SOHA II Tweaks

R13, R14, R15

The stock value for these resistors is 1MΩ. This is a safe value and will work for any value of HFE for a Q5=BC550C. But, it may be possible for many builders to use 1.5MΩ for R15, which will increase the Zi of the buffer to over 1MΩ. This increase will benefit the performance of the tube. To see if you can do this, measure the voltage at the output of each opamp. If this voltage is only a few volts positive or negative you can probably use the 1.5MΩ resistors. Do not use 1.5MΩ for R13 and R14.


R4 and Zeners

Some builders may want to try using 18V 1/2W zeners in the plate circuit of V1. Zeners do generate noise, but a low enough noise zener might work out.

There is an advantage to using zeners in the plate circuit in that the tail current will not affect the plate voltage of the first triode. This means that if you want to tweak the plate currents for sound you can do this without affecting the operating point of the first stage. With the resistor you must set the tail current once and leave it alone.


Small Signal Transistors

The BC550C/BC560C series are excellent small signal devices. But, you can substitute the 2N5086-2N5089 series providing that you rotate the devices 180 degrees. To use these transistors make the following substitutions:

Alternate Small Signal Transistors

Part # Standard Alternate
Q1, Q2 BC560C 2N5087
Q3, Q4, Q8 BC550C 2N5088
Q5 BC550C 2N5089

Warning: Just another note for you: Look at the pinouts on the PDF files before using the 2N series devices. Or you will be very unhappy.



Any 9 pin tube that has the same pinout as the 12AXX series (with pin 9 not used) can work in this amp. Total heater current draw is limited to 300mA. This means two 12.6V/150mA in parallel or two 6.3V/300mA in series.

The other limitation on the input stage tube is the operating point. The triodes must bias to a negative grid (positive cathode) condition for the low voltage, low current conditions. Not all tubes will operate here and they will need to be tested. To do this, insert the tube and let the amp warm up. Measure the voltage at the cathodes (pins 3 and 8) of each tube. If this voltage is >300mV, the tube is likely to work.

Switching Heater Voltages

The 2 x 3 onboard header can be jumpered to select 12.6V parallel or 6.3V series operation. It's also possible to wire a DPDT switch to the header pads instead. The wiring to the switch is identical to the pinouts of the header. See the PS Schematic for jumper information.

Higher Heater Currents

It is possible to operate at 600mA heater current. Three changes are necessary to do this:

  • T1 changes to 15-0-15/1.5A
  • Don't install R3P (or remove it if you are modifying the amp). Jumper its pads.
  • The heatsink for H- (heater regulator) must be increased in size to handle 4W power dissipation. There may not be room on the board for this, necessitating an off-board regulator/heatsink arrangement.