I see that you begin to understand that we should not have sophisticated measuring instruments for doing good work in the project, construction and repair of tube guitar amplifiers.
At the time I was making guitar amplifiers of my own projects (my favorite tubes were EF86 as preamp, ECC83 for baxendale tone control, ECC82 as phase shifter and EL34 as push-pull output - rectifiers: I used BY100 silicon diodes), I had at my disposal that:
- A low frequency generator (sine + square wave)
NB: square wave is very useful to verify the stability of the amplifier.
- A model 8 Avometer
- A high impedance meter Hansen UV47
- An oscilloscope 5 Mhz Heathkit IO-12E
- An adjustable high voltage power supply (self-made)
- A variac
- A load resistance 8R 50W (self made).
I had not tube tester because it was unnecessary, we had commitment to quality tubes, they where not tested, I simply replaced them.
No need of distortion meter, the polarization of the final stage was ajusted for minimal anode quiescent current with no visible crossover distortion on the oscilloscope.
To repair and modification of old tube amplifiers, you need more of a capacitor tester (at the time, it was useless, capacitors were new)
A signal tracer can also be very useful.
And above of all, you must have a lot of knowledge, documentation and experience in the field.