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 Post subject: Cooling tubes inside shields - please close
PostPosted: Nov Fri 18, 2016 10:55 pm 
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Joined: Jul Sat 23, 2011 9:33 pm
Posts: 792
Location: Long Beach Ms. USA 39560
There are seven pin and nine pin tube shields with corrugated metal inserts to cool tubes for longer life, but they are hard to find.
The regular plated tube shields can be painted black and probably will help reduce the tube temperature, but the layer of paint inside the shield becomes a barrier to heat transfer.
Bluing as used on old guns (sports department of local stores have it) will turn the shield black without creating a barrier to heat transfer. For a "pretty" appearance I use the bluing inside and out and then a light spray of black on the outside of the shield.
Pat

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Pat W5THT
Unhappy tubes blush while unhappy power FETs scatter plastic


Last edited by pehamel on Mar Wed 08, 2017 7:38 pm, edited 1 time in total.

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 Post subject: Re: Cooling tubes inside shields
PostPosted: Nov Wed 30, 2016 2:29 am 
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Location: Fort Mohave Arizona 86426
Any idea as to what the difference in life is between shielded and unshielded?


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 Post subject: Re: Cooling tubes inside shields
PostPosted: Nov Wed 30, 2016 2:55 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11544
Location: S. Dartmouth MA 02748-1225 USA
With a filament/cathode that runs at a bright yellow heat, what effect does cooling do on a miniature tube with glass to metal seals that were once molten? Save the socket? If a tube is running so hot as to need an IREC shield then it is time to review how hard it is driven...


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 Post subject: Re: Cooling tubes inside shields
PostPosted: Dec Thu 01, 2016 9:34 pm 
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Joined: Jan Tue 31, 2012 2:17 am
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Location: Hamilton, ON, CA
Although the thermal conductivity of paint is likely much greater than that of steel, I would doubt that its overall thermal resistance is of any significance considering its thickness of only a few thousands of an inch compared to that of the shield; and it would be effectively nil when compared to the air gap inside the shield. Direct contact tube shields endeavor to improve transfer heat to the tube shield by conduction from the glass, then followed by convection from the shield.


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 Post subject: Re: Cooling tubes inside shields
PostPosted: Dec Fri 02, 2016 5:55 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9107
Location: Santa Rosa, CA
Quote:
If a tube is running so hot as to need an IREC shield then it is time to review how hard it is driven...


I think these shields were intended for low-level tubes, like oscillators and pre-amps. Not for 6AQ5s.

Rich


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 Post subject: Re: Cooling tubes inside shields
PostPosted: Feb Wed 01, 2017 6:12 pm 
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Joined: Feb Sun 10, 2013 3:31 am
Posts: 43
It's the physics of black body radiation. Also, for an inverse example, consider a t-shirt, which does not have great thermal conductivity. On a sunny day, a black T-shirt vs. a white T-shirt heats the body inside much more. IERC tube shields also allow more convective heat removal.

Ted, KX4OM


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 Post subject: Re: Cooling tubes inside shields
PostPosted: Feb Sun 26, 2017 12:53 am 
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Joined: Mar Sat 28, 2009 8:27 pm
Posts: 691
Location: Georgia
That is the reason that auto radiators and transistor heat sinks are black. Some idiots put black roof shingles on the house down here in Georgia and have a 170 degree attic to contend with.


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 Post subject: Re: Cooling tubes inside shields
PostPosted: Mar Wed 08, 2017 7:37 pm 
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Joined: Jul Sat 23, 2011 9:33 pm
Posts: 792
Location: Long Beach Ms. USA 39560
Supposedly the 170 degree attic has no mould (too hot).

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Pat W5THT
Unhappy tubes blush while unhappy power FETs scatter plastic


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 Post subject: Re: Cooling tubes inside shields - please close
PostPosted: Sep Tue 26, 2017 10:00 pm 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 1869
Location: Thornhill, Ontario
I've noticed that 12DT8's and 12AT7's in FM front ends run very hot. It doesn't seem to harm them, but a decent heat sink/shield seems like a good idea.
Cheers,
Roger

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Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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