I quite often have Quad II amps to service and among the things I have to do is check the resistor values (circuit here
http://www.keith-snook.info/amplifie...-Schematic.pdf). All but two of them can simply be measured in situ provided you're prepared to wait a few seconds for the odd capacitor to charge up (and assuming those capacitors aren't leaky when cold).
But the feedback resistors R10 and R11 - a 100R and a 470R - form a delta with part of the output transformer secondary. Because this is pretty much short circuit to DC it's hard, in principle at least, to deduce the values of R10 and R11 from the three delta measurements. One of these is roughly zero and the other two are much the same (100R//470R = 82R5) revealing little about the indicidual values.
Fortunately, as long as we do measure close to 82R5 across the resistors, we're saved by three things - a) these low value carbon comp resistors rarely drift at all, b) if they do drift then they go high which so if we measure 82R5 it's never because one's gone high and one low and c) Quad took to fitting high-stab resistors to later amplifiers and these are even more stable than the low value carbon comps.
Cheers,
GJ