This concerns L150's, L150A's that I own, (and other models that I don't own anymore), but probably applies to most 70's JBL's with stock Mylar caps on mid and high pass circuits.
I know the general consensus is that metalized polypropylene caps like Solen, Dayton and the higher up films Mundorf, Clarity, all provide "better" (or different) sound when trying to upgrade, or replace, capacitors. I've tried those, tried Audio Cap theta film, they're OK, haven't tried teflon yet.
I have found after a lot of messing about, that I like the stock yellow Mylar polyester caps JBL used.
Between all these different caps out there, it's possible the ESR, or other measurements, are different, not just the TYPE of cap, so the ESR could also contribute to the sound, by maybe altering crossover points, impedance, etc., just guessing here. I do know that when I tried some of the new caps mentioned, the left to right balance evened out a bit more.
I just think the stock mylars have more tone, smoother--3 way system doesn't sound so much like 3 different speakers, rather the blend is much better. Sure they don't have the detail of some other caps, nor the extra "air" that seems to be added by some caps. But something about the tonality of Mylar sounds right to me. Seems like most all my LP collection sound better as opposed to maybe half sounding OK with "other" caps.
I'm concerned though, that over time, they can drift, not as drastic as an electrolytic, but still over time there might be some ESR drift? Or maybe even altered by previous owners cranking the crap out of 'em with low powered amps.
My question is, even if I had a tool to measure ESR, I don't know what they measured originally. So I have nothing to use as a guide. Yes I know I've heard the lower the better, but I'm not so sure that applies in every configuration. Not to say one should purposely buy a higher ESR cap, but rather should try to replace with the correct replacement cap as close as possible. I have heard some Mylar caps arrive from the factory with a bit higher ESR already.
I have found some new Carli caps, and some new Erse caps, both Mylar with the values I need.
Has anyone had experience with these for direct JBL replacements? Kinda getting tired of pulling the boards, and also would like to hear other experiences.