Ideally you want the tops and bottoms of the 10Khz square wave to look like that in post #7. Depending on the make of your output transformers you may or may not be able to achieve that. One way you can dial it in is to put R13/R14 on a pot temporarily instead of a fixed resistor. Run the square wave through it and adjust the pot until you get the square wave tops to look as much like post #7 as you can. Measure the pot and then install the closest size fixed resistor of that value. This is somewhat easy to do for a well wound transformer, and quite a bit more difficult for one that exhibits more winding resonances or has other anomalies.
Also, because speakers represent complex loads, you need to test HF stability under different types of loading conditions. You want a stable amp under any type of speaker load you can throw at it (because you certainly don't want your amp to exhibit instability at certain frequencies when actual speakers are connected and playing). You would do these tests at 1 watt output:
Of course, and this is very important, measurements only tell some of the story. You need to listen to it for a while too and see if you like it. It usually takes me a week or so to really get a feel for how the amp sounds.
Also, because speakers represent complex loads, you need to test HF stability under different types of loading conditions. You want a stable amp under any type of speaker load you can throw at it (because you certainly don't want your amp to exhibit instability at certain frequencies when actual speakers are connected and playing). You would do these tests at 1 watt output:
- No speaker load
- Resistor only load representing the output tap impedance selected (ex: 8 ohm resistor on the 8 ohm tap)
- Paralleled cap with resistor load (varying sizes of capacitor from say .1 uF to 16 uF or so)
- Cap only load (varying cap sizes from .01 uF to 16 uF)
Of course, and this is very important, measurements only tell some of the story. You need to listen to it for a while too and see if you like it. It usually takes me a week or so to really get a feel for how the amp sounds.