How to change a MOJOTONE Marshall kit into a 1968 Marshall Plexi

Today we will be revising a Mojotone 100 watt kit amp into a 1968 Marshall plexi. The revisions will include the ability to play at low volumes without losing tone.

[@phillamoore157]

What an amazing video. I've never seen someone break it down like that. It would be very nice (albeit horribly impractical) to hear a tone test with each of those mods. The amazing part of a plexi is that almost every tone us guitar players love is based on that. So, I suppose as long as you're working with a "true" plexi platform, you can achieve some amazing tones. It's the "learning how to be proficient guitar player" that always get's in the way 😂

[@dougmorisoli6482]

Appreciate that you noted importance of good solder iron with adequate heat (and tip). You could do another video on noting all the solder joints on the board that need a redo. I wish I learned this early on. I once replaced wiring and seloector switch in my Les Paul and later had problems from bad solder joints due to cheapo ppencil point solder iron that wasn't up for the task.

[@sglee4708]

I just found you great video I'm new at this and I'm getting ready to start my first 50 w Marshall I have two years electronics training from Vo-Tech School

[@FeeserAmplification]

Fantastic. If you have any questions, please do not hesitate to ask.

[@sglee4708]

@@FeeserAmplification well thank you sir that is very generous of you I'm a little late starting I'm 56 now

[@stephenchow5161]

very nice. i love these marshall mod vids

[@roncarter2188]

The 250pf cap on the paper&oil caps is that on the end that's grounded? And what does that cap do? Thanks great video.

[@bradt.3555]

You allway's lose tone at lower volumes since a lot of the "Marshall" tone comes from what the speakers do at higher volumes. Blending the 2 channels is also a big part of the super lead sound. Not at all what I was expecting. The original circuit with a output attenuator should get you closer to a super lead. Personally the bass model like Duane Allman used is the ultimate Marshall tone, but there ya go, my 2 cents worth.

[@sambekins]

It is only a few values changed that takes it from 1959 specs to bass spec. .01 in the P.I. VS .022 .01 vs .022 V1 some 1959s had .o68 also in v1 and .068 across v2 and the values in the cathode flower change 56k and 250pf vs 33k 500pf and 1959 has either a 500 or 250 pf across the 1 Meg vol pot on ch 2. I prefer a hybrid spec a cross between lead and bass.

[@bradt.3555]

@@sambekins Yea , I've looked at both schematics many, many, times. The few values that change make a pretty substantial difference in tone. Because of parts tolerances back then Marshalls tended to sound different from one another. Marshal tone is kind of a generalization of what the person saying it is, based on the Marshall (s) they've heard. Back then people didn't care as much cause people weren't "chasing" other peoples tone like today. Rumour is Duanes had some part changes. Just saying of all the marshalls I've heard I prefer Duanes tone. Actually these day's I lean way towards Dumble sounding amps anyway. Unfortunately a big part of the tone was the volume they were played at, and even with an attenuater that can't be duplicated. A speaker at hi volume act's very different than at low volume.

[@sambekins]

@Brad T. yes they do which is why 25 watt gbs sound like they are dying full tilt. THEY ARE! they can only handle the hot cold cycle so many times before one day they quit. A 100 marshal at full tilt is putting out 145 or more depending which pt is in it ive seen 100 Watters do damn near 200 with a a plate voltage of 503. Duanes amp was a strait 1986 with the P.I. caps being lowered to .022. It was like all of us the tone in his hands. But Duane was special also.

[@user-qm7nw7vd5s]

What a beast! Love that point-to-point wiring, although the soldering is obviously by an amateur, using way, way too much solder. Great materials, and elegant chassis layout nevertheless. 👍

[@SuperSeanXXX]

That's a depth mod on the feedback resister he did

[@cardbored_]

I’ve heard that the wire runs can really affect the sound/noise. Is there any truth to that?

[@FeeserAmplification]

This is correct. However all engineer technician are aware of this issue and there are simple fundamental techniques that can resolve those issues within the circuit design.

[@Frankentoane]

Isnt it too dark sounding unlike a classic marshall? From what ive seen theres too much preamp plate snubbers.

[@djlittle7349]

Wow very good bro

[@roncarter2188]

Looks like there are some of those cheap silver mica caps. If they cost 50 cents each then you are asking for trouble, speaking from my own experience with those caps.

[@FeeserAmplification]

Excellent observation. Please let me know if you have a component preference.

[@deanallen927]

I'm not very good myself, but sorry - I see a 1/2 mile of wire in there that just looks like trouble. It also looks very much modded at the start.

[@danieln2967]

What bright cap value do you recommend? Mine is a 50W kit with PPIMV. Thanks!

[@FeeserAmplification]

I like to use the 500 pico farad silver mica types in all my circuit designs.

[@SuperSeanXXX]

Umm I don't have to know the values but did you have to use the smoothing silver micas on the plate resistors of v1 because of fizz oscillation

[@FeeserAmplification]

This particular amplifier was being voiced for the 1969 Marshall Plexi. To achieve this, I had to make revision to that particular amplifier. The first step was adding the the dual 500 pico farad at the anodes on V1. This was only one tiny step in a multi-step process.

[@SuperSeanXXX]

@@FeeserAmplification I understand you were voicing the amp...but stock 69 or 68 Marshalls usually don't have bypass caps on the plate resistors unless the highs are unbearable or it's oscillating....the reason I'm asking is because I have a 69 build that I am trying to voice as a 68....the 68 has lower filtering and an 820 ohm on v1b cathode with .68u cap....but I am getting oscillation...I put a 100pf where you put the 250pf on this amp but it didn't change anything....I put a 250pf across the plate resistor 82k of the phase inverter and that seemed to knock it down about 85 percent....do you know how I could stop it completely?

[@FeeserAmplification]

@@SuperSeanXXX Take the amplifier to your local technician and they will detect and fix the oscillation.

[@pedrolourenco2707]

very Friedmanesk!

[@icepick859]

Can you do a video on jose master volume with diodes?

[@FeeserAmplification]

Absolutely. I will be creating a full line of how to build ,high gain amplifiers soon. I will include all types of diode clipping circuits as well.

[@andyhayes7828]

'Here in California' 😂

[@beatlejuice5951]

I have a jcm800 clone and this video is making my OCD go mad in wondering if mine sounds as good as a real 80s non reissue 800.... 🙄

[@FeeserAmplification]

Lol 😂

[@bluesky6361]

White lab coat is a hoot. Probably wears it to bed.

[@dsvet]

It's an ESD smock. You've obviously never worked in an ESD protected area before..

[@beatlejuice5951]

Are you familiar with Organdoner amplification? Or Granger Amps?

[@FeeserAmplification]

I regret that I do not know these amp builders. Are they California based?

[@beatlejuice5951]

@@FeeserAmplification no, he's a good guy named Tim Robinson out of Gainesville Florida, I found him because honestly I'm a younger musician working a shit job in Portland Oregon and I've always wanted a jcm800 but I could never afford one, at least right now, so he built me a clone with a couple of mods, gridmaster and a depth knob, and it was 1200 bucks, its pretty amazing but yeah Organdoner amplification, he does some great clones man.