Is it OK for Can Type Capacitors to Get HOT?

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tablebeast

Well-known member
Joined
Feb 8, 2005
Messages
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Location
Forest City, NC, USA
I just bought an Eico ST-40 Tube Integrated Amp and its all original (even the tubes!) except for a couple of new 25uf-50v Spragues in the power section. I have had it set up for about four hours now and it sounds gorgeous. I heard this was a rather hot amp temperature wise, but I was just checking it out and the can capacitors seem really hot, not too hot to touch, but just waving my hands over the parts they are the hottest part of the amp. Is this normal? Usually I just use my ears to judge whether caps are good or not and these sound fine, but is this a potential hazard? How hot is too hot? I have no qualms about modifying this guy, I'm going to do a ton of stuff to it so that it fits in with my studio monitoring setup just right. But, I only want to change these 'lytics out if its dangerous. Thanks in advance for the advice.

Jesse
 
make sure they are not getting too much voltage - if they were replaced at some point and are not rated properly they can definitely heat up like that, and bad things will happen.

I would replace any caps that were not already replaced, especially psu caps and any in the direct audio path that are electrolytic... but people have opinions about cap replacement. Hot is not good though I can confidently say that.
 
25uf-50v Spragues in the power section.

What is their function in the circuit?

50V can electrolyics in a tube amp power section makes me VERY suspicious......
 
50V can electrolyics in a tube amp power section makes me VERY suspicious......

maybe cathode decoupling but 25uF seem a little low.


how old is this amp? if it have +10 years you should change the filters.
a schematic could be useful too.
 
not enought information

Are the caps getting hot because they are next to the tubes

What is the temp? What is hot?

50 volt caps in the power supply of a tube amp? are the fils DC? IOf not then are they cathode bypass caps? trace the circuit it is a tube amp it should be easy.

Before you mod it look up its value. You might be better off selling it in close to stock form then building a tube amp with modern parts.

found this
http://www.triodeel.com/schindex.htm

and this
http://tech.groups.yahoo.com/group/eico/
 
The 25uf 50V Spragues are decoupling caps. I didn't mean to confuse anyone. Its not these guys that are the problem. The hot caps are the multisection filter cans, but checking out the amp further it appears that its just heating up because the chassis is hot as well. The whole thing is pretty hot. I think this is normal for this model so it appears that I was just being paranoid. As for modifying this guy, thats the reason I bought this specific model to begine with. Its not worth much more than the $200 I paid for it(the original Mullard and Westinghouse tubes in it are worth more than the amp actually), thats why I picked this one to buy and mod the shit out of. For mods I am going to remove all the channel/stereo mode switching, EQ, loudness and filter switches plus replace the RCA jacks with 1/4". I'm also going to install proper power / standby switches and a grounded AC cord (I shocked myself twice already hooking up the audio cables!) Anyway, I'm not too worried about the caps any more, all seems normal. I'm probably just going to leave the bulk of the electronics alone.
 
I have never seen a can get hot.
If it's capacity is down, then ripple current might heat it up, but it would reallty have to be dried out.

If there is a problem with a rectifier, then maybe half of a pure ac wave might be hitting it, but if this were the case, I do not think the amp would sound that good.
It would probably have a terrible ac hum coming out of it.

But get a cap checker and test that thing if you can.

If the cap is near tubes, you might consider putting one of those cardboard insulator tubes over it, maybe diy it from something around the house.
\
Heat is never good for lytics.


-
 
I've seen failing cans get real hot. They can explode not too long after that if you let them go and they're bad enough. There's a great photo from the 1930's of a can cap exploding to illustrate the eye-gouging potential. Looks like a couple of M-80's going off.

It's probably the whole chassis heating up though. Check the resistance of the caps (should be at least 1M; hopefully much higher) and the capacitance after they are turned off and drained.
 

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