tip147 142

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I've had the same problems a few months ago with an amp using the same transistors. The amp work for about one minute then the transistors went hot and burned (!!!) out. I think these devices must be cooled extremely, anyway after trying it out a few times I decided to start something new with MJ15003 / MJ15004 devices, because the amp was "overdimensioned".
I'm still working at it right now (I've had some delay with going to school ;-) ).
I would suggest to cool the devices better with bigger cooling elements or with fans. Otherwise you can use other (stronger) devices. Or what about the combination of TIP142/147 with MJ15003/15004 as output devices; that should work better.

What's in fact the reason you use other transistors (bdv66 67 not available??)

best regards,

The bias may not be set up properly on these. I built an amp (a long time ago) with these transistors and never could get the bias quite right. They kept running away on me, getting far hotter than they should. I had to set the bias lower than the recommended value. The fact they are Darlington transistors may be part of the problem.
Some amps have a temperature compensation device (a diode or transistor) that needs to be in good thermal contact with the output devices heatsink. You may want to check this.
If I am not mistaken the TIP 142 and 147 are darlingtons and I have run across a problem with some darlingtons from different manufacturers having different values of internal emitter resistors leading to bias problems. Try using the specified manufacturers devices.

Indeed, they are darlington transistors.

I agree with PaulB: the temperature sensing device must be mounted properly on the heatsink. The base-emitter voltage of the output devices will drop when the temperature increases (and the temp will increase after a while because of the biasing current), so the biasing current will also increase because
Vbias - Vbe will be higher, if no temp compensating element is used the devices will blow up because the biasing current will be too high. So it's important to place the biasing diodes or transistor in good thermal contact to prevent damage to the devices. If the problem still exists you can also try to put small resistors in series with the emittors (or increasing their value), this will compensate the differences in internal emittor resistors and will reduce the biasing current when "it's runnig away".
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