118 By-pass cap box - Printable Version
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118 By-pass cap box - hunter2115 - 09-12-2013
Looking at replacing the 4 by-pass caps in the box (part#46).
http://www.nostalgiaair.org/PagesByModel/125/M0013125.pdf
It contains three 1uF's and a 2uF. Parts list calls for electrolytics. Do they need to be electrolytics? Seems to me that mylars would be a better choice.
Any input?
RE: 118 By-pass cap box - Radioroslyn - 09-12-2013
I'd go with mylars. Shouldn't matter either way.
Terry
RE: 118 By-pass cap box - Mondial - 09-12-2013
I am also rebuilding the 118 multisection cap right now and mylars are the way to go, especially since several sections are used as RF bypasses. There is plenty of room inside to fit the tubular style mylars.
You can drill a tiny hole next to each of the cap terminals to pass the lead from the replacement cap and then solder it to the respective terminal. The lead from the common ground end can be passed out at the base under the clamp and connected there.
RE: 118 By-pass cap box - hunter2115 - 09-12-2013
I like the idea of using mylars- no need to replace in say, another 80 years??
They are of different voltages. What voltage caps are you using?
RE: 118 By-pass cap box - TA Forbes - 09-12-2013
Mylar caps get pretty big compared to electolytics for values larger than .5 uFd.
RE: 118 By-pass cap box - Mondial - 09-12-2013
I am using 400V caps because that is what happen to have. I would prefer 630V ones, but they are a bit more difficult to find in 1 uF ( I am using two in parallel for the 2 uF cap ). Actually, in circuit they will be only about 200 to 250V max across them in normal use.
During warm-up when the rectifier filament is hot but the remaining tubes heaters are still warming up, there will be momentarily higher voltages across some of the caps, but not significantly more than 400V.
Cap 28 ( rated 450V ) sees the most voltage because before the 6A7 warms up, the cap is connected directly to the B+ supply through 20K resistor 26 without any voltage drop from tube current. The remaining caps see less voltage because of voltage divider resistors.
RE: 118 By-pass cap box - BrendaAnnD - 09-12-2013
I've had no problem getting 1uf and even 2.2uf poly caps at 630 volts. I have a good stash of both of those values.
RE: 118 By-pass cap box - hunter2115 - 09-12-2013
I guess the only question then is:
Will three 1uf's and one 2uF 630v mylar caps fit in the can?
RE: 118 By-pass cap box - Mondial - 09-12-2013
Good question. I know five of these 1 uF 400V fit with plenty of room to spare.
http://www.ebay.com/itm/140318808659?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
RE: 118 By-pass cap box - BrendaAnnD - 09-12-2013
The new mylar (poly) caps are MUCH smaller than those old paper caps. You'll have enough room left in that can to go dancing.
RE: 118 By-pass cap box - jerryhawthorne - 09-13-2013
Of course you can go for the Solen Fast caps. Non polarized high quality, I get the low value mfd caps from AES.
Jerry