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Old 10-29-2020, 12:19 PM
Jon1967us Jon1967us is offline
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Replacing Mica caps with film or ceramic?

I have a number of Mica capacitors in sets that I am working on that are out of tolerance and a bit leaky. Since Mica caps are very expensive does it really make sense to go through the expenditure and try to source equivalent values and voltage ratings which can be upwards of 8 to $15 a piece, or with today’s manufacturing quality why not just use a film capacitor of the same value and rating?

I know that Mica caps present the highest level of stability, but honestly is this so critical in a tube set?

Has anyone simply swapped out a Mica cap for a film or ceramic?
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Old 10-29-2020, 01:16 PM
Electronic M's Avatar
Electronic M Electronic M is offline
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Modern NP0 ceramics are often as good as tube era micas. The big issue issue with ceramics is thermal drift and NP0 are usually stable. For caps 1000pF and higher use film. Look at the capacitance tolerance of the original part and get one with the same or lower tolerance.

Avoid changing tuner/IF micas unless there is significant leakage current.
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Old 10-30-2020, 12:56 AM
Jon1967us Jon1967us is offline
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No Micas in my tuner or IF circuits. Have you actually used the NP0's in place of Micas in your TVs?

Appears that Micas were the only cap available that could maintain stability at high voltage ratings - paper capacitors at the same rating would've been prohibitively large and not as stable in value at that rating. Sounds like they just didn't have the technology that made an alternative practical decades ago.

Looks like Vishay-Ceramite and Kemet make w ceramic with a C0G or NP0 substrate at smaller values, although I'm looking for higher than a KV rating (10pF 1.5kv for a Damper section), Mouser only has a 6KV version in stock. I hope using a cap 400% over spec won't produce it's own problems.

As far as the higher values like 4700 or 3900pF, those are fairly abundant at the high voltage ratings, such as the Wimas or Kemets.
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Old 10-30-2020, 08:59 AM
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I was lucky enough to get some NOS micas from a place I used to work at so if is fairly rare that I have to order ceramics to replace micas. But I do tend to go for NP0 or C0G when I have to buy replacements for micas. I've seen Banderson and others use NP0/C0G successfully. Sweep circuit caps will be more tolerant to more drifty ceramic temp coefficients than RF/IF/Video stages.

One thing I know from electrostatic deflection set collectors is ceramic caps with non-C0G/NP0 coefficients that are rated around 6KV and ran at 6KV will drop in capacitance. When you run a ceramic at close to it's rated voltage there's some piezo electric effect on the dielectric that drifts the capacitance. Having the voltage significantly overated as you propose should reduce that issue.
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