The photomultiplier tubes were the most common type; if I recall correctly, a RCA 931A or Hamamatsu equivalent:
https://www.nonstopsystems.com/radio...etfax-931A.pdf
The Hamamatsu's were less noisy out of the box if you did not expose them to office lighting (even without power). We never did experiments to determine if the performance degradation was due to gain reduction, and also did not experiment to see if the performance would be restored by letting the tubes rest in the dark.