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Gareth Magennis Gareth Magennis is offline
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Default Thermal pads for transistor mounts

Am I right in my belief that putting heatsink compound on silicon pads gives
less thermal conductivity than pads alone?


Cheers,



Gareth.


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Default Thermal pads for transistor mounts

"Gareth Magennis" wrote in message
news
Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


Cheers,



Gareth.

My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.

S.



--
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Default Thermal pads for transistor mounts


"Serge Auckland" wrote in message
...
"Gareth Magennis" wrote in message
news
Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


Cheers,



Gareth.

My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.

S.



--
http://audiopages.googlepages.com


A few years back Yamaha was having higher than normal failures of some amp
channels using the conductive rubber. As part of the repair the techs were
to add silicone grease to the outputs and rubber sheets.

Mark Z.


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Gareth Magennis Gareth Magennis is offline
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Default Thermal pads for transistor mounts


"Serge Auckland" wrote in message
...
"Gareth Magennis" wrote in message
news
Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


Cheers,



Gareth.

My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.



My understanding was that silicon pads already have the optimum contact with
both surfaces due to their squidginess. Putting 2 layers of thermal
compound with less than 100% thermal conductivity between them is only going
to make things worse.

I just seem to remember reading something about this somewhere, but that
does not of course make it true.


Gareth.


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Serge Auckland Serge Auckland is offline
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Default Thermal pads for transistor mounts

"Gareth Magennis" wrote in message
...

"Serge Auckland" wrote in message
...
"Gareth Magennis" wrote in message
news
Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


Cheers,



Gareth.

My understanding is that thermal compound couples the bottom of the
device to the heatsink by filling-in the tiny interstices that would
otherwise exist between the slightly rough device and the slightly rough
heatsink. Consequently, heat is conducted better than it would be if the
device were clamped directly to the heatsink.



My understanding was that silicon pads already have the optimum contact
with both surfaces due to their squidginess. Putting 2 layers of thermal
compound with less than 100% thermal conductivity between them is only
going to make things worse.

I just seem to remember reading something about this somewhere, but that
does not of course make it true.


Gareth.

Do you mean Silicon, or Silicone? Silicon is a pretty hard material and
would behave as I have described. I have no idea about silicone, I thought
it was used in car wax and breast implants.
S.


--
http://audiopages.googlepages.com




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Mogens V. Mogens V. is offline
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Default Thermal pads for transistor mounts

Serge Auckland wrote:
"Gareth Magennis" wrote in message
...

"Serge Auckland" wrote in message
...

"Gareth Magennis" wrote in message
news
Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


Cheers,



Gareth.


My understanding is that thermal compound couples the bottom of the
device to the heatsink by filling-in the tiny interstices that would
otherwise exist between the slightly rough device and the slightly rough
heatsink. Consequently, heat is conducted better than it would be if the
device were clamped directly to the heatsink.



My understanding was that silicon pads already have the optimum contact
with both surfaces due to their squidginess. Putting 2 layers of thermal
compound with less than 100% thermal conductivity between them is only
going to make things worse.

I just seem to remember reading something about this somewhere, but that
does not of course make it true.


Gareth.


Do you mean Silicon, or Silicone? Silicon is a pretty hard material and
would behave as I have described. I have no idea about silicone, I thought
it was used in car wax and breast implants.


The usual white heat transfer compound is a silicone compound.
At least some of those products exibit rather crappy heat transfer
characteristics, plus it hardens and deteriorates with age.

While I haven't use this for audio, serious computer builders use
products like Titan Silver Grease or Artic Silver, which contains very
fine grained silver flakes in a non-silicone compound.
Note that some similar products are silicone based, to be avoided.

Of cause, for some applications electric isolation is needed; a silver
based compound may be less applicable for this
Maybe a hard thin mica or ceramic isolation shimmer with said compound
centered (to avoid contact round edges) can be used. YMMV..

And as stated by others, the point with any compound is filling an
uneven surface, i.e. as little as possible should be applied.
If two surfaces could have a perfect meet, any compund would only result
in lesser contact. A Bit the same as with lipstick..

--
Kind regards,
Mogens V.

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Default Thermal pads for transistor mounts

Mogens V. wrote:

While I haven't use this for audio, serious computer builders use
products like Titan Silver Grease or Artic Silver, which contains very
fine grained silver flakes in a non-silicone compound.
Note that some similar products are silicone based, to be avoided.


I think it is aluminium dust rather that silver dust.


Kind regards

Peter Larsen



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Mogens V. Mogens V. is offline
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Default Thermal pads for transistor mounts

Peter Larsen wrote:
Mogens V. wrote:


While I haven't use this for audio, serious computer builders use
products like Titan Silver Grease or Artic Silver, which contains very
fine grained silver flakes in a non-silicone compound.
Note that some similar products are silicone based, to be avoided.



I think it is aluminium dust rather that silver dust.


You may very well be right. Still is a good thermo coupler

--
Kind regards,
Mogens V.

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Default Thermal pads for transistor mounts



Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon pads gives
less thermal conductivity than pads alone?


Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as much as it
adds something to the thermal path it must increase the thermal resistance at
least by a tiny bit.

Graham

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Default Thermal pads for transistor mounts



Serge Auckland wrote:

"Gareth Magennis" wrote

Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.


Irrelevant to silicone pads as they confrom to the mating surfaces themselves.

Graham



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Default Thermal pads for transistor mounts



"Mark D. Zacharias" wrote:

A few years back Yamaha was having higher than normal failures of some amp
channels using the conductive rubber. As part of the repair the techs were
to add silicone grease to the outputs and rubber sheets.


Bizarre. Using a better grade of pad would have been far more intelligent.

Graham

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Default Thermal pads for transistor mounts



Gareth Magennis wrote:

My understanding was that silicon pads already have the optimum contact with
both surfaces due to their squidginess. Putting 2 layers of thermal
compound with less than 100% thermal conductivity between them is only going
to make things worse.

I just seem to remember reading something about this somewhere, but that
does not of course make it true.


Your understanding is perfectly correct.

Graham

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Default Thermal pads for transistor mounts

"Eeyore" wrote in message
...


Serge Auckland wrote:

"Gareth Magennis" wrote

Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


My understanding is that thermal compound couples the bottom of the
device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device
were
clamped directly to the heatsink.


Irrelevant to silicone pads as they confrom to the mating surfaces
themselves.

Graham

Ah, so it's silicone, not silicon...quite a difference.
S.



--
http://audiopages.googlepages.com


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Default Thermal pads for transistor mounts

Eeyore wrote:
Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon
pads gives less thermal conductivity than pads alone?


Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as
much as it adds something to the thermal path it must increase the
thermal resistance at least by a tiny bit.


Bring back mica and berlyium oxide.

geoff


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Paul[_11_] Paul[_11_] is offline
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Default Thermal pads for transistor mounts

On Sat, 24 Nov 2007 05:45:38 +0000, Eeyore
wrote:


Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?


My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.


Irrelevant to silicone pads as they confrom to the mating surfaces themselves.


My experience is that the pads are worse than just a thin layer of
thermal "grease". The layer should be very thin, and if so, it has
little influence on the overall thermal conductivity. I find that some
of the high thermal conductivity compounds are too thick in
consistency, and don't allow a thin layer. The thermal compound MUST a
thin film! Beware if your grease settles out (like organic peanut
butter) and you get lumpy hard-to-spread grease. It's worse than
useless in that state.
However..... you sometimes need to ELECTRICALLY insulate the device
from the heatsink. If you have this situation YOU CANNOT USE THERMAL
GREASE ALONE. Then you need an (electrically) insulated pad. You
shouldn't need to use both methods at the same time, since the
silicone should conform under pressure. If it doesn't conform, you
need a new pad. We used to use Mica films, but these weren't flexible,
and needed the thermal compound to fill in the gaps with something
thermally conductive.
Just to be controversial, you don't need something all that
thermally conductive.... a thinner film (possibly flatter surface) is
more effective than the high thermal conductivity greases. We used to
use plain silicone grease without the aluminum/zinc oxide. It worked
OK....
Back in the old days they used beryllium oxide too, but it was
deadly poisonous!


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Default Thermal pads for transistor mounts

In article ,
Paul wrote:

Back in the old days they used beryllium oxide too, but it was
deadly poisonous!


Only if you pulverized it and snorted it.

Isaac
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geoff wrote:

Eeyore wrote:
Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon
pads gives less thermal conductivity than pads alone?


Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as
much as it adds something to the thermal path it must increase the
thermal resistance at least by a tiny bit.


Bring back mica and berlyium oxide.


I prefer live heatsinks myself. That helps keep the device die that much cooler.

BeO2 is potentially dangerous (carcinogenic).

Graham

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Default Thermal pads for transistor mounts



Paul wrote:

Eeyore wrote:

Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?

My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.


Irrelevant to silicone pads as they confrom to the mating surfaces themselves.


My experience is that the pads are worse than just a thin layer of
thermal "grease".


Yabbut .... A thin layer of grease doesn't provide any insulation. And I don't use
'grease' either. I use the 'compound' that's filled with aluminium oxide. It has
better thermal conductivity.

Graham

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Default Thermal pads for transistor mounts


"Eeyore" wrote in message
...


Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon pads
gives
less thermal conductivity than pads alone?


Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as much
as it
adds something to the thermal path it must increase the thermal resistance
at
least by a tiny bit.

Graham


That was exactly my thinking. Wharfedale are putting compound on some of
their pads, and I clean it off when I find it.


Gareth.


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GregS[_3_] GregS[_3_] is offline
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Default Thermal pads for transistor mounts

In article , Paul wrote:
On Sat, 24 Nov 2007 05:45:38 +0000, Eeyore
wrote:


Am I right in my belief that putting heatsink compound on silicon pads
gives less thermal conductivity than pads alone?

My understanding is that thermal compound couples the bottom of the device
to the heatsink by filling-in the tiny interstices that would otherwise
exist between the slightly rough device and the slightly rough heatsink.
Consequently, heat is conducted better than it would be if the device were
clamped directly to the heatsink.


Irrelevant to silicone pads as they confrom to the mating surfaces themselves.


My experience is that the pads are worse than just a thin layer of
thermal "grease". The layer should be very thin, and if so, it has
little influence on the overall thermal conductivity. I find that some
of the high thermal conductivity compounds are too thick in
consistency, and don't allow a thin layer. The thermal compound MUST a
thin film! Beware if your grease settles out (like organic peanut
butter) and you get lumpy hard-to-spread grease. It's worse than
useless in that state.


Most of the time, too much grease is used. The grease should be
rubbed around, and you should see visable metal. I have found too
many parts that have dried out over the years, but too much grease
in the first place will create a worsening situation.

greg

However..... you sometimes need to ELECTRICALLY insulate the device
from the heatsink. If you have this situation YOU CANNOT USE THERMAL
GREASE ALONE. Then you need an (electrically) insulated pad. You
shouldn't need to use both methods at the same time, since the
silicone should conform under pressure. If it doesn't conform, you
need a new pad. We used to use Mica films, but these weren't flexible,
and needed the thermal compound to fill in the gaps with something
thermally conductive.
Just to be controversial, you don't need something all that
thermally conductive.... a thinner film (possibly flatter surface) is
more effective than the high thermal conductivity greases. We used to
use plain silicone grease without the aluminum/zinc oxide. It worked
OK....
Back in the old days they used beryllium oxide too, but it was
deadly poisonous!

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Default Thermal pads for transistor mounts

Eeyore wrote:

Bring back mica and berlyium oxide.


I prefer live heatsinks myself. That helps keep the device die that
much cooler.

BeO2 is potentially dangerous (carcinogenic).


So are live heatsinks with a PD of 180VDC a centimetre apart ! At least
chances are that the DC fuses have long ago popped when the PCB components
burned and tracks vapourised, eh Graham ;-)

geoff


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Mogens V. wrote:

I think it is aluminium dust rather that silver dust.


You may very well be right. Still is a good thermo coupler


Aluminum is 40 times worse than diamond.


Difficult cutting them tiny flakes off a diamond for the heat
compound..


You will never make it in high end audio with that attitude, do you want
flakey sound? - if you want clean, transparent sound, then you cut them in
thin slices and use them like mica was used. Single ended designs may be
attractive for budget conscious ....


Kind regards

Peter Larsen



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Peter Larsen wrote:
Mogens V. wrote:

I think it is aluminium dust rather that silver dust.


You may very well be right. Still is a good thermo coupler


Aluminum is 40 times worse than diamond.


Difficult cutting them tiny flakes off a diamond for the heat
compound..


You will never make it in high end audio with that attitude, do you
want flakey sound? - if you want clean, transparent sound, then you
cut them in thin slices and use them like mica was used. Single ended
designs may be attractive for budget conscious ....


But audiences are all flakes too. And musicians.

geoff




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geoff wrote:

Eeyore wrote:

Bring back mica and berlyium oxide.


I prefer live heatsinks myself. That helps keep the device die that
much cooler.

BeO2 is potentially dangerous (carcinogenic).


So are live heatsinks with a PD of 180VDC a centimetre apart !


Only if you open up the box. No different to most electronics really. And far
less dangerous than a CRT.


At least chances are that the DC fuses have long ago popped when the PCB
components burned and tracks vapourised, eh Graham ;-)


Blame that ****wit Paul Belcher whose bright idea that was. I'm sick of being
criticised for his stupid ideas.

Graham

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geoff wrote:

Eeyore wrote:

I prefer live heatsinks myself. That helps keep the device die that
much cooler.

BeO2 is potentially dangerous (carcinogenic).


So are live heatsinks


NO. They're not carcinogenic.

Graham

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"geoff" wrote in message
...
Eeyore wrote:

Bring back mica and berlyium oxide.


I prefer live heatsinks myself. That helps keep the device die that
much cooler.

BeO2 is potentially dangerous (carcinogenic).


So are live heatsinks with a PD of 180VDC a centimetre apart ! At least
chances are that the DC fuses have long ago popped when the PCB
components burned and tracks vapourised, eh Graham ;-)

geoff


The EV P3000/2000 etc have live heatsinks less than 2mm apart, they seem to
do just fine.


Gareth.


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isw isw is offline
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Default Thermal pads for transistor mounts

In article ,
Eeyore wrote:

geoff wrote:

Eeyore wrote:
Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon
pads gives less thermal conductivity than pads alone?

Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as
much as it adds something to the thermal path it must increase the
thermal resistance at least by a tiny bit.


Bring back mica and berlyium oxide.


I prefer live heatsinks myself. That helps keep the device die that much
cooler.


And adds greatly to the capacitance of whichever electrode is connected
to the case. Usually a bad thing, design-wise.

Isaac
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isw wrote:

Eeyore wrote:
geoff wrote:
Eeyore wrote:
Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon
pads gives less thermal conductivity than pads alone?

Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as
much as it adds something to the thermal path it must increase the
thermal resistance at least by a tiny bit.

Bring back mica and berlyium oxide.


I prefer live heatsinks myself. That helps keep the device die that much
cooler.


And adds greatly to the capacitance of whichever electrode is connected
to the case. Usually a bad thing, design-wise.


Completely irrelevant at audio frequencies. Since the device case is almost
invariably the collector with bipolars and this is usually the power supply, it
is indeed totally irrelevant in such cases.

Have you actually ever designed an amplifier ? I have designed a number of them
and several hundred thousand of them in total have been made.

Graham



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Default Thermal pads for transistor mounts

In article ,
Eeyore wrote:

isw wrote:

Eeyore wrote:
geoff wrote:
Eeyore wrote:
Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon
pads gives less thermal conductivity than pads alone?

Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as
much as it adds something to the thermal path it must increase the
thermal resistance at least by a tiny bit.

Bring back mica and berlyium oxide.

I prefer live heatsinks myself. That helps keep the device die that much
cooler.


And adds greatly to the capacitance of whichever electrode is connected
to the case. Usually a bad thing, design-wise.


Completely irrelevant at audio frequencies.


Well, probably, but the active devices need to have a bit more bandwidth
than that.

Since the device case is almost
invariably the collector with bipolars and this is usually the power supply,
it is indeed totally irrelevant in such cases.

Have you actually ever designed an amplifier ?


Ummm, yup. Stabilized them to prevent oscillation from too much
capacitive coupling, too. Not all amplifiers use the topology you
mentioned.

And the difference between theta-j-a for a "hot" sink and a properly
insulated one is really quite small. If it makes the difference between
an amp that works and one that doesn't, I'd suggest that the heat sink
is too small for reliable operation. There's a lot of power in the
filter caps of a large amplifier; having it present on large chunks of
aluminum is not a good idea.

Isaac
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isw wrote:

Eeyore wrote:
isw wrote:
Eeyore wrote:
geoff wrote:
Eeyore wrote:
Gareth Magennis wrote:

Am I right in my belief that putting heatsink compound on silicon
pads gives less thermal conductivity than pads alone?

Do you mean silicone pads ?

There's simply no purpose in using heatsink compound with them. In as
much as it adds something to the thermal path it must increase the
thermal resistance at least by a tiny bit.

Bring back mica and berlyium oxide.

I prefer live heatsinks myself. That helps keep the device die that much
cooler.

And adds greatly to the capacitance of whichever electrode is connected
to the case. Usually a bad thing, design-wise.


Completely irrelevant at audio frequencies.


Well, probably, but the active devices need to have a bit more bandwidth
than that.


Anywhere between 10 and 40 MHz for modernish designs but capacitance on the power
supply nodes is GOOD anyway !


Since the device case is almost
invariably the collector with bipolars and this is usually the power supply,
it is indeed totally irrelevant in such cases.

Have you actually ever designed an amplifier ?


Ummm, yup. Stabilized them to prevent oscillation from too much
capacitive coupling, too.


Stability results from closing the feedback loop with adequate gain/phase margin.
It has nothing to do with 'capacitive coupling' or SHOULDN'T do !


Not all amplifiers use the topology you mentioned.


All amplifiers using bipolar devices have the collector as case which means that
the power supply is the node that will typically be connected tot he heatsink. In
the instances of QSC style designs (grounded collector), the heatsink is earthed
anyway so there's no problem there either.


And the difference between theta-j-a for a "hot" sink and a properly
insulated one is really quite small.


Nonsense.

Take an MJ150xx type device dissipating an average of say 50W (which is quite
typical). Adding a thermal washer will typically increase the interface resistance
by about 0.4C/W
..
https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

That'll take the junction temperature 20C higher.



If it makes the difference between
an amp that works and one that doesn't, I'd suggest that the heat sink
is too small for reliable operation.


Reducing device temp is ALWAYS beneficial.


There's a lot of power in the
filter caps of a large amplifier; having it present on large chunks of
aluminum is not a good idea.


It's a perfectly fine idea when it's inside the case. There's LOTS of live bits
inside an amplifier including potentially lethal AC mains voltages., there's
nothing special about a heatsink that means it can't be live. You're not meant to
be pokoing about in there.

Graham

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