voltage problem in 400watt amp

jojo_tennis · 6th September 2005, 09:58 PM

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

jaycee · 6th September 2005, 10:18 PM

+-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?

Eva · 6th September 2005, 10:56 PM

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...

anatech · 6th September 2005, 11:29 PM

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.

-Chris

amplifierguru · 7th September 2005, 12:04 AM

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!

Cheers,
Greg

Enzo · 7th September 2005, 03:33 AM

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?

6th September 2005, 09:58 PM	#1
jojo_tennis diyAudio Member Join Date: Aug 2005	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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6th September 2005, 10:18 PM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	+-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?

6th September 2005, 10:56 PM	#3
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	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...

6th September 2005, 11:29 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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. -Chris

7th September 2005, 12:04 AM	#5
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	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! Cheers, Greg

7th September 2005, 03:33 AM	#6
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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?