Sorry, I should have read Rob's procedure better myselfLittlewyan wrote: ↑Thu Oct 26, 2017 9:06 pm...So it says to measure the centre tap voltage and then plate voltages, but I'm sure I'm right in saying that you need a pretty precise meter for this?
I don't know why he say to do it like that.
He writes in step 2 You can also measure the voltage drop directly by putting your meter probes on the plate pin and center tap but this is much more dangerous but I can't see why he thinks it's more dangerous.
I suggest to directly measure the voltage across the resistances of the OT half primaries; use probe clips if concerned about regular probe tips slipping off the terminal and shorting across terminals.
The risk of probe tips slipping is greatly reduced if they're kept sharp; I use a whetstone.
Sharp probe tips are very beneficial in making quick accurate measurements with minimal risk of the probe slipping.
You charts look fine; the slight treble hump may be due to the low setting of the pre gain and the parasitic 'bright' cap across the control (that's why hifi amps don't have high value volume controls).
Your tests are good but it may be better to also use a volume control (eg 500k) on the output of your sig gen, as it's quick to just sweep the control to change the level and source impedance, and it may better replicate the intended use, eg perhaps the 'all controls at 10' set up would show a problem if the source was 5mV @ 10k impedance and there was an inductive load.