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Old 31st July 2004, 05:08 AM   #1
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Default Safe Working voltage of output Transistors in class AB amps with +/- supply

Safe Working voltage of output Transistors in class AB amps with +/- supply

Please let me know the Safe Working voltage of an output Transistor having Vceo 90v in a class AB amps with +/- supply, my amp is working fine since long with a +/- 60 v supply,
What is the maximum voltage that I can go upto with a transistor with Vceo 90v, does a value of emitter base resistor of 100E or lower help in using the device to higher voltages nearing the breakdown voltage?
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Old 31st July 2004, 06:22 AM   #2
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It's depend also on SOAR parameter of these transistors. Practicaly we can to say, that save voltage is approximately 2/3 - 3/4 Uceo.
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Old 31st July 2004, 06:53 AM   #3
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You are very lucky.

Normally you take the Vceo of the device and subtract 10% right off the top to allow for line voltage fluctuations, divide whats left by two. A 90V part would handle no more than 40.5V

A friend of mine has a 20+ year old Leach on 57V with MJ802/4502

BUT

The transistors were selected.

The transformer is a ferro-resonant CV type, it outputs the same from 85V ~ 140V in.

Most people don't have a curve tracer and cases of transistors to sort through.
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Old 31st July 2004, 06:58 AM   #4
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Default Don't forget +/- 60 volts is 120 volts peak.

Rajeev , when the voltage swings you will have up to double your 60 volts across the transistor ( 120 volts ).
Most often the 90 volt rated Tr will handle much more unless they come in graded voltage ratings when the max Vceo may only be slightly higher than rated. With the base resistor it will be slightly higher .
So you may be working on the edge of breakdown. Additionally you have to consider mains fluctuation. With a 15% increase , not uncommon for us , it could mean disaster if you are working at the Vceo . This also depends on the voltage swing . So with high supply voltage ( high mains voltage) the amp could still survive if you are playing at low levels. SOAR ( dissipation limited) operation is also applicable and is important as maximum power dissipation is at lower output power levels. It will most likely die quietly !

I would use at least 160 volt (Vceo) transistors with a +/- 60 volt supply .
Cheers.
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