Dear Colin, Generally speaking, what is it that causes output transistors to blow? Do they tend to die a natural death? Bad drivers? Emitters? Problems with the rectifier circuit? Assuming there wasn't shorted speaker wires or excessive moisture. The amp bias and the +/-77 voltage was fine before they blew. It happened on power up. According to my Peak Atlas DCA-55, 4 of the 12 outputs test bad. I’m in Canada but i guess that won’t make a difference! Thank you. Geoff Deighton in the UK
Hi Geoff, Over heating and working out of the SOA safe operating area are normally the cause, most well designed and made semiconductors should last 20,000 - 100,000 Hours. Now it could be something like the driver, the PSU. Lets see the circuit; maybe we could help? You will also need to match all the devices to get them to share properly. This is very important and often not done then you get current hogging and bangs. Thank you for the question.