So I finally got my hands on a decent-ish signal generator. A few online reviews of this device haven't been particularly kind to it, but the shortcomings discovered are well outside the range that I'll be using it at anyway. For my purposes it's 5-stars excellent.
This is the device, an AliExpress-special, it's called a "MHS-5200A":
It hits all the high notes the previous two signal generators I was using couldn't... so today I was able to test my response out to 100kHz.
Which revealed something extremely interesting... my volume control, which is simply an attenuator on the input, is having an effect on frequency response.
Not entirely surprising I suppose, given that the combined effect of the volume control and the capacitance of the valve would form an RC network.
So to recap, this is the relevant section of the circuit:
With a sig generator that will easily give 100kHz (and more) and at a nice 200mV RMS (which the previous couldn't) it means I can test the frequency response with the attenuator at maximum (equivalent to maximum volume on a finished amplifier) as well as with the signal being attenuated.
The test was taken from the output of the driver stage - in other words, the signal went in the initial gain stage, through the concertina, and then through the driver, before going into a X100 probe and then to the oscilloscope.
The result was interesting... and confirmed my suspicions. Volume controls come with hidden extra features. Check freq response graph below... I tested at 5kHz intervals from 1kHz up to 100kHz, so this graph doesn't contain any extrapolations.
So the green trace is the freq response out to 100kHz with the volume control set to maximum and a 200mV RMS input. I had a probe on the input as well and my spreadsheet normalized for slight variation in signal voltage on the input (verified as being the same whether connected to the amp or running open-circuit)
The red trace shows the response with the volume control at -10dB... so the signal is going through some carbon track.
Probably not enough to make an audible difference, but interesting and informative.
I'm also quite rather pleased that my naive circuit built using valves that are everyday quality rather than audiophile spec, managed to turn in such a good response with zero attenuation dialled in.