What about different cores for better cooling?

Magnet wire resistors, enough to make any engineer scream.
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ottoamps
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Joined: Mon Feb 20, 2006 3:06 am

What about different cores for better cooling?

Post by ottoamps »

Hey Dave and guys,
I may as usual be showing my ignorance here. Not that I'll be able to afford any of Dave's products anytime soon, but I like to think about new things, and this seems promising to me. Wouldn't a heat conductive core, one that may even exit the top of the chassis to create a stack make sense to help these things perform better? I imagine different dielectric properties and/or permeabilty may change the way the resistors function too much, either too much inductance or capacitance. Glass or copper would certainly help to dissapate heat though.
If we wanted to get real fancy you could even mount a heatsink to one end, or maybe convection liquid cooling through a loop.
Whatya think?
Edu-ma-cate me...

ps. just occurs to me a "standard" model could be immersed in non-conductive coolant.
dave slagle
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Post by dave slagle »

Hey,
Wouldn't a heat conductive core, one that may even exit the top of the chassis to create a stack make sense to help these things perform better?
Sure, I have toyed with using copper, but anything conductive will make a nice capacitor to boot. Not that these things are perfect by any means, but why risk it.
Glass or copper would certainly help to dissipate heat though.
When it comes down to it, these things do not heat that much. Even run at 250 circ mils per amp the current only ups the temperature a few percent, so I suspect radiant heating from the tubes will cause more of a change.
If we wanted to get real fancy you could even mount a heatsink to one end, or maybe convection liquid cooling through a loop.
Ideally you would want the temperature to stabilize, My hunch is the inner temperature of the chassis is fairly consistent so you just need ot plan for that to be the operating temperature of the resistor. (not saying that is an easy thing to spec)

dave
DowdyLama
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quasi-scientific hypothesis

Post by DowdyLama »

> When it comes down to it, these things do not heat that much.

Dr. Dave is correct, but, FWIW: I'm beginning to suspect that his uber-cool cathode resistors perform best drawing smallish amounts of current. Again, I'm merely proposing a quasi-scientific hypothesis here.

I've only compared in the cathode of the the 3C24/25T [6 mA] vs the 75TH [24 mA]...both tubes used as A1 drivers with a B+ of approx 375 vdc.
Without a doubt, the Intact resistors sound better than Mills MRA-12s in both cases - however, the difference between the sound in the 3C24's cathode is HUGE relative to the difference I notice in the 75TH.

I've experimented quite a bit with the 75TH in this set-up; and I preferred it drawing 23-24 mA with the Mills resistors in the cathode; but with the Intact resistors, I thought it sounded better drawing 18 mA...and, I'm convinced this is not a headroom issue.

Obviously, the 3C24 has a much cooler cathode to begin with, but has anyone else experimented with this?
I haven't actually taken any under-chassis temp measurements - it's definitely hotter under the chassis with the 'THs going, but I've felt the resistors and they don't feel significantly hotter.
ottoamps
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Post by ottoamps »

Are you still running these bypassed Jim?
Just thinking still along thermal lines, and while the average resistance of the resistor may stabilize at a specific average temperature, if current is swinging through these the resistance will vary with the current as the windings rapidly heat then cool. Same thing as you get with CC resistors, but being copper windings I'd think it could be even more pronounced, especially if they impart a clarity to the amp through the lack of noise. The Teflon doesn't impart much in the way of temperature ballasting, like the cement in a standard wire-wound would.
If you're bypassed though, current variation should be pretty minimal.
DowdyLama
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Post by DowdyLama »

> Are you still running these bypassed Jim?
All of what I wrote involved cathode resistors that were bypassed - although one of the set-ups was only 90% bypassed [75TH drawing 24 mA].

> if they impart a clarity to the amp through the lack of noise
There's definitely an increase in clarity - it's one of those 'black backgrounds become even blacker' things...
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