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.
Quick question. What gain setting are you using your pre at? Also what is you system gain structure? Are you using a main preamp or are you attenuating the valve pre connected to the F4?