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Old 16th December 2010, 04:26 AM   #1
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Default can cap - rectifier spacing

Looking for a rule of thumb on spacing. How far apart should a rectifier tube like 5AR4 and a can capacitor be to prevent shortening cap life? If you have to go closer is a simple metal plate between the two going to provide a decent heat shield?
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Old 16th December 2010, 04:34 AM   #2
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Many older units had a metal shield with an asbestos sheet glued to it, between the tube and capacitor. Metal alone isn't too good.
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Old 16th December 2010, 10:01 AM   #3
DF96 is offline DF96  England
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If you put metal next to a hot valve, blacken the side facing the valve so that it absorbs heat and does not reflect it back into the valve. Keep the side facing the capacitor shiny, so that it does not radiate too much. Ensure there is a good heat conduction path into the chassis (to keep the metal cool) - but then you have to keep the conducted heat away from the cap.

A bit of space is better! Just make sure the leads to the rectifier don't spray buzz around from magnetic induction.
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Old 17th December 2010, 12:08 AM   #4
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I am guessing that 1/2" is not really adequate right? I am thinking of shooting for more like an inch or two and using cooling holes around the recto.
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Old 17th December 2010, 02:01 AM   #5
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How is a cylindrical object (the cap can) going to reflect heat (in any significant quantity) back to the source of heat (the rectifier) which itself is cylindrical and will itself reflect off a large proportion of any heat radiated from a line source toward it???

I think an inch (or even better, 25.4mm) would be ok, more is better. But more for the sake of the capacitor than the rectifier.
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Old 17th December 2010, 04:01 AM   #6
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The cap is indeed the concern. By putting the cooling holes it is hoped that the increased convection would draw the heat up and away from the cap.
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Old 17th December 2010, 11:12 AM   #7
DF96 is offline DF96  England
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Holes won't much affect radiation, which is significant for a power valve like a rectifier. They will reduce the glass temperature a little, which will reduce heat radiation from the glass itself but do nothing about the heat coming straight from the anode through the glass.

I wasn't talking about the cap can, but the metal screen. Incidentally, why do JJ cover their cans in black - about the worst possible thing to do. Do they assume we all use SS rectifiers?

I would leave a couple of inches gap as a minimum.
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Old 17th December 2010, 11:12 PM   #8
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Thanks, DF. Appreciate the help.
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Old 17th December 2010, 11:48 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by DF96 View Post
Holes won't much affect radiation, which is significant for a power valve like a rectifier. They will reduce the glass temperature a little, which will reduce heat radiation from the glass itself but do nothing about the heat coming straight from the anode through the glass.

<snip>

I would leave a couple of inches gap as a minimum.
I'll second all of that - for me 2" is the minimum for GT style envelopes, for bigger G types I generally allow even more. The components inside doing all of that radiating are generally in a well defined plane and do a better job of heating things nearby than you would think from the shape of the envelope.
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Last edited by kevinkr; 17th December 2010 at 11:52 PM.
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Old 18th December 2010, 02:08 AM   #10
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Quote:
Originally Posted by kevinkr View Post
I'll second all of that - for me 2" is the minimum for GT style envelopes, for bigger G types I generally allow even more. The components inside doing all of that radiating are generally in a well defined plane and do a better job of heating things nearby than you would think from the shape of the envelope.
yeah, I am with Kevin. 1/2" is definitely not enough. 2" would be good.

holes in the chassis around the rectifier only help if there is cool air down there. If there are power resistors down there then you have just made the problem worse. Helps for the resistors though.
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