[GrimJosef]

Microphony is an important issue but I can, perhaps, understand why valve tester manufacturers didn't include it as one of the standard tests in their machines (besides the fact that, as you say, most traditional testers put AC through the valves).

The basic problem must be "How can a meaningful number be assigned to the degree of microphony in a valve ?". Without a number it wouldn't be possible to say whether a given valve passed or failed the test.

Your anecdote about the DK valves is interesting. I'm not clear, though, whether the problem with the DK91 was that all of them were more microphonic than the DK32s, so the basic design needed changing, or was it that a fraction of the DK91s were fine but the rest were too microphonic to be usable ? If the latter then your tester would be handy for picking out quiet ones and using only those in sets where low-microphony was needed.

I've heard it said that one of the reasons for Mullard changing the design of the ECC83 from the early 'long plate' metalwork to the later types with shorter anodes was to reduce the microphony. This sounds plausible as, in general, larger structures are 'floppier' than smaller ones and their resonant frequencies are lower, bringing the problem into a part of the audio spectrum which it's more difficult to suppress with vibration isolators. Sadly some members of the guitar-playing community actually liked the microphonic nature of the earlier '83s and the long-plate ones can now fetch a premium price if only the seller can locate a buyer who likes their 'sound'.

Cheers,

GJ