Inital Setup

Follow this procedure to bring up your SOHA II. 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 the Test Point and Jumper Finder.

SOHA II Test Point and Jumper Finder (PNG)
SOHA II Test Point and Jumper Finder (PDF)

Sequential Start Up

This procedure assumes that you have not strapped jumpers J1, J2 and J3.

Power Supply

Connect the power section (transformer, switch, fuse, etc) and the 15-0-15 windings to the terminal block at the rear left of the board. Do not insert the tubes.

Verify the following voltages:

Test Point Voltage
TL+ +15V
TL- -15V
TH -12.6V
TB+ +103V to +125V

The voltage at TB+ will be close to 125V when the tubes are not installed and will pull down to near 103V when they are.

If these voltages are satisfactory, then power down, allow the caps to bleed. Bleed TB+ with a 10k resistor to ground. Strap jumpers J1 and J2, but not J3.

WARNING - 100VDC is significant high voltage. Every precaution for handling HV should be exercised.


This section assumes that you have wired the ε12 circuit along with the buffers. Do not connect headphones.

It should not be necessary to preset the O/P quiescent current trimpots, P2L and P2R because the biasing won't allow the O/P devices to draw too much current. But you should, nonetheless, try to do this step quickly.

  • Place the DMM leads across R10L with the scale set to 3V.
  • Power up and adjust P2L to set the idle current.
  • Use this forumla to calculate the current Iq = VR10 / 2.2.
  • For example, if you measure 220mV the Iq is approximately 100mA.
  • Power off and let the PS bleed down.
  • Move the DMM TO R10R and repeat the process with P2R to get the same idle current in both channels.
  • The design center idle current is 100mA, but you should have a wide adjustment range.
  • After you set the idle currents, power off and power on again and pay attention to the ε12 delay. You should hear the relay click in after about 10-30s, indicating that both buffers have stablized their DC output.


  • Bleed the HV supply using a 10k resistor from the TB+ test point to ground. Ensure that this voltage is below 10V before proceeding.
  • Strap J3
  • Set the P1P trimpot to its halfway position.
  • Place the DMM on 300V scale and attach to either TB+L or TB+R and ground.
  • Power up the amp.
  • Adjust the P1P trimpot slowly until the test point reads about 60V.
  • Power down.

Input Stage

  • Set the heater jumper HTR for the tubes you are using. See PS Schematic for the jumper diagram.
  • Insert the tubes.
  • Power up and verify that both tubes light properly. If you are using 6.3V tubes and they glow really bright and then die you have set them to the 12.6V heater setting.
  • If there is any smoke or smell of burning components power down and find out what component is having difficulty.
  • The voltages in the input stage are fairly low. In general there are few components that will be stressed enough to burn up.

Setting the Tail Current for the Input Stage

  • Set the DMM to 3V and attach across R6L. Adjust P1L until you get the desired current. The typical value is 2mA.
  • Use this formula: It = VR6 / 200.
  • For example, to get 2mA VR6 will about about 400mV.
  • Do the same for R6R using P1R.
  • Go back and measure TB+L and TB+R. These should be at least 60V. Higher is ok.
  • Measure TPL and TPR.
  • The difference between TB+L and TPL (or TB+R and TPR) should be approximately 19V. If it is you're finished.
  • If it isn't, go back and check the tail current in R6 to make sure that it is 2mA.
  • This voltage difference doesn't have to be exact. Approximately 19V is good enough.