Can’t comment on wear and tear. I updated my JD-9 with new V-Caps ODAM, Tele tubes, Burr Brown OPAMPS, damping foam pads and transformer shield. Made a considerable improvement after cap break in. Sad part is that I stream 95% of my listening.
Look up the Audiokarma "Official JD9 thread". It is a huge source of info for upgrade mods for the JD9. My JD9 II has upgraded caps, Burr Brown opamps, fuse, damping material and NOS RCA black plate tubes.
One of the good things about the JD9 is the excellent capacitance settings available for moving magnet carts. You can actually select a setting of zero if appropriate your cart. An example is a cart that has a capacitance spec of 100 picofarads. Well the phono cables will usually use up most or all of the "100" picofarads so, adding more with the preamp may not be desirable. A setting of zero could be the best choice in that case.
If one wants to go down the DIY Route, then taking time to learn the methods for testing the Components for their values will be the way to go.
Heat, Humidity and the Breakdown or aging of materials used for the the assembly are all factors usually considered. Even a Solder Joint has a limited life?
Have a look at Tutorials covering Basic Component Fault Finding.
As a strong suggestion and an option that was not available at the time of original ownership, as an addition to any work being undertaken.
Replacing the RCA's on Both Chassis (Phon' and Pre'), as well as the Cable Terminations, to a Low Eddy Design, is an expense will worthwhile of incurring. If done as a DIY activity the cost is not too much.
Additionally using a PC Triple C wire for the Signal Path within the Phon' can also add real time benefits to the sonic. Add the PC Triple C Wire to the Interconnects using the Low Eddy RCA's and the sonic might just jump to a night / day betterment.
Phon's I have been very used to experiencing and very impressed with, have been taken to another level with the above suggestion adopted for the Signal Path.
My wish today is to hear PC Triple C used for a Cart's Coil and a SUT, I have heard it in use for every other role where Wire is to be used, and its presence is always perceived as a betterment
First thing is if you've been running the same tubes for 20 years then they probably need replacement. There are lots of options for 12AX7s - I really liked the Sovtek 12AX7LPS. Low noise, readily available, sound great.
I did all my own upgrades on my JD-9, and they're easy to do. It's possible your output caps have deteriorated in 20 years, but they don't get stressed much in the JD-9 so they're likely fine. I replaced my OEM caps with Jupiter Beeswax and it made a noticeable improvement. Again, there are many choices.
Opamps might fail but don't wear out. I originally installed the Burr-Brown (OPA627) op amps and they were better than the OEM but not dramatically. The big change came when I bought 6 Burson V5i on sale; they were majorly better.
If you can solder, you can easily DIY all these changes. Good luck!
All triode tube units will present a capacitance at the input, if the gain stage is a simple common cathode topology. This is due to capacitance between the anode and grid, primarily, and to a lesser capacitance between the grid and cathode. This is called Miller capacitance. The 12AX7 has very high miller capacitance owing to its high gain factor. So add 50 to 100pF to whatever you think is cable capacitance. A cascode at the input avoids miller capacitance, but usually you won’t have a 12AX7 in a cascode. Transistors and op amps exhibit a similar phenomenon.
@armstrod just replaced my JD-9 Burr-Brown 637 OPA’s with a set of 6 Burson Single V5i’s. Definite all around improvement. Well worth the $115 at Parts Connexion. Appreciate the suggestion in the thread.