josephkk wrote in message
...
On Thu, 21 Apr 2011 21:15:25 +0100, Baron wrote:

josephkk Inscribed thus:

On Mon, 18 Apr 2011 17:36:42 +0100, Baron
wrote:

N_Cook Inscribed thus:

1.8mm slabs of porcelain under TO220 devices.

I would check that ! Its more likely that they are Beryllium_oxide !
http://en.wikipedia.org/wiki/Beryllium_oxide

Tried grinding a clearance notch on the side of one
pad but got nowhere as ceramic.
Not only that but I had no trouble holding the slab in fingers while
trying to grind a slot, it was barely getting warm - good thermal
insulator or small grinding wheel not generating heat as not cutting
into the material ?

The dust from grinding or abrading Beryllium_oxide is highly toxic !

BeO is very expensive. I would be very surprised to find any in such
application.

I wonder how much 12 dozen TO3 BeO insulators would be worth ?

The last time i saw a price they were over US $1 each, but that was over
30 years ago and for military use. Between the now rabid regulation and
the expected cost i would be rather surprised to find them in commercial
instrument amplifiers. Of course with Chinese manufacture such bets may
be knocked into a cocked hat.

++++

I'm assuming boards soldered up in China but assembled into the cab in USA.
So these insulators would likely be fitted in the USA. They obviously were
not made for the job, wrong cross-section area, unnecessarily thick, hole
not used - perhaps they just had them laying around. That grey silipad could
have been used with FET legs preformed to accommodate.