Q: measuring thyristor chopped AC with clamp amperemeter?

Ahhh...What you need is for the current probe transformer to drive a scope. This means that the current transformer must be "terminated" and calibrated. What you will see then, are partial sine waves; a sharp step from zero current (at a time after line zero crossing) and the balance of the sine wave until it gets to zero. Depending onthe configuration, this "pulse" may be half-wave (only one half of the cycle) or full-wave (two pulses per cycle). The peak of that waveform will be the peak current, and an analog meter with a current clamp would read the average. A digital meter is more likely to miss the pulse or othewise "alias" the sampling. To make a current transformer for this, one can get a 50 amp filament transformer and use the secondary in series with the thyristor; the primary is loaded witha resistor as follows: Start with the voltage ratio of the transformer; say 240VAC in, 2.5VAC out) and run it backwards, 50 amps in for 500mA out ==> 1 ohm load would give 500mV for that 50 amps (fairly close and might be good enough for the first approximation).