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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 9th Apr 2019, 10:34 pm   #1
ColinK!
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Default Replacement Caps

Being new to rebuilding valve radios, I'm looking for a little advice about replacement caps'. I'm wondering where a direct value replacement isn't available, more often the case these days, is it better to go up or down to the next nearest value.

I'm currently rebuilding a Murphy TA160 and have found replacements for some of the cap's, but just as an example I'm wondering about C56 a 4uf electrolytic, do I go down to 3.3uf or up to 4.7uf ? Being that the paper caps are, Im guessing nearly 70 years old, I'm replacing them too, but again have the same issues with nearest values, up or down ?

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Old 9th Apr 2019, 10:54 pm   #2
bikerhifinut
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Default Re: Replacement Caps

It probably doesn't make any real difference, I'm assuming also that the capacitors used may well have been 20% tolerance so the values may well be close to either of your alternatives.

A.
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Old 10th Apr 2019, 7:30 am   #3
Diabolical Artificer
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Default Re: Replacement Caps

Up, electrolytic caps often have a -10%/+20% tolerance, from that it makes more sense to go up, but it doesn't really matter in most applications.

Andy.
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Old 10th Apr 2019, 8:31 am   #4
Lucien Nunes
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Default Re: Replacement Caps

Up, generally. In many applications, such as HT decoupling and signal coupling, the capacitor simply has to be large enough to do the job. For economy the manufacturers will have used the smallest value meeting the requirements. A significant increase won't make any difference except size and cost, whereas a significant decrease might affect performance. The main exception is the HT reservoir, where using a vastly oversize cap can stress the rectifier.

In tuned circuits the actual value is critical, however the original caps are more likely to be mica or ceramic and probably won't need replacing anyway.
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Old 14th Apr 2019, 12:39 am   #5
ColinK!
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Default Re: Replacement Caps

Many thanks for your replies. Feeling a little more confident now and have ordered a pile of Caps, so hopefully I can get on with replacing them over the coming long weekend. Was lucky with the HT cap, found the exact value replacement, 16uf, which I believe is the correct one for the specified rectifier valve UU6. Unfortunately the valve was missing when I acquired the set, so has some diodes and dropper resister fitted temporally until I can locate one, but may become permanent.
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