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Old 15th April 2012, 05:31 AM   #31
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Many links to cutaway Sprague Atoms, see whats inside...
Sprague Atoms @ Mouser [Archive] - The Gear Page
Not bad, they cut up some scary fakes for comparison too.
But even the real Atoms seem to contain a much smaller
modern capacitor inside..

I note prominently on the outside: 65C. - Six Five See!
Atoms might be the right shape and size, but I would not
be caught dead using a low temp cap in that application.
Too hot for 65C cap just sitting in my car in the summer.

We can hope the temp printed on the cover is for show,
as those are a legacy style for restorations. The real cap
inside might be a much higher temperature grade? But
we have no way of proving that assumption....

I siliconed modern Nichicon 105C caps into the original
Mallory paper tubes, for reason of the mechanical size.
At least I can guess reasonably sure what I hid inside.
Fakes inside of fakes is a possibility too, I suppose...

Last edited by kenpeter; 15th April 2012 at 06:00 AM.
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Old 16th April 2012, 01:47 PM   #32
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what would happen if i replaced the 50uf/ 50v atom with a f&t 47uf/50v cap
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Old 16th April 2012, 02:07 PM   #33
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Why suggest a weak 50V part to replace one that has failed,
when we have both measured a higher voltage at this point?

Last edited by kenpeter; 16th April 2012 at 02:11 PM.
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Old 16th April 2012, 02:47 PM   #34
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well someone said using a higher voltage might raise the bias volts. or wouldn't this bother anything? this is new to me i am more musician than amp tech.
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Old 16th April 2012, 02:52 PM   #35
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Quote:
Originally Posted by cowboy 1 View Post
what would happen if i replaced the 50uf/ 50v atom with a f&t 47uf/50v cap
Electrolytic caps typically have a tolerance of +/- 20% or more. Back when that amp was made +100/-50% tolerances were typical. IOW, there is no meaningful difference between 47 and 50uF, except that 50 is not a modern, standard value. The only reason to use those Sprague caps is if nothing else has long enough leads. It is their only favorable characteristic. The F&T is a much better cap.

I would definitely use a larger voltage rating; minimum 63V, better 80 and almost everybody in the biz just puts in a 100uF @ 100V. The only real constraint on the value is that the bias supply must charge up as fast or faster than the heaters and B+. 100uF is not a problem, more than that is, so don't go bigger unless you're religious about that standby switch.

During installation, remember the bias supply is negative and the positive side of that cap goes to ground. You can measure bias volts at pin 5 of the power tubes, use the balance pot to get them equal (or near 'nuff). Do this on standby without tubes. Then hit the switch and see that they are still equal and in that amp pretty close to where they were on s'by.

Much of your testing can be, and since you're having problems, should be done with a 100W lamp in series.

As previously mentioned, there's no reason to use a temp rating under 105C in a tube amp, You can often get away with 85C, but why ....
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Old 16th April 2012, 02:52 PM   #36
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can i go to a 100v cap, its hard to find one in the right values. then not knowing if blue atoms are real or fakes where can one go?
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Old 16th April 2012, 03:01 PM   #37
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Yes you can go to a hundred volt cap.

Don't worry about getting blue atoms, unless your trying to do a full out concours restoration.
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Old 16th April 2012, 03:20 PM   #38
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ok thanks guy's
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Old 16th April 2012, 10:38 PM   #39
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Quote:
Originally Posted by cowboy 1 View Post
well someone said using a higher voltage might raise the bias volts. or wouldn't this bother anything? this is new to me i am more musician than amp tech.
Higher uF might become a problem if taken too far.
Anything up to 100uF would not be "too far" in my
opinion. Bigger values will provide a smoother bias.
But this negative bias has reach full charge faster
than the High Voltage supplied to the plates. Bigger
caps charge slower, so yeah, you can go too far.

The next common value above 50 is 68uF. I tried
66uF (as a pair of parallel 33's) and worked fine.
I had no good reason to use 33's, except that it
saved me from waiting on parts I did not have.
The worst offence, merely that is does not look
original to use two smaller radial caps where one
medium sided axial type is expected. Does not
cause any electrical problem...

Higher Volts and higher temp can only be good.
They do not affect the sound or operating point,
only living much longer under harsh conditions...

You can't take reliability too far, except price.
Any simple device that's expensive and popular
becomes target for fakers. And fake parts are
the opposite of reliable.

Last edited by kenpeter; 16th April 2012 at 11:00 PM.
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