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A DC-Coupled Tube Amplifier With Futterman Output Stage for Dynamic Headphones

A DC-Coupled Tube Amplifier With Futterman Output Stage for Dynamic Headphones

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.
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).

If you use the output with the cap there is no need to adjust anything at all afterwards. The DC output however drifts away around several tens of millovolts and needs to be used carefully. I mainly used this output to check the sound during short periods, not recommended for using ‘blindly’ at any time.

I did not use a volume pot in the amp itself. I use a separate tube pre-amp to do the job. Adding a volume pot into the amp itself is of course no problem. I would use an Alps 100k dual pot (a blue one, around 25 dollars); these work very fine.

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