substituting capacitors

[timmy]

i recently priced some of the caps i need for my tube tester and crt tester and they are expensive and not in stock. so i have literally hundreds of electrolytics of all kinds of values all new so i was wondering if anyone knows if i can do the following,for a 450v 50uf i have a 450v 47uf and for a 450v 5uf i have 5- 450v 1uf in paralle and for 250v 20uf i have 2- 250v 10uf in paralle would this be acceptable for use in these testers as replacements. i dont think this cap would be close enough but ill ask you guys ,for a 0.1uf 1000v i have a 5.6nf 2000v metallized cap would this be close enough to use?i dont know how criticle these caps would have to be not to effect performance thanks

[stusnyder]

for a 450v 50uf i have a 450v 47uf ----thats not enough uf...alittle bigger would be ok

for a 450v 5uf i have 5- 450v 1uf in paralle -- Ummm I would'nt do that..

for 250v 20uf i have 2- 250v 10uf in paralle -- I would'nt do that either.

for a 0.1uf 1000v i have a 5.6nf --- that won't work, since 10,000 mmfd = .01 uf


I tend to replace exact values, or as close as possible.


For real good prices check out justradios.com they're in canada, so it takes about a week to get a order via their airmail option. They have nice kits of components, and are very reasonable compaired to mouser or digikey.

[timmy]

ok but if the numbers come out the same for the volts and uf what harm could it do, just curious... i have a short light showing dim with a good tube being tested so im thinking an electrolytic maybe leaking or shorted.

[stusnyder]

because when you put electrolytics in parallel, in theory it should work, in reality the caps can slightly differ from each other, and then you have a balance problem. Best to replace with the single proper cap. You can go a bit higher on uf and voltage, but never lower. Also I would suspect your tester if the short light showed up on every tube tried, but on just one, it's probley the tube.
That link I gave you has a good page all about capacitors and what and not what to do.
I usually check my tubes using 2 different testers.

[timmy]

well this tube is supposed to be a nos tube still in box and tested befor i got it and when i tested it i tapped it lightly and the light didnt flicker at all. but this short light is neon and its not lit bright it only looks like just alittle of one side lights but if you look at a neon bulb the whole thing lights up when powered full but this one seems to me its just getting alittle to glow like it is. i thought when changing caps the voltage should stay the same but the uf can be higher.

[stusnyder]

try some other tubes in it first. You might have a bum tube. It's suppose to be NOS and tested but do you know that for sure? On caps it's fine to go a bit higher on voltage and uf, but never lower.

[timmy]

well i just spent some time on checking resistors around the short light and found a few resistors way out of tolerance, got them and going to change them and see what happens, ill be back and thanks for the info. tim

[timmy]

changed resistors that were bad but still got the light and i am checking the same 6at6 tube thats in the tester and switched them around and still got the light i got a spare 6at6 but they both read the same and both have the light. as soon as the tube lights up thats when the short light comes on so i guess its caps now because there really is nothing else to change, theres not much in here at all. i thought it might have been because of testing the same tube thats in the circuit itself but there is a listing for this very tube, heater 6v sensitivity 90 socket 7d. i read on the DIY site someone had the same problem with the exact same tester and changed the caps and it worked fine no short light, so its on to caps now.

[timmy]

well so far i have it figured out thanks to some members but thanks anyway. waiting for some caps and see if this cures the problem,it should because theres nothing but caps and resistors.

[Reece]

As to electrolytic values, older ones used round numbers like 20, 30, 50 mfd. etc. These days you won't always find round numbers like that but numbers like 47 mfd. This is the modern equivalent for an old 50 mfd. electrolytic and will work fine. Truth is, the old caps and the new ones may have a 20% tolerance, so either one could be higher or lower than marked, anyway. On a new cap stick with the old voltage rating or higher, and a microfarad value close to the old one or a little higher.

[timmy]

i got 2 450v 50uf electrolytic to replace and did not get them as yet but i i have 1 450v 47uf but dont think i have another,oh well ill figure something out. well got the caps and the bk is good as new now, no more false short light.