• 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.

dBv in for full power out??

The old professional Vu meters had very tightly controlled electronic time constants, and very tightly controlled meter needle ballistics. All this meant that all the professional Vu meters responded to continuous sine waves the same . . .
And they responded the same to dynamic voice and dynamic musical instruments.
Here, for once, there was a standard.

The peak signal to a recording studio (magnetic tape or Shellac), or an AM station modulator, or an FM station modulator
has to be accurately controlled. The professional Vu meters helped them to do this.

Then, some manufacturers said we can build cheaper, less expensive Vu meters.
They read out correctly to a continuous sine wave.
But they did not read the same as the professional meters, when the signal was dynamic voice and musical instruments.
So much for quality!
 
Preamp measurements as promised for Kenwood Basic C1.
Maximum signal (input or output) that can be processed without distortion = 5.8 VRMS.
Volume knob position for 0 dB gain is about 6.2 (out of 10).
Volume knob position for +10 dB gain is about 7.7 (out of 10).
Volume knob position at 10 gives +15.6 dB of gain.

This is only the measurement for one preamp model, but I am assuming it is not that atypical. If others have similar measurements, please share.
The measurements were done at 1 kHz only with the tone controls set to flat.
Similar results were obtained with Tuner, CD, and Tape, inputs. Phono inputs were not measured.

Given the help from all of you and these measurements I like the guidance from Ray of 0 to +3 dBv input sensitivity for full output power from an amplifier.

Thanks all, and I hope this discussion is helpful to others building amps.