Right, although I've worked with TWTs, klystrons, etc. in terrestrial equipment and have some familiarity with them, I've only casually thought about the TWTs in the Voyagers and in space use generally.
Given the remarkable longevity of the Voyagers, what I'm surprised about is that there hasn't been much discussion about their componentry and why they've been so reliable. For example I've seen nothing written about the engineering involved in Voyagers' TWTs and why they have been so reliable.
For instance, what is the cathode material, barium, strontium, thorium oxide, etc. used in these TWTs? Was its selection criteria based on emitters with the lowest work function/highest emission at the lowest temperature with preservation of the heater life foremost in mind, and or was it based on oxides with highest ruggedness—least affected by cathode poisoning, etc. Discussions about Richardson's laws and cathode emitters is something I almost never come across these days let alone how they've played a role in the engineering of Voyagers' longevity.
Whilst component manufacturers consider these matters, terrestrial users generally don't, we just reach for replacement parts when components fail. Perhaps I'm just not reading the right material but given the remarkable performance of these spacecraft, I'm surprised we're not focusing on the science and engineering that's made that all possible.
No doubt those who're involved in space engineering are focused on these issues but it seems to me not much information has filtered down to even people like me who have some limited knowledge of the technology let alone the general science-reading public.
Using the Voyagers' history and notoriety would be an excellent way to interest students in the physics of TWTs not to mention the material science and the engineering used in their design and manufacture.
When one considers it, there's a lot of fascinating science and engineering involved in making this 'relic' from the vacuum tube era function and keeping it so.