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Old 8th September 2015, 01:41 PM   #1
6V6dude is offline 6V6dude  Australia
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Default Replacing caps 50uf with 47UF

I'm about to recap my electrolytic caps in tube amp. Original values are 50uf/25V, 25uf/10V, etc. Can't really get these odd values so plan to use
47uf/25V, 22uf/25V and so on. Slightly lower in capacitance but I figure the tolerances of the old caps would be much greater than that anyway, maybe 30%.
I thought I better ask for confirmation since everything else I've recapped in the past had "modern" values apart from filter caps on which I just go bit higher in mf if I don't have the exact value. I guess small difference in polyester caps values wont' make different either? Thanks for any feedback.

Last edited by 6V6dude; 8th September 2015 at 01:47 PM.
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Old 8th September 2015, 01:50 PM   #2
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Completely fine.
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Old 8th September 2015, 02:21 PM   #3
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Electrolytics now have +-20%. In the old days they were +20 -50% so standard values will be fine.
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Old 8th September 2015, 02:24 PM   #4
Mike P is offline Mike P  United Kingdom
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Quote:
Originally Posted by JonSnell Electronic View Post
Electrolytics now have +-20%. In the old days they were +20 -50% so standard values will be fine.
What he said. Electrolytics are very low tolerance so don't stress about it. If I'm faced with an unusual value I usually just go up to the next standard value.
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Old 8th September 2015, 09:14 PM   #5
Alkis is offline Alkis  Greece
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Why do you want to replace them?
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Old 9th September 2015, 11:13 AM   #6
6V6dude is offline 6V6dude  Australia
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It's 1963 Playmaster amp. They all leak. The amp still works fine but doesn't sound as good as my other one (same one but with radio). Seems to have slightly annoying midrange so I think it "might" be caused by the failing bias caps (oscilaton?). I haven't checked if the tubes are different brand than the other amp or how strong they are yet, but the caps need to be done. Here's the pic of chassis, all the blue caps are small electrolytic, plus will change filter ones as well. All leaking, amazing it still sounds very clean.
Click the image to open in full size.
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Old 9th September 2015, 03:21 PM   #7
Alkis is offline Alkis  Greece
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I'd check the tubes for low emission. A tube rolling with the tubes of the other amp is a good start and before that, you could measure bias voltage at the same check points in both amps.
And don't forget to check out the resistors. Allen Bradleys absorb moisture and by time they get higher resistance. Combine this with aging and you have an other schematic.
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Old 10th September 2015, 02:36 AM   #8
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6V6dude,

One other matter. On your photo, locate that grey wirewound resistor, sort of left of middle, mounted across the capacitor can. Move it away! That resistor might get pretty hot, and heat is a particular enemy of electrolytic capacitors. Check that particular cap. I cannot quite see, but if the rubber round the terminals is at all cracked or hard, replace (as you perhaps intend to anyway).

General capacitor info: One sometimes find small 6,3V electro's in equipment. Although modern capacitors are of far higher quality, I still never use a voltage rating lower than 16V. Perhaps in-grained bad experience from past low voltage caps having gone phut, but modern caps are also so much smaller.
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Old 10th September 2015, 10:27 AM   #9
6V6dude is offline 6V6dude  Australia
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Click the image to open in full size.
That cap near the grey resistor is 32uf/300V so I might relocate the new replacement if the resistor is getting hot, yes good point. As for checking the bias of the tubes in the other amp, that's a great idea. Will check the resistors yes. Some of mustard caps have spider cracks near ends but apparently these have no liquid inside to dry out so I'm not sure, what do you guys think? Leave them? Otherwise I might just as well replace everything :-)
Here is a pic of the other amp. Same circuit 4x 6GW8 PP with radio. Only a couple of years younger but all caps are original and not leaking or puffing, look like new. Only had one 12AT7 bias resistor that failed, that's all. Funny how different storage locations can affect internal parts so much.

Last edited by 6V6dude; 10th September 2015 at 10:30 AM.
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