Customer had spent hours trying to find/fix a noise problem on their expensive microphone (just outside of warranty) so asked me to take a look, from the explained symptoms I suspected preamp valve noise, this microphone has the Abbey Road Studios REDD.47 preamp built in, and uses EF86 and E88CC valves.
Repeated the problem (intermittent hissing noise), so first checked the PSU to be sure nothing being introduced there, but all good. Very nicely manufactured unit, so valves next on the list.
The valves in the microphone were unmarked/unbranded so of unknown origin. The EF86 was replaced with a pin-compatible and equivalent (but longer in size) NOS E80F from Telefunken, and a NOS Golden Dragon E88CC, thankfully still fit.
The newly fitted valves tested fine, no more noise and a very lovely, sensitive mic worthy of the price tag IMHO!
Chandler only offer 90 day warranty on valves, and have a minimum service handling charge even if was under warranty (probably covering the cost of their valves!) so made more sense to install some arguably better valves.
And the customer is happy to be using the mic again without worrying about nasty noise!