Don't ham radio people still use JFET's all the time ? For hobby use with discrete parts, is there a direct successor to JFET's ? How low do discrete MOSFET input capacitance's get ?
Well, sure, but hams aren't nearly as technical as you might like to think; most of them are, pick up a radio, plug in and start chatting. A few build kits, or even work from schematics -- passed on down over the decades, with often few if any changes; and of them, rarely changes based on core design principles. There
are some quite excellent engineers out there, license in hand, but they are few and far between -- as with any population, the 80/20 rule applies, and you need to apply it several times in succession to get down to the highly technical core.
I hope I'm not being unfair or anything; I had the same surprise many years ago, as I slowly realized some of my favorite tools (COIL.EXE comes to mind, and
http://hamwaves.com/antennas/inductance.html nowadays -- COIL is cited on this page, actually) are the exception rather than the norm, and in fact, not representative of the technical prowess of hams in general. That is, there are many highly-technical practitioners out there who are hams, but the converse is not true.
Another case in point, you also see 2N3866 quite often, which is also mostly obsolete; only Central is making them right now I think -- basically the zombie graveyard of parts too old to die, and you pay for it in kind (several bucks for what's little more than a 2N4401's worth of silicon in a can).
Discrete MOSFETs sadly don't get much smaller than this one,
https://www.digikey.com/en/products/detail/rohm-semiconductor/RUM001L02T2CL/4004581?s=N4IgTCBcDaIE4FcC2AGFBGEBdAvkA and it's still a pretty big device at 3.5 ohms; compared to say a 3N170's 200 ohms that's comparable to CD4000 output transistors, or, compare to 74HC's (and most CMOS devices today) 30-70 ohms.
("Big" in the sense of, you put enough MOSFET cells in parallel, you fill up the die reducing Rds(on) while increasing Q's and C's. A logic gate might be a single cell's worth, if even that. Good RF MOSFETs are painfully small; so small they're hard to use, even with responsible ESD practices in place!)
The confusion is fundamentally not about device availability, but what is providing that basic functionality anymore. The world has moved on, no one needs (or wants, in quantity) things like this -- there are better solutions now, integrated ones. "Better" might be a matter of opinion, but on the mass market, only one matters: cost.
Unfortunately, it doesn't help much with developing an individual understanding of the circuits within those ICs. But since you're just buying a few to play with, it's not a big cost in the grand scheme of things, even if it means buying old stock or salvaged parts. Kinda sad I guess, but it is what it is.
So, to wit -- hams don't use JFETs, they use MMICs, integrated analog front ends (even ye olde NE602 is long since obsolete, though there are similar parts still around), and integrated synth tuner-on-a-chip, or SDR (software defined radio) the whole way. At least, those hams that are either using these (whether they know it or not) in commercial equipment, or those specifically designing boards using them. Hence the popularity of SDR and high density digital modes in some bands these days, for example.
And to be clear, like, JFETs show up in all kinds of things still -- but by far the mass quantity of them is in ICs, for front ends (e.g., JFET-input amps) and bias circuits. There are some token applications still, like electrets as above, or some radio front-ends (CPH3910 for example is widely available and would do a fine job at AM BCB), or bespoke things like low-noise or high-impedance amplifiers, electrometers (probably mostly CMOS though?), or etc. custom stuff; but these are all low quantity examples, and only like automotive stuff is going to amount to any kind of production quantity (millions).
And JFETs are completely obsolete for analog switching, variable resistance, mixing, etc. Most of those applications went down even back in the 70s, when ICs came out to facilitate those functions. JFETs have been paradigm-leaped a few times over now, as analog discrete solutions (e.g. synth panels) gave way to analog integrated (op-amps, OTAs, BBDs, etc.), to digital integrated (FM synth), to wholly software-defined (e.g. a MIDI library + sound font does exponentially more than an electric piano of the 70s, maybe 80s too, ever could). Today, mixing is best done by IC (at least matched arrays, but proper precision multipliers are still available, if rather boutique), or again, by wholly digital signal chain; and switching is done by CMOS analog switches (or sometimes, opto-MOS relays).
I should be living near an electronics market, I'd walk down there and stop at a fresh food market aswell.
That would be quite lovely; and they'd likely have an assortment of, not only vintage capacitors to choose from, but a fine deli, fresh bread or pastry, perhaps a flower shop? Or a book store to sit down and peruse the technical section of?
Alas, those days are mostly over in the US (and, by extension, CA to a similar extent).
Tim