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Building Marshall Lead 100 MOSFET
Building Marshall Lead 100 MOSFET
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Old 5th October 2018, 02:49 PM   #21
jrow is offline jrow
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Quote:
Originally Posted by JMFahey View Post
Which Mosfets did you use? Where did you get them from?
The FETs are a pair of BUZ900, BUZ905 from Farnell. In datasheet they put a power dissipation of 125watt instead of original sk134 and j49, which is 100watt channel dissipation.
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Old 6th October 2018, 03:49 PM   #22
JMFahey is offline JMFahey  Argentina
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Thanks.
Ok, when you measure what I asked earlier we will know whether your amp works reasonably well and just does not "seem" to be that loud or whether we have an Electronic problem.
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Old 3rd November 2018, 11:32 AM   #23
jrow is offline jrow
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Okay,

I've discovered a few things scoping the output of the amp.

First of all, I was WRONG telling that the volume was low. The amp is able to sound strong, but the problem is that a distortion appears at a mid-low volume.

The distortion seems to be like a fuzz pedal. When the volume is low, the amp works without that distortion. It sounds completely clear, but quiet.

But scoping the output, I realised that the origin of distortion wasn't clipping: I injected 1 kHz increasing volume in a preamp, and these were the results.

input 1khz (convert-video-online.com) (1).mp4

As you can see, clipping is in a higher level than the distortion that makes the amp at a mid-low volume.

So, I measured the voltage in output, just before clipping in scope (no speaker attached). The AC voltage in output is 3.7 V. See what happens when I connect at this point the speaker. The distortion appears, and it sounds.

I tried it with a guitar too. When I'm playing softly, or at a low volume, it sounds clean. Even when I scoped the output without the speaker, the amp didn't seem to make the distortion that makes when I put a load to it.

ddd (convert-video-online.com) (1).mp4
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Old 3rd November 2018, 11:39 AM   #24
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And these were my attemps to solve the problem (surprise... no)

I put a TRIMMER in the R54 but I didn't change ANYTHING!!! I mean, the trigger was of 1kOhm, and in the schematic R54 was of 100Ohm. Okay, I set the trigger to 100 Ohm to go up or down... and when I put to 0 Ohm or to 1kOhm, the sine wave doesn't even react.

I put a gain pot (50k linear) in R61 (Originally 10k) but this reacted normally. As I increased gain, output was increased.

The problem is the distortion...Why?!!!
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Old 3rd November 2018, 12:12 PM   #25
jrow is offline jrow
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I missed a thing:

I measured the DC supply. It was perfect (+47.9 to GND and -47.9 to GND) even symmetric.
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Old 3rd November 2018, 12:21 PM   #26
jrow is offline jrow
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If you can't see the video, here are some screen frames.

1kHz - NO speaker.png

1kHz - speaker distortion.png

1kHz - speaker.png

1kHz -no speaker clipping.png
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Old 3rd November 2018, 01:02 PM   #27
voltwide is offline voltwide  Ireland
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the video shows some current limiting effect when loaded with 8 ohms. did you check supply rails under load?
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Old 3rd November 2018, 03:27 PM   #28
jrow is offline jrow
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Hi voltwide,

This is how the rails looks like:

The amp switched on with -inf db signal: +47.9 V // -47.9 V

The amp switched on with low volume and no distortion: +47.7 V // -47.7 V

The amp switched on with maximum volume: +46.0 V // -45.8 V

I think this little drop is normal, isn't it?
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Old 8th November 2018, 11:06 AM   #29
jrow is offline jrow
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Plzzzz help !!!
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Old 8th November 2018, 12:07 PM   #30
Mooly is offline Mooly  United Kingdom
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Building Marshall Lead 100 MOSFET
I have seen two very obscure problems cause distortion like that.

1/ Having a ferrite cored inductive component in series with the load. This can be something as small as a ferrite bead slipped over one of the output leads somewhere. The material saturates and gives this weird distortion... more likely an issue at high frequencies though.

2/ Another inductive issue can be having a shared magnetic component (such as a dual wound ferrite core) in series with each rail. Mutual coupling between the supply currents again causes this odd effect.

Beyond that and its a case of scoping rails and seeing if anything is apparent. The volt drop is normal, however the rail should still remain clean though and with only a little ripple showing.
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