Postby ClubAndCountry » 26 Oct 2009, 01:41
They are not reliability issues - like all JCM800s, they're built like tanks and almost never break.
But there are some performance issues, eg the bleed-through from the distortion channel into the clean - which affects some but not others. There are three different versions, which all look identical from the outside. The best is the earliest, which has less gain but a fatter tone, and also has a post-phase-inverter master volume, like the Vintage Modern. The gain only really kicks in right at the top of the turn on both the gain and channel volume, so some people think they aren't working right, but it's just the way they are. These were made from 1982 until 1984. The second version from 1984 to 1987 has more gain and a more even control range but a buzzier tone and is the one with the worst problem with bleed-though. The third version from 1987 to 1990 is similar-sounding and with the bleed problem cured. Some people like the sound of this one the best too, but I think it's the most solid-state-like and buzzy.
You can identify the versions if it's from 1984 or 1987 - open it up, and have a look for either a dual-gang master volume pot (1st version) or a switching chip (3rd version).
The only actual reliability issue I can think of with them is that the reverb valve (position varies depending on which version) is run at such high voltage that weaker types quite often fail, particularly Sovteks. They're also all getting a bit old now so you can probably expect electrolytic cap failures sooner or later, although mostly JCM800s aren't quite at that point yet. For some reason the type of pots they used seem prone to going badly crackly too, but you can fix that with some contact cleaner/lubricant (possibly more than one attempt!).