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In Reply to: RE: Let's start with mica capacitors posted by sser2 on April 25, 2017 at 16:19:03
Older types
ФТ-1, ФТ-2, ФТ-3 (FT-1, FT-2, FT-3, Ftoroplast high Temperature). Same construction, but FT-1 have axial silver-plated wire leads; FT-2 and FT-3 have silver-plated brass tabs. Electrodes are aluminum foil, and insulation is either 2 layers (200 V) or 3 layers (600 V) of 20-micron film of Ftoroplast-4 (equivalent to DuPont Teflon). One layer of Ftoroplast is never used because it is impossible to produce film without spot irregularities. Capacitance range is 560-0.01 (FT-1), 0.027-0.047 (FT-2) and 0.01-0.68 (FT-3). Maximum temperature 200C. FT capacitors have considerable series inductance. Foils are connected to tabs or wires by thicker aluminum strips; these strips are riveted to tabs, but not welded or soldered to foils. For this reason, FT caps may not perform well at very low voltages, and, if used as coupling capacitors at low signal levels, need at least 10V of DC bias. FT capacitors are non-magnetic.
ФЧ (FCh) - Ftoroplast high frequency. Non-inductive winding, rated for RMS voltage at specified frequency, usually 5 or 10 kHz.
ФГТ-И (FGT-I) Ftoroplast Sealed in Tubular ceramic case, Impulse duty. These are high voltage (kilovolts) capacitors for pulse generators. High voltage rating is achieved by series connection of sections stacked inside the case, and filling the case with nitrogen pressurized at 6 atm. Large sizes preclude their tube DIY use, although there are claims that they are the best-sounding teflon caps.
Newer types
K72-8 Grounding capacitors. One of the leads is threaded for connecting to chassis. Aluminum body, non-magnetic, values in thousands of picofarads.
K72-9. I could not find any difference between this type and K72П-6, other than different manufacturing plants (K72-9 St. Petersburg, K72П-6 Novosibirsk).
K72-11 Kings of Russian teflon caps. Beer can size, steel case with axial screw terminals. The highest values are 4.7 uF 125 V and 2.2 uF 250 V. Some are rated for high frequency, and those may have the lowest tgδ (<0.0008 vs. 0.0015 of regular teflon capacitors).
K72П-3 (K72P-3) Feed-through capacitors designed for filtering off high frequencies. Due to their construction, have virtual zero series inductance and can pass frequencies in the gigahertz range. Body, which serves as grounding electrode, is bolted to the chassis.
K72П-6 (K72P-6) Similar to FT, but have tightly sealed steel body with glass insulators, and silver-plated steel connection pins. Internal leads are 999.9 silver, a capacitor may contain up to 0.45 grams of it.
Foils of all K72 capacitors are welded to leads, so there are no restrictions on minimum voltage.
Follow Ups:
There is a lot of confusion about Russian paper capacitors. For the most part it is due to great variety of types. Also, some capacitor types claimed by sellers to be paper-in-oil are not in fact paper in oil. Here are those that are true PIO:КБГ (KBG) types:
КБГ-МH (KBG-MN) Sealed in Normal metal case, foil electrodes
КБГ-МП (KBG-MP) Sealed in Flat metal case, foil electrodes
ОКБГ (OKBG) Special KBG, foil electrodes
КБГ-М1, M2 axial in metal cylindrical case with 1 or 2 insulated leads, foil electrodesCM (SM) Large transformer oil-filled capacitors, 3.5uF or 5uF
МБГЧ (MBGCh) Metallized paper for high frequency
K40У-9 PIO, foil electrodes
K42У-2 PIO, metallized paper. Technical sheet explicitly discourages their use for signal coupling
K75 PIO+mylar, most metallized, but some foil (K75-12)
Everything else is dry paper.
Dry paper with foil electrodes:
БМ (BM), БМT (BMT), БГ (BG), БГT (BGT), K40У-2, K40П, (K40P), K41.
Dry metallized paper:
MБМ (MBM), MБГ (MBG), MБГO (MBGO), MБГП (MBGP) MБГT (MBGT), MБГH (MBGN), MБГЦ (MBGTs). Everything that has "MБ" is metallized paper. Of newer types in this category is K42Ч-6.
Most paper caps have maximum temperature of 70C. Caps with letter T are good to 100C. Metallized paper capacitors have self-healing property: if insulation breaks down, metallization vaporizes locally, and insulation is restored. All metallized capacitors have a specific distortion called flicker. Metallization does not provide sheer metal layer, microscopically it is rather spots and patches with imperfect contacts between them. Voltage increase improves the contacts and effectively increases the area of metal electrode. Thus, capacity increases ( or "flickers") with voltage. The magnitude of this effect is not very strong, but sufficient to cause audible distortion.
Edits: 04/26/17 05/09/17
Thanks so much for the great info on the Russian caps. I've had a lot of experience with them, and cut quite a few apart, but definitely had some knowledge gaps that you helped fill. I'm copying these posts, and storing them on my hard drive for reference. BTW when referring to the OKBG "special" caps do you have any idea how they are special, and how they differ from the regular KBGs?
twystd
but could not find more specific information. "Special" might be versions to accommodate specific needs of certain end users, for example anti-corrosion paint. It might also mean deviation from ГОСТ (GOST, State Standard). GOST, which was country's manufacturing law, described specifications in minute detail. Any deviation from GOST required special approvals.
...Based on your description, FT series "Teflon" caps should probably not be used in critical audio apps (high inductance and contact potential issues/bias requirement). Considering these characteristics alone , I can't imagine a competent designer specifying these caps for an audio design and yet........they generally receive rave reviews from those that have used them and at least a few around here have a thriving upgrade business based in part on installation of these things in older equipment in spite of physical size. I wonder what's going on.
There is no contact potential issue in FT capacitors. Thermoelectric contact potentials, if any, in brass-silver-aluminum junctions cancel out due to symmetrical construction. The real issue is low pressure aluminum to aluminum contact between foils and contact strips. Aluminum is always covered with a thin layer of non-conductive oxide, making connection problematic at very low voltages.DC biasing is not a problem in most cases, but these capacitors should be especially avoided in circuits with very low signal level and little or no DC voltage, like in some phono stages.
An interesting aside on teflon caps is great technological difficulty of producing them. Teflon variety suitable for capacitors is microcrystalline in nature and very difficult to make into thin film. Even when technological difficulties are overcome, the resulting film has a lot of imperfections (microscopic holes) that further reduce the already low dielectric strength of the material. Film stretching during winding process exacerbates the defects. A lot of R&D went into these capacitors, and we are extremely lucky to be able to buy them at junkyard prices.
Edits: 04/26/17
nada aqui
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