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Old 6th September 2005, 09:58 PM   #1
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Default voltage problem in 400watt amp

hi all
i need help...
i bulid transistor amplifier by mj15003/mj15004 transistors
and i make test when power supply is =+30 / 0 /-30 it is work very good but when i use original power supply = +70 / 0 / -70
the transistors burned
any bady can help me
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Old 6th September 2005, 10:18 PM   #2
jaycee is offline jaycee  United Kingdom
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+-70V rails would be 140V across the transistors. This is the maximum Vceo for these devices, and most likely, they can't take it. Use MJ15024/5 or MJ21193/4 instead.

Also this isn't really enough information - what other parts have you used ? Can we see a schematic?
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Old 6th September 2005, 10:56 PM   #3
Eva is offline Eva  Spain
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Using power transistors near its maximum ratings requires very careful circuit design because any minor oscillation, cross-conduction or thermal runaway issue will kill them. On the other hand, these are very rugged devices and I remember seeing people saying that they were capable to use them with +-70V in a reliable way.

Also, your transistors may be fakes, that would explain the blow-up quite well. In case you don't know what I mean, search the forum for 'fakes', 'fake transistors', etc...
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Old 6th September 2005, 11:29 PM   #4
anatech is offline anatech  Canada
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Another question. Did the bias current increase to a higher level? Is the bias stable or does it rise with time (thermal runaway)?
Okay two questions. It's also possible that another transistor was taken above it's ratings. Check them all.

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Old 7th September 2005, 12:04 AM   #5
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Of course the problem may be simply that the output stage bias currents has increased dramatically with the rail increase from 30V to 70V. Amplifiers with resistive input diff'l tails are particularly sensitive to such changes in supply - their operating currents would more than double sending the output stage bias through the roof!

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Old 7th September 2005, 03:33 AM   #6
Enzo is offline Enzo  United States
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I like anatech's clue, make sure the driver transistors are within spec. They need to be able to take the whole 140 volts as well. A driver breaks down, and it will take out the output transistors.

What is your load impedance, and how many of those MJ output transistors are there in the circuit?
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