I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

yes

you have too much time on your hands

only joking honest

Ron

N Cook wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

The *heatsink* temperature won't be affected by the choice of insulator one tiny
bit.

You actually need to measure the *device* temperature to determine how effective
the heat transfer washers are. I don't know how well calibrated your fingertips
are !

Graham

On Sep 11, 10:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

Your fingertip response change is very steep in the area 32-33 C.
--
John

On Tue, 11 Sep 2007 16:29:38 +0100, "N Cook"
wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

Silicone pads are terrible thermally.

My finger is pretty well calibrated from 50 to 60C. I can stand 50C
forever, 60C for about one second, and interpolate between.

John

John Larkin wrote:

Silicone pads are terrible thermally.

The cheap silicone pads are terrible. There are however some very good silicone
pads too.

The white ones that the OP mentioned he had in another post are normally higher
performance than mica+grease.

Graham

On Tue, 11 Sep 2007 09:01:53 -0700, John Larkin
wrote:

On Tue, 11 Sep 2007 16:29:38 +0100, "N Cook"
wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

Silicone pads are terrible thermally.

My finger is pretty well calibrated from 50 to 60C. I can stand 50C
forever, 60C for about one second, and interpolate between.

John

My favorite coffee temperature


D from BC

Eeyore wrote in message
...


John Larkin wrote:

Silicone pads are terrible thermally.

The cheap silicone pads are terrible. There are however some very good
silicone
pads too.

The white ones that the OP mentioned he had in another post are normally
higher
performance than mica+grease.

Graham


Yes white ones
I'm wondering if they can chemically change over time and/or excessive
temperature , downgrading to be more of a thermal insuilator.
They are not discoloured or hardened or anything different in the
ex-compressed area by sight or flexing, just permanetly deformed , the
ruffling is permanent.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:

Eeyore wrote
John Larkin wrote:

Silicone pads are terrible thermally.

The cheap silicone pads are terrible. There are however some very good
silicone pads too.

The white ones that the OP mentioned he had in another post are normally
higher performance than mica+grease.


Yes white ones
I'm wondering if they can chemically change over time and/or excessive
temperature , downgrading to be more of a thermal insuilator.
They are not discoloured or hardened or anything different in the
ex-compressed area by sight or flexing, just permanetly deformed , the
ruffling is permanent.

Sounds normal to me.

Because they're quite soft, these silicone washers will flow slightly under
pressure and conform to the heatsink and device surfaces very well. This aids
the thermal transfer. It also explains the ruffled edges.

Graham

N Cook wrote:
Eeyore wrote in message
...

John Larkin wrote:

Silicone pads are terrible thermally.
The cheap silicone pads are terrible. There are however some very good
silicone
pads too.

The white ones that the OP mentioned he had in another post are normally
higher
performance than mica+grease.

Graham


Yes white ones
I'm wondering if they can chemically change over time and/or excessive
temperature , downgrading to be more of a thermal insuilator.
They are not discoloured or hardened or anything different in the
ex-compressed area by sight or flexing, just permanetly deformed , the
ruffling is permanent.

Is it possible that it's been caused by some kind of attack by a
chemical substance, silicon grease? Maybe some clown has sprayed
something like WD40 all over the inside. People really do that you know!

Ron(UK)

In article , "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

I didn't quite follow your entire post, but I would have used a IR thermometer
pointed on the devices, then the heatsink, then the room. I never thought
silicone was better than mica, except in the case of a poor fit, or warped
pieces. I have also tried to sand down surfaces that were not flush, and even
remove the oxide on aluminum, to optimize transfer.


grge

Meat Plow wrote:

I prefer silicon pads in high power amps both RF and audio as do quite a
few manufacturers.

When building stuff in the west, the reduction in labour with 'grease-free'
assembly is a not inconsiderable cost saving. It should pay for the extra cost of
a decent performance silicone pad.

Graham

On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.

The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).

N Cook wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices

What the heck *WAS* TOP66 ? I'd have thought that was a plastic TO-66 size
device.

I'm sure those devices are in some other package.such as TO247, TO264 (TO-3P)
whatever

Graham

On Tue, 11 Sep 2007 16:09:33 GMT, D from BC
wrote:

On Tue, 11 Sep 2007 09:01:53 -0700, John Larkin
wrote:

On Tue, 11 Sep 2007 16:29:38 +0100, "N Cook"
wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

Silicone pads are terrible thermally.

My finger is pretty well calibrated from 50 to 60C. I can stand 50C
forever, 60C for about one second, and interpolate between.

John

My favorite coffee temperature


D from BC

It's interesting that the tongue is the most temperature-sensitive
part of the human body, and the most resistant to temperature damage.

John

On Tue, 11 Sep 2007 16:29:38 +0100, N Cook wrote:

Any comments ?

Most people have known for decades that mica and good goop beats
those stupid silicone things hands down.

Cheers!
Rich

On Tue, 11 Sep 2007 12:57:23 -0700, John Larkin wrote:
On Tue, 11 Sep 2007 16:09:33 GMT, D from BC
On Tue, 11 Sep 2007 09:01:53 -0700, John Larkin
....
My finger is pretty well calibrated from 50 to 60C. I can stand 50C
forever, 60C for about one second, and interpolate between.

My favorite coffee temperature

It's interesting that the tongue is the most temperature-sensitive
part of the human body, and the most resistant to temperature damage.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


That's only because it's the wettest part that ever sees the outside. ;-)

I heard some standup comic talking about hot food - "It's too hot to
pick up with my hands, so I'm going to put it into my mouth?" ;-)

Ever blistered your palate with hot pizza? ;-)

Cheers!
Rich

On Tue, 11 Sep 2007 17:30:33 +0000, GregS wrote:
....
pieces. I have also tried to sand down surfaces that were not flush, and
even remove the oxide on aluminum, to optimize transfer.

Unless you're under oil or something, you _can't_ remove the oxide on
aluminum, because it so aggressively sucks up oxygen.

But, fortunately, aluminum oxide (especially in the thicknesses that it
spontaneously forms in air) is practically as good a heat conductor as
pure aluminum. :-)

And it's a much better electrical insulator. ;-D I've heard that
if you get the surface flat enough, and type III anodize it, that
you don't even need the mica.

Cheers!
Rich

On Tue, 11 Sep 2007 20:38:08 +0100, Eeyore wrote:
N Cook wrote:

I was not convinced that for an existing used amp with 4x TOP66 power output
devices

What the heck *WAS* TOP66 ? I'd have thought that was a plastic TO-66 size
device.

I'm sure those devices are in some other package.such as TO247, TO264 (TO-3P)
whatever


The way I had been taught, if you cut of the "C" lead of a TO-220,
and bend the "E" and "B" leads 90 degrees at the right point, it drops
right into a TO-66 spot, and a TO-247, with the same treatment, will
drop into a TO-3 spot.

Hope This Helps!
Rich

On Tue, 11 Sep 2007 21:29:02 GMT, Rich Grise wrote:

On Tue, 11 Sep 2007 16:29:38 +0100, N Cook wrote:

Any comments ?

Most people have known for decades that mica and good goop beats
those stupid silicone things hands down.

Cheers!
Rich


Yup; the silicone is too thick. And the vendors measure theta with
insane clamping forces applied, then lie by another 2:1 for good
measure.

John

"whit3rd" wrote in message
ups.com...
On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silicone pads were better than mica.

The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).



The interface between the device and the heatsink is probably the most critical
piece in the puzzle, and is the piece that a builder/repairer has most control
over. The device temperature is what you're trying to control. You have no
control over the internal thermal impedances of the device... they are
determined by the manufacturing process and materials.
Given a heatsink has adequate capacity for dissipating the heat (heatsink to air
thermal impedance), then that only leaves the device to heatsink interface to
manipulate. In comparison between greased mica and silpad insulators, the
mica/grease combination has the better thermal characteristics.
Mica can be problematic though, mainly because there seems to be no quality
control on the manufacturing process. The thickness of a mica slab can vary
from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per mil, so it
really doesn't need to be so thick. You can usually split a mica slab several
times and still be safe in using it. The thermal impedance goes down somewhat
with thinner slabs, too.

Overall, mica/grease can exhibit almost half the thermal impedance of a silpad.
Given the same device power dissipation, lower thermal impedances mean lower
device temperature.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra

DaveM wrote:

Overall, mica/grease can exhibit almost half the thermal impedance of a silpad.
Given the same device power dissipation, lower thermal impedances mean lower
device temperature.

And this beats mica+grease hands down.
http://uk.farnell.com/jsp/Semiconduc...jsp?sku=681120

0.2C/W

Graham

"Eeysore Gullible Fool"

And this beats mica+grease hands down.
http://uk.farnell.com/jsp/Semiconduc...jsp?sku=681120

0.2C/W


** The 0.2 figure is meaningless until you read how the maker derived it.

1. It applies to a special test rig with 1 square inch area.

2. The pressure used is a massive 200 psi.


As a TO3P pack has a metal tab area of only 0.36 sq inch and the mounting
pressure is more like 50 psi in practice, the 0.2 figure has to be de-rated
by about 6 times.

So it is really 1.2 C/W, at best.

Which thin mica and thermal grease will easily better.


Yaaaaaawnnn......




......... Phil

"John Larkin" wrote in message
...
On Tue, 11 Sep 2007 16:09:33 GMT, D from BC
wrote:

On Tue, 11 Sep 2007 09:01:53 -0700, John Larkin
wrote:

On Tue, 11 Sep 2007 16:29:38 +0100, "N Cook"
wrote:

I was not convinced that for an existing used amp with 4x TOP66 power
output
devices that the silicone pads were better than mica.
Each of the 4 insulating pads had shrunk about 5mm at the tops (hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.

I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz cointinuous sine giving
20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.

Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it
now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a
fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.

Any comments ?

Silicone pads are terrible thermally.

My finger is pretty well calibrated from 50 to 60C. I can stand 50C
forever, 60C for about one second, and interpolate between.

John

My favorite coffee temperature


D from BC

It's interesting that the tongue is the most temperature-sensitive
part of the human body, and the most resistant to temperature damage.

John

That being the case, why is it so easy to burn your tongue on a hot drink or
hot food ?

Arfa

Phil Allison wrote:

"Eeysore Gullible Fool"

And this beats mica+grease hands down.
http://uk.farnell.com/jsp/Semiconduc...jsp?sku=681120

0.2C/W

** The 0.2 figure is meaningless until you read how the maker derived it.

1. It applies to a special test rig with 1 square inch area.

2. The pressure used is a massive 200 psi.

As a TO3P pack has a metal tab area of only 0.36 sq inch and the mounting
pressure is more like 50 psi in practice, the 0.2 figure has to be de-rated
by about 6 times.

So it is really 1.2 C/W, at best.

Which thin mica and thermal grease will easily better.

Hey Phil,

I have used similar high-performance washers in some of my amplifier designs although I prefer to avoid washers ideally using 'live' heatsinks fwiw.

I can assure you that they absolutely do not have 1.2C/W thermal resistance in practice. It's trivially simple to do the measurements. In one design that
comes to mind using the 'rust' coloured washers from Warth, the thermal resistance was measured at ~ 0.3C/W.

Graham

"Eeysore MORONIC BLOODY Fool"


And this beats mica+grease hands down.


http://uk.farnell.com/jsp/Semiconduc...jsp?sku=681120


0.2C/W


** The 0.2 figure is meaningless until you read how the maker derived
it.

1. It applies to a special test rig with 1 square inch area.

2. The pressure used is a massive 200 psi.

As a TO3P pack has a metal tab area of only 0.36 sq inch and the mounting
pressure is more like 50 psi in practice, the 0.2 figure has to be
de-rated
by about 6 times.

So it is really 1.2 C/W, at best.

Which thin mica and thermal grease will easily better.


Hey Phil,



** Drop dead you PITA ****.

Anyone else can go read the damn Bergquist data sheet !!!





........ Phil

Phil Allison (the snipping twit) wrote:

"Eeysore MORONIC BLOODY Fool"

And this beats mica+grease hands down.


http://uk.farnell.com/jsp/Semiconduc...jsp?sku=681120


0.2C/W


** The 0.2 figure is meaningless until you read how the maker derived
it.

1. It applies to a special test rig with 1 square inch area.

2. The pressure used is a massive 200 psi.

As a TO3P pack has a metal tab area of only 0.36 sq inch and the mounting
pressure is more like 50 psi in practice, the 0.2 figure has to be
de-rated
by about 6 times.

So it is really 1.2 C/W, at best.

Which thin mica and thermal grease will easily better.


Hey Phil,

** Drop dead you PITA ****.

Drop DEAD you snipping MORON

" I have used similar high-performance washers in some of my amplifier designs although I prefer to avoid washers ideally using 'live' heatsinks fwiw.

I can assure you that they absolutely do not have 1.2C/W thermal resistance in practice. It's trivially simple to do the measurements. In one design that
comes to mind using the 'rust' coloured washers from Warth, the thermal resistance was measured at ~ 0.3C/W. "


Anyone else can go read the damn Bergquist data sheet !!!

I said I was using WARTH pads. And the figures I obtained were with normal device torquing.

" Thermal impedance range from 0.13 - 0.6°C-in/watt at 50 psi "
http://www.lairdtech.com/pages/produ...e-Overview.asp

" Thermal impedance of 0.13 range °C-in/watt at 20 psi clip force "
http://www.lairdtech.com/pages/products/T-gard-K52.asp

Graham

DaveM wrote in message
. ..
"whit3rd" wrote in message
ups.com...
On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power
output
devices that the silicone pads were better than mica.

The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).



The interface between the device and the heatsink is probably the most
critical
piece in the puzzle, and is the piece that a builder/repairer has most
control
over. The device temperature is what you're trying to control. You have
no
control over the internal thermal impedances of the device... they are
determined by the manufacturing process and materials.
Given a heatsink has adequate capacity for dissipating the heat (heatsink
to air
thermal impedance), then that only leaves the device to heatsink interface
to
manipulate. In comparison between greased mica and silpad insulators, the
mica/grease combination has the better thermal characteristics.
Mica can be problematic though, mainly because there seems to be no
quality
control on the manufacturing process. The thickness of a mica slab can
vary
from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per mil,
so it
really doesn't need to be so thick. You can usually split a mica slab
several
times and still be safe in using it. The thermal impedance goes down
somewhat
with thinner slabs, too.

Overall, mica/grease can exhibit almost half the thermal impedance of a
silpad.
Given the same device power dissipation, lower thermal impedances mean
lower
device temperature.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters
in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra



I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.
I've no reason to believe the original silpads had aged, been affected by
WD40 or anything.
I will assume they are , all manufacturers, all generically bad until a
similar personally conducted experiment, in a real situation, proves to me
to be otherwise.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:


I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.
I've no reason to believe the original silpads had aged, been affected by
WD40 or anything.
I will assume they are , all manufacturers, all generically bad until a
similar personally conducted experiment, in a real situation, proves to me
to be otherwise.

Exactly what is the item of equipment in question?

Ron(UK)

"Eeysore = ASININE, MORONIC BLOODY IDIOT



And this beats mica+grease hands down.

http://uk.farnell.com/jsp/Semiconduc...jsp?sku=681120


0.2C/W


** The 0.2 figure is meaningless until you read how the maker derived
it.

1. It applies to a special test rig with 1 square inch area.

2. The pressure used is a massive 200 psi.

As a TO3P pack has a metal tab area of only 0.36 sq inch and the
mounting
pressure is more like 50 psi in practice, the 0.2 figure has to be
de-rated by about 6 times.

So it is really 1.2 C/W, at best.

Which thin mica and thermal grease will easily better.


Hey Phil,


** Drop dead you PITA evil ****.


Anyone else can go read the damn Bergquist data sheet !!!


I said I was using WARTH pads.


** The Farnell link above that

** YOU ****ING POSTED **

is for BERGQUIST !!!


YOU STINKING ASININE CRIMINAL MORON






........ Phil

"N Cook" wrote in message
...
DaveM wrote in message
. ..
"whit3rd" wrote in message
ups.com...
On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66 power
output
devices that the silicone pads were better than mica.

The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).



The interface between the device and the heatsink is probably the most
critical
piece in the puzzle, and is the piece that a builder/repairer has most
control
over. The device temperature is what you're trying to control. You have
no
control over the internal thermal impedances of the device... they are
determined by the manufacturing process and materials.
Given a heatsink has adequate capacity for dissipating the heat (heatsink
to air
thermal impedance), then that only leaves the device to heatsink
interface
to
manipulate. In comparison between greased mica and silpad insulators,
the
mica/grease combination has the better thermal characteristics.
Mica can be problematic though, mainly because there seems to be no
quality
control on the manufacturing process. The thickness of a mica slab can
vary
from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per
mil,
so it
really doesn't need to be so thick. You can usually split a mica slab
several
times and still be safe in using it. The thermal impedance goes down
somewhat
with thinner slabs, too.

Overall, mica/grease can exhibit almost half the thermal impedance of a
silpad.
Given the same device power dissipation, lower thermal impedances mean
lower
device temperature.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters
in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra



I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.
I've no reason to believe the original silpads had aged, been affected by
WD40 or anything.
I will assume they are , all manufacturers, all generically bad until a
similar personally conducted experiment, in a real situation, proves to me
to be otherwise.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


Surely a degree of faith in what manufacturers say, has to be assumed,
otherwise we would all go around our whole lives with our heads up our
collective arses ! Do you for instance do a complete analysis of every piece
of processed food you eat to see if the manufacturer is lying about how much
salt it contains or how much calorific value it has ? Do you check your
bottles of beer to see if the ABV that the manufacturer quotes is true ? Do
you run a complete set of tests on a resistor type before stocking it as a
spare to see if what the manufacturer says about its tolerance stability is
correct ? No? Neither do I.

I can understand scepticism about 'waved in the air' figures that appear to
have little or no foundation in reality, but figures for these washers that
quote specifics on the applied pressure to get the results that are
published, don't seem to fall into that category to me. I can also
understand manufacturers of some products getting into 'mine's bigger than
yours' specmanship contests, but silicone heatsink pads ? Do you really
believe that these are such a big part of the manufacturer's total output of
products, that he needs to engage in the sort of behaviour that involves
lying, or at least distorting, the 'true' facts about the product, just to
sell a few more than the other guy ?

As it happens, I cannot remember seeing pads distorted in the way that you
describe, so maybe they were from a bad batch or something. Maybe, the amp
manufacturer torqued them up too much, and the heat did the rest. Maybe the
amp was used on midsummer's day in Ibiza at full volume with a blanket over
it, and a fault ...

I agree that many amps have output devices that appear to run hotter than we
might like, and that the thermal transfer across the insulators seems less
than ideal, but if that's the way the manufacturer made it and is happy
with, and the item is not on my bench with failed output transistors, then I
tend to just note the fact with interest, and work on using the "If it ain't
broke, don't fix it" principle.

And to Graham, I HATE amps that have live heatsinks. Just a few days ago, I
had a big AV amp on the bench that nearly threw me off my chair when I laid
the edge of my hand across the two heatsinks ready to take some measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...

Arfa

" Chairman of the CRIMINAL ****WITS "



** Go eat dog **** - you muther****ing nut case.






....... Phil

Phil Allison wrote:

"Eeysore = AMAZING GENIUS


I said I was using WARTH pads.

** The Farnell link above that

** YOU ****ING POSTED **

is for BERGQUIST !!!

Those Bergquist pads are white like the OP's were.

I have not used that variety myself.


YOU STINKING ASININE CRIMINAL MORON

I said that I have used WARTH PADS (rusty red in colour) and in TO-3 size they measured 0.3C/W. They also DO NOT require a high pressure interface.

Graham

N Cook wrote:

I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.

Considering that you don't know how to measure thermal ressitance I will rely on
my own measurements which say that top-quality high performance 'sil-pads' can
outperform grease and mica.

Graham

Arfa Daily wrote in message
...

"N Cook" wrote in message
...
DaveM wrote in message
. ..
"whit3rd" wrote in message
ups.com...
On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66
power
output
devices that the silicone pads were better than mica.

The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.

It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).



The interface between the device and the heatsink is probably the most
critical
piece in the puzzle, and is the piece that a builder/repairer has most
control
over. The device temperature is what you're trying to control. You
have
no
control over the internal thermal impedances of the device... they are
determined by the manufacturing process and materials.
Given a heatsink has adequate capacity for dissipating the heat
(heatsink
to air
thermal impedance), then that only leaves the device to heatsink
interface
to
manipulate. In comparison between greased mica and silpad insulators,
the
mica/grease combination has the better thermal characteristics.
Mica can be problematic though, mainly because there seems to be no
quality
control on the manufacturing process. The thickness of a mica slab can
vary
from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per
mil,
so it
really doesn't need to be so thick. You can usually split a mica slab
several
times and still be safe in using it. The thermal impedance goes down
somewhat
with thinner slabs, too.

Overall, mica/grease can exhibit almost half the thermal impedance of a
silpad.
Given the same device power dissipation, lower thermal impedances mean
lower
device temperature.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate
characters
in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra



I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.
I've no reason to believe the original silpads had aged, been affected
by
WD40 or anything.
I will assume they are , all manufacturers, all generically bad until a
similar personally conducted experiment, in a real situation, proves to
me
to be otherwise.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


Surely a degree of faith in what manufacturers say, has to be assumed,
otherwise we would all go around our whole lives with our heads up our
collective arses ! Do you for instance do a complete analysis of every
piece
of processed food you eat to see if the manufacturer is lying about how
much
salt it contains or how much calorific value it has ? Do you check your
bottles of beer to see if the ABV that the manufacturer quotes is true ?
Do
you run a complete set of tests on a resistor type before stocking it as a
spare to see if what the manufacturer says about its tolerance stability
is
correct ? No? Neither do I.

I can understand scepticism about 'waved in the air' figures that appear
to
have little or no foundation in reality, but figures for these washers
that
quote specifics on the applied pressure to get the results that are
published, don't seem to fall into that category to me. I can also
understand manufacturers of some products getting into 'mine's bigger than
yours' specmanship contests, but silicone heatsink pads ? Do you really
believe that these are such a big part of the manufacturer's total output
of
products, that he needs to engage in the sort of behaviour that involves
lying, or at least distorting, the 'true' facts about the product, just to
sell a few more than the other guy ?

As it happens, I cannot remember seeing pads distorted in the way that you
describe, so maybe they were from a bad batch or something. Maybe, the amp
manufacturer torqued them up too much, and the heat did the rest. Maybe
the
amp was used on midsummer's day in Ibiza at full volume with a blanket
over
it, and a fault ...

I agree that many amps have output devices that appear to run hotter than
we
might like, and that the thermal transfer across the insulators seems less
than ideal, but if that's the way the manufacturer made it and is happy
with, and the item is not on my bench with failed output transistors, then
I
tend to just note the fact with interest, and work on using the "If it
ain't
broke, don't fix it" principle.

And to Graham, I HATE amps that have live heatsinks. Just a few days ago,
I
had a big AV amp on the bench that nearly threw me off my chair when I
laid
the edge of my hand across the two heatsinks ready to take some
measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...

Arfa



Unless big G has a defendable patent on mica then why do I not see
like-for-like , in real situations, comparison tables of BigCorp GeeWizz
SiilyPad versus mica in the performance figures.

"Live" heatsink amps would be ok if the damn things were labelled with big
fluorescent stickes stating such. But of course such amps should only be
serviced by their own service agents is what they would say.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Arfa Daily wrote:

And to Graham, I HATE amps that have live heatsinks. Just a few days ago, I
had a big AV amp on the bench that nearly threw me off my chair when I laid
the edge of my hand across the two heatsinks ready to take some measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...

Arfa


A bit of a zap now and then is sposed to be very good for your heart -
if it don't stop it that is

I`m with you on the live heatsink thing, but it does make mounting
devices a lot simpler (and cheaper for the manufacturer no doubt).

Has anyone any thoughts on using nylon machine screws to mount devices,
instead of the usual tophat spacer and normal nut n bolt?

Ron(UK)

Arfa Daily wrote:

And to Graham, I HATE amps that have live heatsinks. Just a few days ago, I
had a big AV amp on the bench that nearly threw me off my chair when I laid
the edge of my hand across the two heatsinks ready to take some measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...

You're not supposed to rest your hand inside live equipment !

I hope you take more care with LOPTs etc.

Graham

" Graham Stevenson = totally schizo psychopath. "



** The Farnell link above that

** YOU ****ING POSTED **

is for BERGQUIST !!!


Those Bergquist pads are white like the OP's were.



** You are totally and completely INSANE - Stevenson !!!

Someone needs to shoot you * right in the ****ing head* .

Then apply for a Queen's awards for great favours to humanity.


You deserve to die in the most vile agony.




....... Phil

Eeyore wrote:

Arfa Daily wrote:

And to Graham, I HATE amps that have live heatsinks. Just a few days ago, I
had a big AV amp on the bench that nearly threw me off my chair when I laid
the edge of my hand across the two heatsinks ready to take some measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...

You're not supposed to rest your hand inside live equipment !

I hope you take more care with LOPTs etc.

Graham

You're not a proper TV repair man till you lose all the hairs off the
back of your hand!

Ron(UK)

"Eeyore" wrote in message
...


Arfa Daily wrote:

And to Graham, I HATE amps that have live heatsinks. Just a few days ago,
I
had a big AV amp on the bench that nearly threw me off my chair when I
laid
the edge of my hand across the two heatsinks ready to take some
measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...

You're not supposed to rest your hand inside live equipment !

I hope you take more care with LOPTs etc.

Graham


Easy to say, but when you've been doing it for 30 odd years, and are used to
heatsinks being grounded, it's hard to change. And as for LOPTxs, I try to
avoid even getting the backs off TV sets and monitors these days, unless
it's for a mate ...

Arfa