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How many logic gates did the IBM 650 have? I'm used to measuring the complexity of a CPU by transistor count, but the 650 was a vacuum tube machine; the number of tubes would also be an interesting statistic.

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How many logic gates did the IBM 650 have?

It's a rather useless question. When is a gate a gate? Is a wired-OR a gate? Does a 38-input-OR, used to create a zero condition count as much as a two-input? Using a gate count does, if at all, only make sense for machines only build from diskrete gates.

I'm used to measuring the complexity of a CPU by transistor count,

Which as well isn't a great tool. More of a marketing tool - just follow the discussions that came up after the 6502 was disected by the Visual-6502 team :))

but the 650 was a vacuum tube machine; the number of tubes would also be an interesting statistic.

To some degree yes. Then again, in the strict sense, the number of tubes just tells how often a signal had to be inverted, as that's the only logic function where a tube is necessary and can't be replaced. In addition they serve (as two inverters) to build latches (today called flip-flops) and amplifiers for long lines or lines with multiple drains.

Now for the number: The only one I could get so far (from a friend with access to a still complete 650 arithmetic unit) is ~3000 tubes.

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    Re, "the number of tubes just tells how often a signal had to be inverted," It also gives you a rough idea of how many hours per week the machine is down for maintenance. Scheduled replacement of tubes on some of those old machines took some significant time.
    – Solomon Slow
    Commented Apr 16, 2019 at 12:50
  • @SolomonSlow LOL, yeah, you're right. Great addition :))
    – Raffzahn
    Commented Apr 16, 2019 at 15:18
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    While this thought is not pertinent, it came to mind while reading the answer. In the early 1960's, portable transistor radios had become common, and could easily be carried around. One marketing ploy was to include the radio's transistor count on the case. A kid could brag about his 9-transistor radio to his friend with only a 5-transistor radio. It turns out (according to what I heard) that some manufacturers, aware of the greater status given to higher transistor count radios, would wire-in extra transistors, but only connect two leads, making them diodes.
    – RichF
    Commented Apr 16, 2019 at 18:11
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    @RichF Well, yeah, I remember. More so as this is kind of relevant when it's about the so called transistor count in chips - many of what is counted aren't transistors but diodes.
    – Raffzahn
    Commented Apr 16, 2019 at 18:25
  • Even tube count isn't all that great. Tubes could have 1, 2 or even more separate active devices in the same envelope. Most vacuum tube computers made extensive use of dual triodes, in particular.
    – RETRAC
    Commented Nov 10, 2020 at 18:39

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