Vactrol @1mA
18.5k Average Resistance
44k High
4.4k Low
Sample Size: 20 Units
Chinese @ 1mA
4.6k Average Resistance
12k High
0.6k Low
Sample Size: 10 Units
Vactrol @ 0.1mA
3.9M Average Resistance
6.6M High
1.0M Low
Sample Size: 10 Units
Chinese @ 0.1mA
8.7k Average Resistance
14k High
2.4k Low
Sample Size: 10 Units
Chinese @ 0.01mA
34.9k Average Resistance
52k High
21.5k Low
Sample Size: 10 Units
So the Amazon Chinese ones have a far lower resistance range than the Vactrol ones that I bought from AES a number of years ago. The Chinese resistance range squats the 12AX7 down too much and makes it sound like the neon roach tremolo. It also makes the overall volume drop when heavy tremolo is engaged.
Three of the Chinese units never got below 11k no matter how much current I gave them. These would be useless in a trem-o-nator because there isn't enough variation in resistance.