EHHA Rev A Setup


Inital Setup

Follow this procedure to bring up each EHHA Rev A Board. You will need a digitial multimeter. If something goes wrong, disconnect the power connections immediately. You need to be reasonably skilled at using a DVM and accessing test points on a PCB.

Use this wiring digram for two channel operation.

EHHA Rev A Two Channel Wiring

Use this wiring digram if you have hum/buzz that you can't fix through any other means.

EHHA Rev A Two Channel Alternate Wiring

Heater Supply

  1. Connect your transformer to the terminal block labeled 6.3-9VAC. Power up the transformer.
  2. If you have the heater LED installed it should light.
  3. Measure the voltage at the output block between either G connection and the 6.3VDC output.
  4. Adjust the trimpot until you get 6.3VDC (or whatever heater voltage is in the range that you want).
  5. Turn off the power and connect the 6.3VDC and one of the G terminals to the Heater terminals on the amplifer section. Insert a tube and power up the heater supply
  6. If the tube lights measure the DC output of the supply again. This should be what you set in the previous step.
  7. If it is, proceed to the next section.
  8. If it is not, measure the voltage from TP3 on the heater board to ground. This is the input voltage for the regulator. If this voltage is below 8.3-8.5VDC then your regulator will drop out and not regulate properly. Change the transformer and/or diodes.

Rails Supply

  1. Connect the 30-0-30 transformer to the terminal block on the PS board with the same label.
  2. Turn on the power.
  3. If you have the rails LEDs installed they should light.
  4. Measure the +30 and -30 terminals on the block to ground. They should be close to 30V.
  5. If they are proceed wire the +30 and -30 terminals together between the PS and amp sections.
  6. Go to the next step.
  7. If one or both of the voltages are wrong, check all component values and ensure that the right regulator is in the right place.
  8. Figure out what's wrong.

Ground Wiring

  1. Attach the unused G terminal on the heater board to the star ground.
  2. Attach the center terminal on the PS output to the star ground.
  3. Attach the ground terminal (the one not labeled) on the amp PS inlet to the star ground.
  4. Holes are provided on the boards for routing these wires under the board from the terminal blocks.
  5. Make sure that the heater and rail connections are all wired across the boards.

Amp Power Up

  1. Set the trimpot P1 to its halfway position.
  2. Attach a DMM from the OUT terminal to ground set on the 300mV setting.
  3. Turn on the heater and rails supplies.
  4. Look for smoke. If there is none, proceed.
  5. Wait for the output DC to settle into the low mV range. If this does not happen the servo is not working. Stop and turn off the power.
  6. If the DC offset settles then adjust the bias.
  7. Attach the DMM from either TP1 or TP2 to the OUT terminal set on the 300mV scale. This measures across the output resistors.
  8. For the Mosfet version adjust the trimpot until this voltage is 220mV (100mA).
  9. For the BJT version adjust the trimpot until this voltage is 1V (100mA).
  10. Measure the other testpoint to OUT. It should be the same.
  11. After the output heatinks get hot set the bias again because it will change.
  12. If you get 0VDC offset and if you can set the bias then you can attach headphones and listen.