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In Reply to: RE: I tried that circuit but the SS device failed. posted by DAK on December 29, 2018 at 09:15:57
Is the chassis enough of a heastink in this instance? I don't see any voltages/currents on the drawing, but I tend to assume that the chassis is about a 50C/W heatsink unless it's enormous/thick and very well ventilated.
Follow Ups:
Much larger than the SCA35 which the circuit was made for. The B+ is from dual 5v4gb rectifiers so a slow startup would help the high voltage spike. So its a mystery. Maybe, soldering damaged the unit's reliability? It worked for a couple of weeks.
My take away is don't try this type of scheme with a custom build. Go with the bulletproof method, a cathode bias resistor, or a negative fixed bias voltage.
If we examine the SCA-35 design, we have a 95 ohm cathode resistor with 16V across it, so call it 170mA of current and 16V of bias. That's about 3W of dissipation, so the chassis is not going to be enough. I would go for a heatsink that's 20C/W or less under these conditions.
" The [chassis] heat sinking so provided is more
than adequate, as the 337 dissipates less than
2.5 watts under worst case conditions."
I put the EFB in an SCA-35 some years ago, with the 337 mounted to the chassis as called for in the article. I've got maybe 100+ hours on it, and have never had an issue with heat.
The Texas Instruments data sheet for the 337 rates it at 125 ºC, with a built-in thermal shut-down.
Perhaps DAC's 337 was damaged during soldering, as he has speculated.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
It's important to think about lifetime of these parts in the thousands of hours. Running at 100C,they will fail prematurely. When stuffed into a small vintage amp chassis that is cooking hot on its own, your ambient temperature may be more than 50C, hence the recommendation for a bigger heatsink.
Another fun problem is when the tubes themselves heat up the chassis where you are trying to sink heat, this just makes things even worse.
...because my totally unscientific "the chassis feels cool to the touch where the 337 is mounted" really doesn't tell much, I realize.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
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