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Old 7th June 2005, 01:15 PM   #21
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Default m250

hienrich kumusta? these sanken transistor you have mention ive been using these about ten years ago with my 150w amp which was taken acctually in the electronics digest... the output stage was using (2sa1216a & 2sc2922a) or (2sa1215a & 2sc2921a) they only differ in Vce. its input stage was using an op amp LM301AN... the low power supply requirement of the op amp was scaled down with the use of limiting resistors and further regulated by zener diodes.....

musta bay?

alan
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Old 7th June 2005, 01:43 PM   #22
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well, we still have differences in doing things but arrive on the same
place.

many service manuals of knowned manufacturers, places test points
on there service schematics and even voltage drops across these test points
meaning, you place your test probes on those points and measure
those assigned voltages when ever doing a servicing...

when doing it your way it is safe but you should also know the
current consumptions of the input and the voltage amplfier stages,
and add them up together with the output stages so you have your
total current consumption....
but what if there is this someone who knows not more on current
consumptions, without scopes and only done his projects by a given schematic and a PCB layout but without construction manuals.....??

so still VBE measurements are essential no matter which bipolar
are used....

rgds,

macweb
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Old 8th June 2005, 12:25 AM   #23
anatech is offline anatech  Canada
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Hi macweb,
No. What you are doing is incorrect. Period. And another period!

What is important to know is the actual current through each device, so your idea is correct. Your method is totally wrong.

How do I know? I've serviced gear for over 25 yrs. Warranty for many brands and LARGE power amps. The only accepted method (where emitter resistors exist) is to measure and monitor the voltages (in mV) across the emitter resistors. You want to know the outputs are sharing current equally. You pick an average one and set your bias current from that. End of story.

Now, some manufacturers want you to replace a fuse with a resistor and measure the voltage across that. Others (like Fosgate car amps) want you to look for crossover distortion under load, normally something like 20KHz across 4R at low power. I guess you could use a current probe but that's a bit out there for normal techs.

-Chris
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Old 8th June 2005, 10:16 AM   #24
AJT is offline AJT  Philippines
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Default built my first ss amp in1975!

my first solidstate amp was a pioneer 707 clone! it was a quasi-complimentary design with bootstrap capacitor and a single 38volt rail,

then a complimentary amp with 2n3055/mj2955 in the late or early 80's with a supply voltage of +/-32volts.

i built my first leach super amp around 1984 using japanese transistors and scaled down psu rails of around +/-60volts dc!

with all the years of experience behind me, repairing power amps like soundcraftsmen, GAS, hafler, and many others of japanese make, i think i am qualified to comment about testing methods!
i can spot where something is amiss!


the m250 is typical design topology that is open for a lot of possible modifications.
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Old 9th June 2005, 04:14 AM   #25
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Does anybody think this amp would survive with +/-75v rails? This amp is going to be my next project and a 110vct toroid would be much more easy to get.
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Old 9th June 2005, 04:21 AM   #26
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not if you don't change the transistors from the schematics!
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Old 9th June 2005, 06:31 AM   #27
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Which do you recommend to replace them with? I would like to stick with TO-3's because of the heatsinks I have. Finding an 80vct toroid for the originals will be easier than sourcing different heatsinks for plastic devices.
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Old 9th June 2005, 06:31 AM   #28
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ok, just realized your were reffering to the other transistors in the circuit rather than the output stage. Which in particular would I need to swap for higher voltage ones?
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Old 9th June 2005, 06:36 AM   #29
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everything except the protection transistors, and bias setting transistor need to be replaced with a higher Vceo rating, at least 250 volts DC.
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Old 16th June 2005, 11:03 AM   #30
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Quote:
how about replacement for the TIP121? do you have any idea what transistor instead of using TIP121?


mje270 is a good replacement, comes in to-77.
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