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jrow 6th January 2018 11:22 AM

Building Marshall Lead 100 MOSFET
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Hi there,

I've thought of making the Marshall Lead 100 Mosfet because of the opinions and reviews. They're so positive, and people say that it's probably one of the best solid state sounds, similar to tubes.

But I just want to build the power amp, because I'm going to use another preamp for it. I know that it's not gonna change the sound a lot, but I just wanna try that circuit.

Here are the schematics:

Attachment 655038

First of all, as I don't have much more experience in electronics (I've only built another 100 watt amp with 2SC5200), I don't know the resistor wattage, except for the 5watt (0.3ohm). I'm going to put them of 1/2 watt all of them. Is it a good idea?

I have the +49, gnd, -49 power source with a toroidal transformer and 2x4700 uF capacitors (63V), I think that's right.

Lastly, I don't know the function of the first two diodes (D1, D2). They are not necessary, aren't they? Just if the preamp is built, isn't it?

Do you think it is a good project idea?

Thank you so much for helping, JROW

The full schematics of the 3210 head.

Enzo 6th January 2018 07:36 PM

Your biggest issue will be what are you going to do about the 2SK143 and 2SJ49 MOSFETs?

JonSnell Electronic 6th January 2018 07:57 PM

D1 and D2 clamp any over voltage on the first transistor. The function is to avoid/sink static/over voltage to within the rail voltages, avoiding damage to the transistor.

GeorgK 6th January 2018 08:00 PM

The Marshall power amp will certainly do its work when built properly, but I am not sure what should be so special about it? (Except that it uses long-obsolete MOSFETs?)
Apart from that, 1/2W resistors should do nicely.
The diodes at the input are for protection. You can leave them, but I would keep some form of voltage clamping, especially if you use a tube pre.

jjman 6th January 2018 09:25 PM

I owned that Marshall. The gain channel was useless but that has nothing to do with its power amp section. My understanding is that MOSFETs function in a manner that is somewhat similar to tubes is some respect. I've also read that this fact does not automatically mean they sound like tubes. I don't know if I ever cranked the clean channel nor if the output section was capable of overdriving or sounding like tubes.

GeorgK 6th January 2018 10:13 PM

This is just a power stage, nearly as simple as can be. Using MOSFETs rather made Marshall save a few components.

I remember trying one of the Mosfet series in a shop when they were available new, in the late 80s or so. Had only little playing experience then, but I did not like it, did not buy it - and did not regret it. They were meant as a cheap(er) alternative to the "real" JCM800 series amps.

To see them on ebay for over $400 is ridiculous IMHO.

Yes, there are two power MOSFETs in the amp. For Marshall this was reason enough to write it on the panel. I doubt they make much difference.

Marshall later chose a different approach with their Valvestate amps, as they were working with current feedback on the speaker then. Don't know if this made the difference, but I have better memories of the Valvestate amps than of the Mosfets.

Enzo 6th January 2018 11:31 PM

I liked those power amps, to me the MOSFET output stage somehow sounded more "effortless" than their regular bipolars. Just my opinion. I loved the old Trace Elliot amps for similar sonic reasons. That the Marshall had a less than stellar preamp was a separate issue to me.

jrow 7th January 2018 11:26 AM

What about buying both 2SK134 and 2SJ49 in ebay? (or in similar pages). Do you think it's probably they give me a fake one? (20€ pair)

If they're fake, will it be so different?

And also I've thought about buying some equivalents like IRFP240 and IRFP9240.

Mooly 7th January 2018 11:33 AM

The IRFP's definitely 'aint equivalent to the 2SK/2SJ devices. They are a different technology with totally different characteristics.

Lateral FET's are available from the likes of Profusion:
Lateral MOSFETs | Profusion

jrow 7th January 2018 11:53 AM

Something like BUZ900 and its respective P channel?

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