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Why are good capacitors so expensive, and why don't they make these any more?
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vk6zgo:

--- Quote from: Benta on December 31, 2023, 12:04:05 am ---A small note:
Polystyrene caps are history since 20+ years.. PS film hasn't been produced since then. NOS parts can be found.
PP is an excellent alternative, but prolly not for RF.

--- End quote ---

In earlier days Polystyrene was regarded as "just the duck's guts", & for low values that was pretty true, but as capacitance & voltage rating increased, they became somewhat inductive.

Back in the early 1970s we replaced two cruddy old Hunt's metallised paper 0.47uF capacitors which were part of the "Sub modulator" of a 1959 Marconi high level modulated TV transmitter with two nice, "high tech looking" Polystyrene caps of the same value.

When we put a video sweep signal through it, the result looked like a fish---a nice notch filter.

We had to get some other type, which ended up being two then brand new, but old tech, waxed paper caps, which worked without a hitch till the transmitter was retired in 1991.
vk6zgo:

--- Quote from: T3sl4co1l on December 30, 2023, 11:14:41 pm ---; tantalum are excellent timing / sampling capacitors for LF/DC purposes (well behaved within rated voltage range, lowish leakage); etc.
Tim

--- End quote ---

Back in the day, tants were regarded as the answer to relatively high capacitance value, along with small size problems.
My work made an "in house" "vertical synch detector" which was used with remote analog TV sites.
The Studio could turn the Tx on or off by inserting or removing the vertical synchs from the video signal.

As vertical rate was 50Hz, a RC network was used of the correct time constant, using a nice highish value bead ceramic.
With the first such installation, we started getting "no start" or "sudden switch off" alarms, so headed off on a road trip to the site.
Pulling the device out of the rack & testing it failed to show a fault, so back it went into service, where after around half an hour, it promptly turned the transmitter off.
Out on the bench, it worked perfectly, but with an oscilloscope monitoring various parts of the circuit, we noticed that heating it caused what looked like a radical change in capacitance, hence failure of the D.U.T, & cooling it with "canned cold" brought it back.
We replaced the tant with a hulking great "Polyester greencap" & the device was solid as a rock!


T3sl4co1l:
JFETs are mostly obsolete now, but you can play with them if you like. I haven't shopped for disc caps that small before, but I can say SMT chips are abundant, *shrug*.

Tim
Kleinstein:
Disc capacitors can be a bit hard to get, as they are getting less and less used. One may still get lucky and may even consider sources like Ebay for a sets. There is a resonable chance to get NOS instead of fakes as the prices are moderate and NOS can still be around, espeically outside China. The values around 100 pF (what one may need for audio) are still available at the major distributors.
Smaller, more hobby oriented shops may be an option too - though I don't know in America.

For the tiny values there is just no way around SMD, alone from the parasitic capacitance of the wires. A quick hack for a few pF an below are 2 parallel wires, cut to a suitable lenght (~ 10 mm/pF)  - however the typical PVC isolation is a rather poor dielectric.
Zero999:

--- Quote from: T3sl4co1l on December 31, 2023, 05:35:56 am ---JFETs are mostly obsolete now, but you can play with them if you like. I haven't shopped for disc caps that small before, but I can say SMT chips are abundant, *shrug*.

Tim

--- End quote ---
What are you talking about? Cut open any electret microphone capsule and you'll find a J-FET.
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