Yes, that is exactly how you describe the process.
You adjust the 100R in order to kill the noise on the output. What I do is use
mili-voltmeter on the terminal of S2 and try to set the voltage in there under
3mV. I think anything above 20mV is auditable, Anything under 10mV is dead
silent. Another very important factor when you change the tube and try to compare
them is that they all will have slightly different gain. So here is what I do.
My tuner has 1kHz calibration tone, it is convenient and I do not need to bring
a generator or test CD).
1) Drive the
calibration tone and measure the AV at the speaker (I prefer to do it electrically not acoustically)
your procedures to replace the tubes
3) Drive the
calibration tone with the new tube and measure the SAME AV at the speaker. If
the AV not same then use the post OTP attenuator to make it the same.
There is some “kind” into it. Some tubes you do not mind to drive
louder (mostly dark tubes: PX25, RE604, YO186, 2A3 with
single plate for
instance) and some you would like to run softer (type 45, 30, 2A3 etc…).
You will end up with whatever you prefer eventually. Be advised that the output
of Fundamentals, HF and Insertion channels will greatly impact how you would
like to hear your MF.