I've got a Marshall 3520 Integrated Bass System 200W head that I really like, but I'm finding that it is too loud for home use.  The volume controls (separate treble and bass volumes) are barely above zero and the slightest movement takes it from silent to way-too-loud.

Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model?  I've attached power amp schematics of these two models and encircled the differences between the 3510 and the 3520.

If so, would this be as simple as desoldering one leg of R17 and R22 while leaving everything else in place and undisturbed?

I think Billy Gibbons also used the Marshall Lead 12 to record My Head's In Mississippi.

The tag on the back of my Nifty Fifty (for 120 V mains) states the power consumption is 40 W and it's fused at 500 mA.  The power will remain the same, and (as Jazz P Bass stated) the current draw will be half.

For what it's worth, I really like the G10 Greenbacks in my Micro Stack.  I found their slightly reduced high end makes the amp's treble control more usable.

I should mention that my clip was recorded with an SM57 microphone and I think it sounded quite representative of the sound in the room.  As you said, different recording methods will tend to emphasize different parts of the frequency spectrum, and some may not portray the "real" sound as well as others.

If you do get around to trying out different op amps, I would love to know what you thought ... maybe even some clips if you can.

If you get a reply, I'd love to hear what Johan Segeborn says about this as he's quite the champion of the Lead 12 (and probably the reason I bought mine).

That trailing fizzle is definitely present on mine as well.  I've attached a quick recording of what mine sounds like at two different volumes.  The first time is with Gain 7.5, Volume 2, Treble 6, Middle 10, Bass 10 and the second half is identical except volume on 8.  I might be imagining things, but the fade out at the higher volume seems a little smoother, but I didn't repeat it multiple times to find out how consistent it is.

Edit:  I realized that the playback volume on the file was quite low, so I bumped it up a little.  Same recording as before.

I just re-read your thread on the Marshall Forum.  I remember having read it a while back, but didn't have any insights to offer ... and I suppose the same is still true.

I'll try to get a recording of mine later today and post the results here.

I totally agree that you have to know and accept the limitations.  I'm very happy with my Lead 12 within its inherent limitations, but in my opinion the settings used in that video do not produce a very flattering tone.  At maximum gain it does get harsh and having the treble wide open just makes it sound fizzy especially with the stock Celestion G10D-25 speaker.

Not that the speakers could change the amp's clipping behaviour, but I've replaced the stock Celestion G10D-25 speakers with Celestion G10 Greenbacks and I love what they've done for the tone of this amp ... strong, controlled low end, nice pronounced mids, and clear, but smoothly tamed highs.

For what it's worth, my settings are generally Gain between 4 and 8, Volume 2 (playing in a small room at home), Treble between 4 and 6, Middle 10, Bass 10 and I'm usually aiming for something ranging from AC/DC to Ramones in terms of tone.

I don't have a solution, but I can tell you that my Lead 12 (a 3005 Micro Stack) sounds very similar on the fading trail of a sustained chord.  I've always assumed this is simply the solid state devices' way of (not very elegantly) transitioning out of clipping as the signal diminishes below the clipping threshold.  I could be wrong and there may be something amiss, but I'm far from an expert so I can't really offer any input on where to look to attempt to change this behaviour.

In any case, good luck and welcome to the forum!

In the top-right corner of the schematic it specifies that IC1 is an MC1458.  This is a dual op amp that provides the two gain stages shown in the schematic as IC1a and IC1b.  Pin 8 connects to the positive rail as shown and Pin 4 (not labelled on the op amp) connects to the negative rail as shown below R7 in the schematic.  This is the same op amp application used in many solid state Marshall amps of this era.

As suggested, this circuit can use a variety of dual op amps.  I've read about people trying many different dual op amps in these applications.

I've been considering building this amp too, and would be interested to follow your progress.

I don't own a 5203 and I'm not an electronics expert, but I have several small, solid-state Marshalls from the same time period.  Can you read the model number from the top of the transformer?  What input voltage are you using?  The following may be useful for comparison, but keep in mind that mine are 120V input.

My 3005 (Lead 12) uses a 4942 transformer and its schematic specifies +/-14.3V output with 120V input.  I've measured +/-14.8V (29.6V between terminals) output with 122V input.

My 3505 (Micro Bass) uses a 4937 transformer.  I've measured +/-17.3V (34.6V between terminals) output with 122V input.

The only photo I could find of the 5203 showed a 5010 transformer which is the SEMKO model number shown on the schematic of the 5002 (Lead 20) which uses the 4937 in USA for 120V.  So based on this, I wonder if the required secondary voltage of your 5203 is the same as that of my 3505?

Hopefully someone with more direct experience can offer some feedback.

I like the tones I'm getting playing my bass through this amp, so I'd like to make it as reliable as I can with the 4 ohm load I'm running it into.  I'm also quite new to tinkering with electronics, so I'm really enjoying the learning that goes along with any modification process.  This site has been a fantastic resource for knowledge as has the very highly recommended "Solid State Guitar Amplifiers" by Teemu Kyttala which I've been slowly making my way through.

Just wondering now ... would it be better to replace the Lead 12 transformer with a higher VA rated unit,  but the same secondary voltage (so as to avoid changes to the rest of the power stage circuit) along with improving the heat sinking of the output transistors?

That's what I suspected and that's why I've been looking into using the transformer from the 3505 MicroBass as it's intended to produce 30W with a 4 ohm load.  It's also the same transformer shown on the schematics of several other Marshall solid state amps in the 20W to 30W range some of which drive 8 ohm loads and others that drive 4 ohm loads.  Do you think my intention of trying to replicate this power amp for use with my 4 ohm load makes sense in this case?

I've done some more investigation that I'd like to get your feedback on.  I'm thinking about making changes to the power stage of the Lead 12 so that it matches the power stage of the MicroBass since the MicroBass is intended to be used with a 4 ohm total load.  On the attached schematic of the power stage I've shown the component differences between the Lead 12 and several 20W and 30W Marshall solid state amps.  Some things came to my attention that led to some questions:

- In some cases (C8, R12, C14, & C15) the two Bass amps use one component value while the two Lead amps use another component value.  Is it reasonable to think that these differences are related to the "tone" rather than the power output?

- I measured and compared the secondary voltage of my Lead 12 and MicroBass amps.  The MicroBass is about 17% higher.  Is it correct to assume then, that the MicroBass power stage operates at rail voltage that is about 17% higher than the +/- 19V shown on the Lead 12 schematic (about 22.2V)?

- The schematics of the 20W and 30W amps all show the same T4937 transformer while the Lead 12 uses the smaller T4942 transformer.  Is it the higher rail voltage that results in the greater power output?  Considering these four amps as "known" quantities, the lower rail voltage of the Lead 12 into an 8 ohm load produces 12W, the higher voltage of the Lead 20 and Bass 20 into an 8 ohm load produces 20W, and finally the higher voltage of the MicroBass into a 4 ohm load produces 30W.

- Where the Lead 12 uses 1K resistors from the +/- 19V rails the others all use 2K7 resistors.  This difference seems to correspond very closely with the different voltage drop if I assume the power stages operate at 22.2V and drop to the same 16V preamp rail voltage as the Lead 12.

- R18 has me a little puzzled.  Here the two Bass amps use the same component value, but the two Lead amps do not.  Furthermore, the Lead 12 uses a lower resistance than the Bass amps, while the Lead 20 uses a higher resistance than the Bass amps.  So I'm not sure how to determine an appropriate value here.

- There are two component differences between what is shown on the MicroBass schematic and my MicroBass amp.  While the schematic shows the use of a 500mA fuse (same as the schematics for all of these amps) the board of my MicroBass states that it should be a 1A fuse.  And the schematic shows a W005 rectifier while my amp has a 2W02 rectifier.  So I'll probably go with the higher voltage 2W02 rectifier and the 1A fuse.

- Lead 12 schematic shows that C17 & C18 are 25V electrolytic capacitors.  Is this too close to the higher 22.2V rail voltage with the 30W transformer?  Should I replace these with higher voltage caps if I swap the transformer to produce the higher rail voltage in trying to mimic the power stage of the MicroBass?

Thanks as always for everyone's input.  I'm really enjoying the learning process with your guidance and advice.

Quote from: g1 on November 28, 2018, 11:05:30 PM
That is the case here.  Above the speaker jacks it says 8 ohms.
Below the jacks, it says "Do not use loudspeakers of less than 8 ohms.  IE. - 1 x 8 ohm or 2 x 16 ohm."

This is correct for the Lead 12 that I'd like to use.

The rear panel of the 3505 head says ""Do not use loudspeakers of less than 4 ohms.  IE. - 1 x 4 ohm or 2 x 8 ohm."  So I'm just wondering what I can do to modify the Lead 12 head to work with the output impedance that the 3505 is designed for.  The heat sinking is the obvious difference and I'm certainly planning on improving that area.

Thanks also for the caution about swapping the transformer and the suggestions to check the rectifier and fuse as well.

I'll compare the two schematics again to identify any differences, but from what I recall the Lead 12 and 3505 have extremely similar power section circuits and components.