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In Reply to: RE: Capacitor posted by rhoni on July 09, 2014 at 17:19:28
I am not used to seeing temperature ratings on film capacitors of any type, but perhaps I've never noticed the markings. (Even the Solens cap illustrated in Chip's photo shows no temperature rating.) Electrolytics are usually rated for temperature, but not film caps in my experience. I've used quite a few MIT RTX in the past and present, and I've never ever experienced a failure of any kind. Do you have reason to believe that the ambient temperature inside your amplifier will exceed 85 degrees C, even assuming that would eventually cause film capacitor failure? If the chassis is decently ventilated, and since the cap is used in the input or driver stage (not near the much hotter output tubes), I would suspect you have no need to be concerned, regardless of your choice.
Follow Ups:
I looked at Relcap's datasheet and it seems that polystyrene is good up to 85C but the voltage needs 50% derating. The polypropylene is good upto 105C without derating.
I wish i had access to an IR thermometer but i don't think the internal temp will be anywhere near 85C although all the cathode bypass electrolytics are 105C rated on the PCB. I run my vtl without the tube cage which helps quite a bit with the heat buildup.
What do you think of the RTX?
The reviews that i've read range from good to horrid. I do believed that polystyrene is a good dielectric.
Russian NOS FT3 teflon are cheap compared to other boutique teflon. However i dont think they will fit in my existing space. Otherwise i would go with the FT3.
MIT RTX used to be my favorite film cap, until teflon caps became available. The RTXs are not great sounding at first and require a very long break-in period, if you believe in that phenomenon. Sadly, I too have been inhibited from using the Russian teflons because of their gigantic size relative to the boutique teflon capacitor alternatives. Those Solens teflons mentioned by Chip47 may be a lot less expensive than REL or V-caps, however. I've never investigated them extensively. Whatever is the composition of the Mundorf Silver oil, Silver-Gold oil, etc, they sound good too, IMO of course. Perhaps at this point I would prefer the Mundorf to RTX. Where larger values are needed (e.g. > 0.47uF) I have used the Mundorf Silver Oil, just because the teflons (that fit) cost a fortune. But everyone will tell you a slightly different story.
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