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Discussion Starter #1
hi
i did the iso tranny on my old pine amp- now i gotta do the filter caps, theyre a bit leaky, and the amp hums a lot.
now heres where my theoretical ignorance really shines- the stock ones are .05uf 600v- i cant yet locate this particular value at either my local electronics place, nor at the tube store. guy at nuttech handed me a pair of polypropylene caps at .047 600v and said theyd work- but i dont believe him and in any case theyd provide less filtering- whereas i think it would be wiser to up the cap values-
somebody once told me to go double at 200v- so will a .1uf 200v cap be suitable? i would assume that if 600v was stock then i should stay with that.but if a lower voltage value is cool i can get them here.
does it need to be electrolytic? i did see this-
http://thetubestore.com/ca-mo-01uf-600v.html
a search online is not even showing me any links to caps of the stock value-
and id like to avoid if at all possible buying online

any thoughts are well appreciated- thanks
 

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Yes keep them electrolytic, the .047 would probably work fine. If .05 is a must then you could series 2 - .1mF's to get the .05mF.
 

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but is the lower voltage safe?
 

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Discussion Starter #5
but is the lower voltage safe?

that is one of the questions i have- i really dont know- i dont think it would be, but perhaps that voltage was not necessary to begin with. trouble with these amps is they were thrown together with very cheap components.

once i inquired at the hoffman forum about them, before i think this forum existed- here is an excerpt of the response i got-
These are Hot Chassis amps. They are not legal today, and should not have been legal when they were made. They snuck-in under regulations written for radios, which do not have user-accessible electrical connections (radio waves go in, sound comes out). The power filter caps must be about useless. And they were undersized to begin with (these were the very cheapest amps on the market, no penny left un-squeezed). Read the values and go at least twice the uFd at 200V.

thanks paulS- unfortunately the .047 caps i have are 600v, but are polypropylene- orange drops, and i havent been able to find the .1 at 600v in electrolytic- the tubestore has them in the film in oil type. theyre electrolytics start at 20uf at 600v-
i really need to broaden my horizons and start buying online, but so far every attempt ive made at this is an odyssey of frustration-
im up for the day know so im gonna start doing some looking.
thanks guys.
 

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Discussion Starter #6
i just had a potentialy dangerous thought- the voltages should be double the supply voltage- if my supply voltage is 115v- then i assume that anything above 250v would be safe-:confused:
 

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Discussion Starter #7
ok ferget the voltage thing lol- i read that i can increase the capacitance substantially, but this will increase the sustain characteristics of the amp....:smile:
so ill have to see what value of electrolytics i can find at 600v locally.
 

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Hi,

0.05uF is too small to be a filter cap. That's a coupling cap.

If you have hum and the voltages are about OK, look to replacing te big power supply caps. They will be 10uF and up.

Cheers!
 

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Discussion Starter #9 (Edited)
lol- geek thanks- yu know i assumed these were filter caps



but then looking at pics of old radio restorations i found that the caps were in here



connecting on the underside here- two 50uf caps at 150v



that can is riveted in place and ill have to drill it out- too daunting of a task i think right now-
bah
lol im an idiot
 

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ok ferget the voltage thing lol- i read that i can increase the capacitance substantially, but this will increase the sustain characteristics of the amp....:smile:
so ill have to see what value of electrolytics i can find at 600v locally.
Fraser, the .05 caps are NOT electrolytics! They are coupling caps, quite a different animal. They are also not filters.

Filter caps are the caps used in the power supply after the rectifier that smooth out the rectified DC voltage so there's no horrible hum. In your case the filter caps are inside that can - 50 uf at 150 volts. There are two of them inside with a common negative (-) terminal. They are called electrolytic because they use a wet paste inside that's called an electrolyte.

They are made this way because it's a method of making big values of capacitance with high voltage ratings in a small package. If they were made of the same stuff as those .05 caps they'd be big as your head!

There's a general rule about caps. You can usually use something a bit bigger and you can ALWAYS use something with a higher voltage rating! The voltage rating is the highest safe voltage across the cap at which it will operate safely.

It's like a tire's maximum pressure rating. You run your tires nowhere near the highest safe rating and you don't even care unless some dork puts too much air in the tires. Anything under the max is fine and anything over the max means it will blow up.

Filter caps from the "golden years" typically had a tolerance of -20/+80%. So if the old ones were 50 uf you could use a 47 uf no problem. Just make sure they are rated for 150 volts or higher.

Your chances of finding a can style at Nutech are pretty slim. Cans are a real small sales volume type of part nowadays. Only tubeheads like us use them. It's cheaper and easier to buy individual caps and mount them on a solder tie strip or something under the chassis. It'll work the same. You can leave the old can in place to make it look original.

If you must have a can try http://www.thetubestore.com They're just down James St. near Burlington St. and although they normally take shipping orders on the 'Net they don't mind local guys coming in and paying cash. If you look on their site you'll find a JJ 50/50 at a much higher voltage rating. In the old days a higher voltage rating meant a cap that was probably physically too big but the ways they make them today have improved so much that the higher rated can is likely the same size or even a bit smaller. Make sure you get a clamp to mount it. You're gonna have to do a bit of metal work to put it in there.

As for the coupler caps, if your filters are rated at 150 volts then there can't be any voltages higher than that in your amp. Since it ran directly from the wall voltage with no transformer that could step the voltage up then the voltages will actually be somewhat less than 150 volts.

This means that the guy at Nutech was right. 200 volt caps were fine. Different types of caps are sold in standard voltages. Since anything higher is fine you just pick the next higher standard for your circuit, in this case 200v. Also, capacitance values go up in standard increments. Nowadays no one sells a .05 uf cap. The standard value is .047. Siince the typical tolerance of these type of caps is +/-10% .047 is close enough! Besides, the nature of a couplng cap means it's not at all critical. The value of the cap determines the amount of bass that can easily get through but for a coupler the rate of change is so small that you have to make changes of at least 10 x any value for even the best of bat ears to notice any difference.

Long winded again I know but I really think if you're gonna work on your own amps it's necessary and worthwhile!

Have fun!:smile:
 

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Discussion Starter #11
hi bill- thanks, long winded responses are best lol
where my trouble began was when i tried to find replacements for those coupler caps- the ones he gave me were correct, but i wasnt looking for those- i wanted the filter caps lol- and they didnt look like any filter caps-so when i looked at thetubestore, of course i found nothing that looked suitable- now i know that i shoulda had the whole thing apart before i started hunting down parts-
what ill do is exactly as you suggest- the guys at the tubestore are used to me wandering in off the street, and that jj can would be easiest for me, as i could remove the old one and install the new one in the same fashion- less chance for mistake- providing its pretty much a drop in and rewire as before kinda thing.
the only experience i have with amps is simply swapping parts and as with everything i do, the learning curve is more of a steep frustrating cliff.
what i did was reassembled the thing, and once i tied the new wiring in place and out of the way, the hum is reduced to a really insignificant amount-
i just sat and played it for an hour- and its actually pretty nice-

thanks bill-:food-smiley-004:
 
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