Notice that the M7 has a polarizing voltage of 85V too. Also that heaters are at 11V8 rather than 12V6.
I have a few pages from a BBC training manual which gives a good overview of the valve era gear , especially the mic amps , it only contains simplified circuit diagrams , I'll try and dig up the reference numbers as theres a vast amount of non audio related stuff to wade through in the site .
Ive tried applying above 60 volts to a capsule I have and sure enough it starts to deform , the drill holes in the back plate become visible on the membrane as it colapses. its temporary as soon as the voltage stops the membrane unsticks itself .
An interesting example of undervolting, maybe the ME1400 would make a good candidate for a low noise preamp , good examples seem to be a little cheaper than regular EF37a's which are gone upto a saucy price .
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