The best recipe for taming a JMP or jcm is to stay away from the bright caps and just put a 10 band eq pedal in front of it or a booster with tone control if you want to keep it simple. Your bright cap contributes a lot to the preamp gain as well so you will affect gain as well by snipping the bright cap. If you want to change caps just change the bright tonestack cap from 220 pf to 1nf to shift the mids  or make it switchable

Hi Stuart, I enjoy your videos alot and always watch for new ones. I have two observations: 1.Testing an amps tone is very complicated. Your proceedure was looking at what happens to the treble boost when you made the mod but that was not showing what was happening to the rest of the freq band when you changed the potential divider. Surely the lower tones were also increased. You were not seeing that as you were just testing higher frequencies. I am thinking you need to test a complete sweep of the whole range using a spectrum analyser so you can see the effect across the whole range. 2. Having said that, I totally agree with other comments about this amp needing to be played at volume to see how it really performs (I have one in my collection and its a firm favorite for lead work). Suggestion: Why didn't your customer use an equalizer pedal at the front of the amp  so they can shape the tone for lower volumes as they like. At least that way they wouldn't be screwing up the tone of one of the best lead amps responsible for so much great rock music in the 80s and beyond. Then if they want to play loud they can switch the pedal off which is better than a mod you can't switch in and out.

FYI: All factory sweeps are done with the tone stack and volumes at maximum and the signal generator used to set the level for clipping 😉

Hi Stuart! First let me say I really enjoy watching your stuff and I learn a lot from it. In case of this amp I think it would have been a good idea to just remove the bright cap from the gain pot and leave the treble peakers alone. The gain bright cap is especially effective in lower gain settings (as seen in the video) and removing it doesn’t mess that much with the higher gain tones. I assume with your mods the amp might get a little muddy at higher gain. A way to give it more bass response might be a „fixed depth circuit“ in the negative feedback. Greetings from Berlin, Roman

They were voiced for les paul type guitars hence the rolled off bass and boosted treble

An odd design flaw with this combo. The play through at the end was very pleasant; very Marshall-"esque." That mod indeed tamed the ice pick to the eardrum treble. The owner should enjoy the new voicing of this JCM 800. Well done, Stuart. Cheers!

Nice one Stuart!

Excelent!👏👏👏

you should only remove that 1nF cap on the 1M pot and if that doesn't help, than you should remove the 470pF cap

Stuart,  this is a thought provoking video.  I didn't really comprehend much of what you were saying about frequency. This will cause me to do some study of tone stacks and how they function. The tool you were using seems quite complicated as well. I am appreciative of the video you did on schematics and it has helped me greatly.  I'm wondering if you might be able to do something that would teach us more about your tools as far as how and why you use them.
Thank you for this as it does give much to think about.
Mike

Try injecting a signal on the grid of v1b and see how it responds. Thanks for making these videos.

watching so many of your videos from usa i am picking up your british accent.   :).

Marshalls , esp 800 , are made to be played loud . At decent volume the magic happens . Note clarity at volume is what they are all about .  Normal that at low volume they sound bright/thin . People should not buy these amps to play at bedroom levels , its useless to do so , then complain about a thin sound . Marshalls are all about cutting through in a band mix . Best results are obtained with decent amount of volume and with proper cab and speakers . Must get the power amp tubes cooking in harmony with the pre amp . Its a good well known proven recipe when in the right hands . Still , appreciate and enjoyed the informative video .

I've never seen this done the way you demonstrate it Stuart, Very interesting indeed. When I was playing around with a Fender Twin I used a Tone Stack Calculator which is a free download .. The idea is it takes all the common commercial amps (Fender, Marshal, Vox etc) and you can play around with the responses of Treble, Mid & bass curves.  One of the issues with guitar sounds is it's 'subjective' as there are so many elements in the chain that can alter the end result. It'll be interesting to know what your customer's thoughts are with the altered, but still good sound from the JCM-800 .

Hi Stuart. Thanks again for another great and informative video. I was wondering, where exactly in your dummy load do you have the probe tip and ground cable connected to ? Is it just at the input jack ? Thanks

My 100 watt marshall 2203 will vibrate my house thru a 4x12.     i dont do it often but when I do it still intimidates me.

Oddly enough, my JCM800 from ’83 needs the treble all the way up to sound good through a V30 cab. I think something might be off, and the Bad Cat Unleash could also be eating some of the high end 😊

Interesting way to do some mods. Easy to see the effect of what you're doing with the oscilloscope. Sometimes I find that square wave testing can be helpful as well... very easy to see/hear treble boost, bass cut, etc. but it's not well suited to seeing it in terms of dB.

Hi Stuart,indeed,  the bass control doestn't give much tonal change. You could change the 22nF(at baspot) to a 47nf or 100nf. This gives  much more Bass control. Greatings Guus.

mullard i63 at v1 problem solved!