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In Reply to: RE: heat dissipation posted by raq on November 29, 2007 at 04:45:31
The voltage dropped across the common cathode CCS is the bias voltage that the tube requires to operate at the current set by the CCS. I was just estimating 50v for sake of discussion. Although your math is correct, when it comes to power output tubes I am more conservative regarding heat sink size for two reasons. The first is reliability going forward. You decide that you might like to goose the current up a bit when you get the newest latest greatest KT-88s in your amp and you forget about the heat sink temp and device reliability.... The second is that with a constant current source in the cathode, an internal tube short won't necessarily cause a mains fuse to blow, because the current will be limited to the CCS value, but now the CCS potentially has the whole B+ voltage across it and the attendant heat dissipation. The current source won't survive 80mA at 500v for very long in any case, but might long enough for you to notice there is no sound in that channel and shut the amp off.
You could certainly use one larger, taller heat sink as you suggest. Look at Digi-Key HS117-ND, for example. There is a way to mount the "top" MOSFET on the bottom of the Cascode CCS PC board so that it can be attached to the heat sink and the PC board can also be attached to the heat sink with stand-offs to make for a secure neat installation.
Kevin Carter
K&K Audio
www.kandkaudio.com
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