SWR California Blonde '98

[lowell]

Enzo,

This particular bias transistor is also thermally tracking the heatsink temperature, as it is mounted between the outputs on the heatsink. I'm sure you know this. That is my concern, is that YES it's maintaining a voltage difference between the bases, but it is also reducing that difference as the heatsink temperature increases. Just wanna make sure I'm doing things right. I wasn't aware of the hfe of 40. Now that I look at it, http://www.onsemi.com/pub_link/Colla...MJE15028-D.PDF, I see that I was reading the DC Current Gain LINEARITY, not the DC Current Gain itself. My bad.

Regarding the thermal insulator: I realized the stuff is in my bin and that it's something I purchased many many years ago. It's in my drawer with all my transistor mounting kits etc.. So, probably not gonna figure out what it is. Oops.

However, I did recently order this graphite stuff: tgon 9000 | LairdTech
I got the .51mm thickness. Here's a Mouser link: http://www.mouser.com/ProductDetail/...6917dU00K3j3k0
I had called them asking to purchase a sheet of thermal insulation so i could cut my own pads. This stuff isn't like Sil-pads in it's feel... it's more like Mica... it flakes and breaks quite easily. I can use my fingers to break pieces off.

Opinions? I've never seen an amp with this stuff in it.

EDIT: ummmm... that graphite stuff from Laird is apparently a great electrical conductor, not an insulator AT ALL. Guess the mouser service rep didn't know what she was doing. And I took her word for it.

[lowell]

Found these but wow... a 11.5x12" sheet is $60!!

http://www.mouser.com/ProductDetail/...7jrIFZUXZxGv4f

[J M Fahey]

That looks like kapton film, both because of the Polyimide label and the K letter in the name.
Not the best thermal conductor (nor the worst, but being available in very thin sheets sure does help.
That said, I would use it **plus** grease, go figure.
FWIW next time I visit my insulation materials supplier will check for thinnest available Mylar or Kapton sheet and test it.

Expensive?
Itīs practically square 30x30 cm so you can cut 25*16=400 TO220 insulators (15 cents each) or 17*10=170 TO218/247 ones (35 cents each).
Admittedly more expensive (I think) than bulk micas but not by much and we are talking cents anyway.
Of course you can cut custom shaped (Sanken module or LM3886/TDA7294 or any weird shape) pads and thatīs invaluable.