I've had an A-4 since 1973 (second owner) and it has three filters. Incredibly, it also has all original tubes. I only know that because the tubes are labeled "Collins" (made by RCA) and replacements wouldn't have that logo. It's been amazingly reliable. It was designed for lower line voltage (like 110VAC, and now we have 120VAC or a bit higher) and some Collins fans use means to reduce the voltage to what it was supposed to be, but I've never done that. I use it at least a few times a week. Incredibly, it's still very well calibrated (I've done zero with that) and if I dial it to 15.000 MHz when it's off and have an antenna connected and then turn it on, the first thing out of the speaker is WWV at dead zero-beat. Considering it's now 62 years old, I'd place that in the "amazing" category. Dial still tracks within 1 kHz across each band, and it's nearly impossible to overload. Probably as good as it is because nobody every screwed with it (it lacks the "golden screwdriver" problem) and it's always been in a controlled environment (never cold, never hot, never humid). The most interesting thing to me is it can easily "hear" anything the most modern gear can, and sometimes better. That's not "sensitivity" (which on HF doesn't matter a lot) but just good design. Of course, an R390A is every bit as good, but I hate the way they tune! My hand cramps up thinking about it (I had one and sold it 30 years ago...good RX but wow, not designed for "tuning around").