Building Marshall Lead 100 MOSFET

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Administrator
Joined 2007
Paid Member
So the diode removal has proved it was limiting.

I'm going to add the following comment based on the fact you mention being able to get the volume 'high' even with a 27 watt bulb fitted (presumably the bulb is in the incoming mains feed)… and that is to check the current limiter components are correct, in particular that the 0.33 ohms really are 0.3 and not 3.3 or something like that. Also check that the 470 ohm and 1k5 to each limiter transistor is correct.

If you think it plays loud with a bulb fitted then it certainly shouldn't be limiting under normal conditions. The bulb will absolutely kill the power available (as it should) and so it should play far louder even with the limiter connected and running on full supply.

36 volts rails should deliver around 50 watts RMS into 8 ohms and the peak current in each output transistor would be around 3.5A approximately. That would develop a voltage of just 1.1 volts across each 0.33 ohm and that would not be able to activate the limiter.

So its great you have proved the limiter seems to be the problem, but something doesn't add up because it should really be cutting in at even full output.
 
Administrator
Joined 2007
Paid Member
I think you have to, but it would also be good to try and find out why it behaves as it does. That would involve using a scope to look at the limiter action under real conditions and to see at just what peak output levels the protection triggers at. It would also show if there were any other issues such as instability that could be causing a problem.

Are you sure the speaker loading isn't to low an impedance ? That would draw more current than expected.
 
I think you have to, but it would also be good to try and find out why it behaves as it does. That would involve using a scope to look at the limiter action under real conditions and to see at just what peak output levels the protection triggers at. It would also show if there were any other issues such as instability that could be causing a problem.

Are you sure the speaker loading isn't to low an impedance ? That would draw more current than expected.

ok. First of all happy christmas to all of you!!

I've scoped output. What I thought it was loud... it wasn't, really. But as I used it with guitar distortion, I could not distinguish the clear guitar sound, so I thought it was the preamp distortion.

When I scoped output before clipping, the AC Voltage in output was 14V.
When I scoped output playing loud but with distortion, which is not annoying in guitar, 39V AC.

W(rms) = 14*14 / 8 ohm = 24.5 Watt.

Something is still wrong here....

Please help!!
 
I stumbled upon this thread with google because my love of the mosfet lead 100 is too strong! Great job so far. I have one which I love dearly but it picks up a lot of radio noise, so I'd love to learn from someone who knows the ins and outs of the amp to recreate one that's quieter and more reliable. Or at least the preamp if it can run off a DC supply as I'm a bit scared of working with AC power circuits and have a rack mosfet poweramp I can use it with. Did you recreate the whole preamp including the push/pull EQ knob?

ok mate, but I don't have list of components. All I have done is posted in this thread. Also, wait until I finish the amp without problems, bc I will upload the new schematic.
 
When I scoped output before clipping, the AC Voltage in output was 14V.
When I scoped output playing loud but with distortion, which is not annoying in guitar, 39V AC.

W(rms) = 14*14 / 8 ohm = 24.5 Watt.

Something is still wrong here....
definitely.
I mean those measurements, they are not compatible at all so one or both are wrong.

14V RMS at clipping will give 28V "RMS" (measured on a regular multimeter) TOPS, and in practice somewhat less.

No way you get 39V "RMS" under any condition, not even with continuous tone perfect squarewave and even less with a regular Guitar played by a Human.

so:
1) connect 8 ohm load
2) scope output
3) inject 440Hz or 1kHz sinewave.
4) check that at low volume it is still a sinewave (i.e. the preamp is not clipping)
5) slowly rise volume until it visibly clips on both peaks, then lower slightly until it just stops clipping
6) measure AC voltage and do the Math.
7) post results.
 
definitely.
I mean those measurements, they are not compatible at all so one or both are wrong.

14V RMS at clipping will give 28V "RMS" (measured on a regular multimeter) TOPS, and in practice somewhat less.

No way you get 39V "RMS" under any condition, not even with continuous tone perfect squarewave and even less with a regular Guitar played by a Human.

so:
1) connect 8 ohm load
2) scope output
3) inject 440Hz or 1kHz sinewave.
4) check that at low volume it is still a sinewave (i.e. the preamp is not clipping)
5) slowly rise volume until it visibly clips on both peaks, then lower slightly until it just stops clipping
6) measure AC voltage and do the Math.
7) post results.

All these measurements were in the output of the amp with the 8 ohm load attached. The 14V AC RMS is a real measurement of the voltmeter, just before clipping.

The 39V AC was a wrong measurement because I put the maximum gain in preamp and the amp was obviously clipping, but It was really loud. Obvously I did this in a short preiod of time to prevent damaging FETs.

I've found a post in which a guy replaced the original pair of sk134 and j49 with other ones and had to make some arrengements.

Can you please tell me what measurements do I need (like drain-source current or drain-source voltage) to make the adjustment in this circuit?

Thanks for answering!!
 
Administrator
Joined 2007
Paid Member
Try this...

With NO load attached adjust the output to give 10 volts rms as measured on the meter. The scope will show around 14 volts peak (28 volts peak to peak).

Now connect your 8 ohm dummy load to the output.

The amplitude should remain unchanged. Does it ?
 
If you truly want the "Marshall" sound then you are going to have to splash out on a 100 watt Marshall valve amp.

The valve pre amp Marshalls sound ok but don't match the full valve versions.

The point is that I wanted to make a solid state amp with good reputation. I made it, as you can see in earlier posts, but I have to change some stuff to fit another pair of FET's that aren't the original ones.

My intention is to learn a bit of electronics (this is my second amp project), and this amp is perfect because it can be made with few components and mosfet gain is interesting.
 
Wrong resistor!
This is a 33 Ohm.
You need 0.33R.
You must see R33,NOT 33R.
 

Attachments

  • IMG_3372.jpg
    IMG_3372.jpg
    600.5 KB · Views: 138
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.