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Old 10th February 2011, 04:32 PM   #1
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Default Vibrating capacitor

Hello,

I've been slowly restoring an old console PP tube amplifier. I have no idea what make/model it is. Input tubes are 12ax7's, the output tubes are EL84's, rectifier is 5U4.

I replaced all the paper caps with orange drops and I got a major improvement in sound. Now I'm looking at the electrolytics and wondering if I should just leave them alone. There's two big metal can caps, each with multiple caps in them. One's a Sprague (40 mfd 450v, 60 mfd 450v) and the other is Sangamd (two 40 MFD 350v and two 50mfd 25v).

I can feel the Sangamd one vibrate when I put my finger on it, does this mean it's going bad or is that normal?
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Old 10th February 2011, 04:35 PM   #2
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A capacitor shouldn't vibrate. Are you sure its not a transformer vibrating, which although undesirable is quite normal.

Do you have access to an oscilloscope ? It might be worth looking at the ripple across the caps.

Are the caps visual or hidden within the chassis ?
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Old 10th February 2011, 04:47 PM   #3
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I can feel just the slightest vibration on the transformer, but that cap vibrates like crazy. I can't feel any vibration on the Sprague cap next to it and only a little vibration on the chassis, so I'm pretty convinced that specific capacitor is the source.

Both electrolytic caps are visible. If I do end up replacing one or both, I'd like to hide the replacements in the chassis and leave the old ones there for looks.

I do have an oscilloscope, I'll hook it up when I have a chance and see if anything looks unusual.
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Old 10th February 2011, 04:51 PM   #4
kevinkr is offline kevinkr  United States
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You should replace those electrolytics fairly immediately, they are if any equally or even more unreliable than the coupling caps you have already replaced.

The vibration hopefully is from the power transformer, if not it is a pretty good indication of internal stress in the capacitor and is a clear sign that it should be replaced. Is it getting hot?

Suitable replacements can usually be found at antique electronic supply. Depending on cost you might want to consider JJ types instead if you can make them fit physically speaking.
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Old 10th February 2011, 04:52 PM   #5
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If the caps were inside I was going to suggest changing them anyway.

If outside I was going to suggest opening them up and fitting donor caps inside the cans.

However, you seem to have another solution which is equally viable.

If the amp. is working there is probably not a lot wrong with the caps. Electrolytics do dry out in time and fail, however, this usually causes them to go short circuit and blow a transformer or a few diodes. If they dry out and go open circuit they usually cause the amp. to hum like crazy unless another cap. is keeping the ripple down in a CLC or CRC circuit.

I would certainly replace them anyway.
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Old 10th February 2011, 05:49 PM   #6
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Thanks for the advice. I guess I'll go ahead and replace them just to be safe. I also need to add a fuse and replace the power cable. And hopefully I'll figure out how to get rid of that "old electronics stink" it emits once it heats up.

How critical are the values? For some reason a 40uf cap is $20 but a 47uf one is only $2.
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Old 10th February 2011, 05:54 PM   #7
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Bigger is better.

150uF 450V caps are also peanuts these days.

Size (not value) is only important if you want to preserve the aesthetics of the amp.

If you look at the tolerance of caps they are generally +/- 50% so a bigger cap is going to make little difference.
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Old 10th February 2011, 05:54 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by blue lander View Post
Thanks for the advice. I guess I'll go ahead and replace them just to be safe. I also need to add a fuse and replace the power cable. And hopefully I'll figure out how to get rid of that "old electronics stink" it emits once it heats up.

How critical are the values? For some reason a 40uf cap is $20 but a 47uf one is only $2.

Depends on whether you have a tube rectifier or not - if not I'd go for the larger cap. Replacement multi-section caps are readily available and IMLE the JJs work pretty well and aren't very expensive.

The stink will go away with use, perhaps a good tube & chassis cleaning is in order as well. None of my tube electronics emit any smells unless something is amiss. (In the cases of restored stuff like my G36 MKIII, and TD-124/II they were stinky for a while, but this waned with use.)
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Old 10th February 2011, 06:33 PM   #9
Knarf is offline Knarf  Denmark
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Having the first cap after the rectifier hum or vibrate may also be due to a faulty rectifier, if the device uses an early semiconductor diode bridge. You want to check that one out, and possibly replace it if you intend to use the amp a lot. Early bridge SS rectifiers will frequently have become very lossy and unreliable with age, and will heat up excessively during use. This might not cause the amp to stop playing, so having sound is not an indication that everything is OK. This problem just lowers the B+ to some degree, at least up until the bridge shorts out and kills the mains transformer.

I restore vintage European tube radios as part of my tube habit, and generally I don't switch them on for even a brief test, unless I have replaced all paper and electrolytic capacitors, and given any SS rectifier a very solid looking over. Tube rectifiers are usually fine, of course. The large PSU electrolytics in cans I usually disconnect and leave in place for looks, and simply fit new ones out of sight below the chassis. Might be an alternative if you cannot find cans, which fits your chassis.

Just my $0.02 worth.

Frank.
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Last edited by Knarf; 10th February 2011 at 06:36 PM.
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Old 10th February 2011, 06:56 PM   #10
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Thanks, everyone. The Rectifier is a Dumont 5U4GB tube. I hadn't considered that it might be going bad. It looks like the max input capacitance for that tube is 40 uf...
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