WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
My AES catalog from 1994 lists the OPT-102 as a 25 Watt PP with a 6600 Ohm primary imp. However with your measurements I calculate that it is only 2592 Ohms! Primary volts divided by secondary volts, square that and then multiply by the secondary Ohms. 90/5=18, 18x18=324, 324x8=2592. If it is supposed to be 6600 Ohms, the voltage ratio should be approx. 28.7. Check your readings again.