Thanks Tassie ... very much appreciate your input!

I have decided to go the route of trying the log pots for the treble and bass volume controls.  You are correct, the pots are the 24mm Alphas.  I've ordered the three different values that were available in log taper (4.7k stock value, along with 22k, and 100k).  Based on the results of the circuit modelling, it seems there's nothing to loose by trying the 100k pots first.

I love the idea of the push-pull, but I'm going to keep this simple.  I'll be very happy if I can just get some finer control at low volume levels.

Hey Phil,

Thank you so much!  My understanding is improving slowly, but steadily.  If you'll indulge a few more, somewhat rambling, lengthy questions ... if not, I totally understand and thank you again for everything you've shared.

I tried modelling a few different potentiometer values (4.7k, 47k, and 470k) to gain additional understanding and found that the higher value pots produce the same range of sweep when the upper and lower limits are treated as a percentage of pot value.  For example, the total gain range is the same for the 4.7k pot swept from 5 ohms to 4.7k ohms as compared to the 470k pot swept from 500 ohms to 470k ohms.  This is as I would have expected since the voltage divider should operate the same as long as the proportions remain the same regardless of the absolute values.  Am i correct here?

One thing I did not expect is that the higher pot values seemed to have a slightly stronger low-bass response (from about 40 Hz to 200 Hz) as compared to the lower pot values.  See the attached "Overlay 1" where the blue traces are 4.7k pots, the green are 47k pots, and the red are 470k pots.  The plots are essentially identical above 400 Hz, but show that the higher value pots provide more bass extension at corresponding settings.  Is it correct to assume that this is due to the higher resistance to ground of the high-value pots is allowing more low frequency content to pass through rather than bleed off to ground?

Lastly, I tried modelling the same three pot values swept from an initial resistance value of 5 ohms up to their respective maximums (5 ohms to 4.7k, 5 ohms to 47k, and 5 ohms to 470k).  In these plots I did see that the higher pot values indeed provide greater range at the quiet end of their sweep, as shown in the "Overlay 2" attachment.  This again is as I would have expected, but my question is which sweep is more representative of what a real potentiometer would produce?  In other words is the lowest attainable resistance of a potentiometer some particular value regardless of the pot's maximum value or is it a percentage of the pot's maximum value?  Or (as I fear) does it vary from pot to pot and manufacturer to manufacturer meaning that this has all been an exercise with no practical value?!

Thank you again!

I very much appreciate everyone's input on this as I'm quite new to the world of modifications.  I have a couple of questions for those with more experience that myself (which is just about everyone!):

As my end goal is to make the volume controls less sensitive at very low settings, is there something preferable about adding resistance (either fixed or a pot) before the power amp versus changing the volume pots from linear taper to log taper?

I have found volume settings that I am quite happy with, but I would really like to have finer control in this area and it seems like log taper pots would accomplish this without adding components or making permanent modifications.

If I may ask another question to further my understanding of this circuit:  It seems like the two volume pots (VR10 and VR11) are not located between their op-amp's input pins and output pins, and so should not affect the gain of these op-amp stages.  Is it correct to say that these two volume pots control the signal passed to the power amp as voltage dividers rather than by altering the gain (and therefore clipping behaviour) of the op-amps?

Thank you again for all your input and insight.

Thanks Tassie, that's a nice solution in a single cable.  If the volume control in the effects loop had worked out, this would have made a very neat and tidy installation.  Unfortunately, since the boost and volume controls are after the effects loop in this circuit, this didn't work out as well as it would have in a circuit where the effects loop is located after all the preamp controls.

Earlier today I remembered that I had modeled this preamp circuit in Micro-Cap so I took a look at it again.  I used the stepping analysis to model the effect of sweeping both volume controls from 0 to 100% in 10% steps using both linear and logarithmic tapers.  I've attached the plots and it seems the linear taper produces a huge jump in the first 10% of the pot's rotation, where the log taper uses over 70% of the pot's rotation to cover the same range.

I don't have much experience using Micro-Cap, so I'd be happy to share the file with anyone who's more familiar with it and could verify whether my analysis is valid or find any mistakes in my use of the AC analysis tools or the model itself.

I've written to an engineer at Marshall to get their input on whether swapping from linear to logarithmic taper pots would make the low end of the volume controls less sensitive to small movements.  I'll post an update if/when I get a response.

Thanks Will!

I probably should have clarified in my original post, that while I would like to reduce the volume (or have better control over the volume at the quiet end), I DO want the preamp to be pushed into clipping.  I like the tone and overdrive grit that I'm getting with the first gain control at maximum and most of the EQ knobs boosted to about 3 o'clock.  So, while lowering the gain of the first stage would certainly reduce the overall volume, it would also not overdrive as much and that's less desirable to me than the too-sensitive volume controls.

I think I'm going to order the log pots for the volume controls and see how that works out.  I'll post back here if/when I try it.

Yup, my dumb mistake.  I realized after reading the first few replies that the transformer is in fact different between the 100W and 200W models.

Today I tried the "pot-in-a-box" in the effects loop, but it is quite finicky and also affects the way the boost control responds and hence the tone.  This amp doesn't have a conventional single master volume control.  It has separate treble volume and bass volume with a variable crossover.

I've attached the preamp schematic, and it seems there are three gain stages and their corresponding controls (boost, treble volume, and bass volume) after the effects loop.  Would it be correct to say that the placement of the effects loop within this circuit limits the effectiveness of the "pot-in-a-box" to control the overall output volume and causes it to affect other aspects of the amp's tone?

I also noticed that the treble and bass volume pots (VR10 and VR11) are linear taper rather than audio taper.  Would this tend to make them more sensitive at the lower end of their rotation as compared to audio taper pots?  If so, would it be worthwhile swapping these for audio taper pots?  I suppose this thread has become more about managing the quiet end of the amp's volume range rather than reducing the amp's power output.

Thanks again to everyone for your input.

I'm thinking about building a simple DIY volume control to put in the effects loop rather than buying a more complex (and expensive) volume pedal.  Is there an ideal potentiometer value if I go with a simple input->pot->output arrangement?  Any drawbacks to such a simple circuit?

Thanks again for talking me down earlier!

Thanks for the replies all!  This is the first amp I've owned with an effects loop ... never even occurred to me to try a volume pedal in the loop.

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.