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Old 27th March 2012, 07:28 PM   #1
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Default Final output stage amp marshall

i can't find the MJ3001/MJ2501 or an equivalent (MJ2501 Equivalência Transistor ou SMD, MJ3001 Equivalência Transistor ou SMD)... i think that is possible change the final stage, occasionally i have here 2sc5200/2sa1943, and I can found others transistors on old radios/tv/scrap circuits.. any suggestions? thanks
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Old 27th March 2012, 07:56 PM   #2
CBS240 is offline CBS240  United States
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Here is the datasheet, it is a Daralington transistor in TO-3.

Perhaps some of these will work?
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Old 27th March 2012, 08:41 PM   #3
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Hi CBS240, yes, its possible, but its hard to find this type of transistors in the stores around here...
So I'm trying to found some solution before try buy this transistors on another country (shipping takes too much time, maybe a month)
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Old 2nd April 2012, 04:56 AM   #4
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Hello everybody! after work hard for some days, I finished the amp.. by the way, I used TIP142/147 replacing the MJ3001/2501, works great ...

But now I have a problem with preamp... With very low gain, sounds clear, but with local radio on background, and when I put more gain, it emits a high frequency, very loud, very, very, very loud (like: zuimmmmm.)

I'm dissapointed with this preamp, I double checked too... :/

I dont know if i messed up assembling the preamp.. the diagram seems pretty confused, but I think that I did it right... In attachment there is a piece from diagram that explains the "ground" on preamp, I followed that...

Anybody can help? Thank you
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Old 2nd April 2012, 07:34 AM   #5
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tip 142-147 will work there but never make it in long run its actually a terrible device ... drop in replacement and as used in later or bigger Marshall is the MJ 11015-16


Edit : as seen from the schematic there is no trimmer to adjust the bias ....Obviously if you replace transistors with something else bias will change and will need to be readjusted often when this is happen we have to replace one of the resistors on the Vbe area with a trimmer .... you need to verify bias according to the schematic cause higher bias will result to higher temperature that heatsink will not manage and eventually will fail and lower bias will increase distortion
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Last edited by east electronics; 2nd April 2012 at 07:39 AM.
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Old 2nd April 2012, 04:08 PM   #6
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Quote:
Originally Posted by sakis View Post
tip 142-147 will work there but never make it in long run its actually a terrible device ... drop in replacement and as used in later or bigger Marshall is the MJ 11015-16


Edit : as seen from the schematic there is no trimmer to adjust the bias ....Obviously if you replace transistors with something else bias will change and will need to be readjusted often when this is happen we have to replace one of the resistors on the Vbe area with a trimmer .... you need to verify bias according to the schematic cause higher bias will result to higher temperature that heatsink will not manage and eventually will fail and lower bias will increase distortion

I checked Vbe (from tip142/147), is about 0.83~0.87 V... I think that is normal... In max volume, the temperature are on 29ºC

...but it seems that it dont have a problem with final output stage... I made a little test here, injecting signal from a external preamp pedal in the C8, and works fine...
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Old 2nd April 2012, 05:42 PM   #7
CBS240 is offline CBS240  United States
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Hi

It shows 12mA bias on the diagram. For 12mA bias, you should measure ~4mV across the 0.33R output emitter resistors. Each Darlington has two forward PN junctions, two Vbe's to overcome cutoff. Each device should require 1.1-1.2V in order to conduct bias. So you should see 2.2-2.4V between the bases, or the Vce of transistor 4.
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Old 3rd April 2012, 12:32 AM   #8
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Quote:
Originally Posted by CBS240 View Post
Hi

It shows 12mA bias on the diagram. For 12mA bias, you should measure ~4mV across the 0.33R output emitter resistors. Each Darlington has two forward PN junctions, two Vbe's to overcome cutoff. Each device should require 1.1-1.2V in order to conduct bias. So you should see 2.2-2.4V between the bases, or the Vce of transistor 4.
I got 1.94V on Vce TR4
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Old 3rd April 2012, 04:23 AM   #9
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I put the preamp in a separate pcb now, but the noise keeps popping up ... with low gain in the "VR1" the guitar is clean, but when I raise a little more, a low-frequency noise appears, and more gain I increase, the frequency becomes higher, with a very high volume too, and at some point, when I increase more gain, the noise disappear but the guitar signal disappear too.. I recorded a sample mp3, is attached...
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Last edited by felipemuniz; 3rd April 2012 at 04:29 AM.
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Old 3rd April 2012, 10:05 AM   #10
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It seems like you have marginal stability problems .

Check C13 ...220pf.....this helps keep the power amp stable

Check all the feedback components/wiring around IC1B .
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