  1. Locate and drill or punch the holes in the top plate for the tubes.
  2. Locate and drill the hole for access to the B+ trimpot.
  3. Locate and drill holes in the front panel for the headphone jack and volume pot.
  4. Locate and drill holes for mounting the transformer, power inlet, power switch and fuse, and input jacks on the back panel.

Board

  1. The SOHA II PCB is designed to insert into the third slot from the bottom in the Hammond enclosure. This position leaves room at the top for the heatsinks and at the bottom for small boutique caps and standoffs to fasten down the board.
  2. Attach the drilled front panel to the enclosure.
  3. Slide the board in from the back pushing the nose of the headphone jack and volume pot through the holes in the front panel.
  4. Use the spacers that come with the jack to space out the nut from the front panel.
  5. Fasten the volume pot with its nut.
  6. Mount the transformer with the wires up and towards the back. See the Board Wiring section for a diagram.
  7. Install the power inlet, power switch, and input jacks into the rear panel.
  8. Wire the transformer, power inlet, power switch, and input jacks as described in the Board Wiring section.