DC-Coupled Tube Amplifier

https://web.archive.org/web/20140223023023/http://headwize.com/?page_id=746

Biasing the Output Stage

The first 2 stages are auto biased. So the only variables are bias 1 and 2 of the output stage. Using an ECC88 the optimal bias for the minus pole is around 1.5 Volts leading to 15 mA of current in the lower half of the ecc88. So BIAS 2 = 1.5 Volt. For the plus pole the optimal bias is around 2 Volts leading to a quiescent current of 12 mA for the upper half of the ECC88. The difference in quiescent currents between plus and minus just makes the amplifier putting out 0 mV of offset at the output.

I’ve put in small 4.7 Ohm resistors in the SEPP where you can measure the idle current. Ideally you measure 55 mV and 70 mV over these resistors to get 12 mA for the upper half of the SEPP (that goes to the plus 90 Volt) and 15 mA for the lower half of the SEPP (that goes to the -90 Volt supply). The corresponding biases for the ECC88-halves are around -2 and -1.5 Volts respectively. The best way to go is to set up both biases to these voltages and tune one of the biases till you have 0 mV offset at the output. (Note: why the currents of upper and lower halves are not the same? Because if you try to do this you get several volts on the output, a characteristic of this Futterman solution. In a normal 8-Ohm Futterman amplifier this can not easily be seen because the phase-splitter draws almost no current compared to the big SEPP output tubes of the like of EL519′s).