Best High gain Mod/ Master Volume for a JMP 50 MK II

[plexitone]

Question: can I pull one set of power tubes and run as 50w? Need to rebias?

Yes, but remember to set the amp one impedance below the cab (set amp to 8 ohm when using a 16 ohm cab for example). No need to re-bias.

It probably won't sound that good; tends to lose bass and treble.

[Roe]

Hi All,

Perhaps this is the wrong forum but I will ask the question anyway.

I would like to mod my JMP 50 Mark II Non-Master into a high gain shredder style amp.

Can you suggest a mod? Perhaps throw a schemtaic over the fence?

As far as non-master to master volume...

...
Danny

search larmar master volume and jose a. mod.

[Roe]

I bought a 1983 Marshall JCM 800 2203 this weekend!

Question: can I pull one set of power tubes and run as 50w? Need to rebias?

...

I don't recommend it but you can if you double the load and watch for redplating. voltages will go up when it draws less current

[xtian]

What is the resistor and cap pair there that has been added on?

It looks like the stock attenuator/bypass cap after the first gain stage. Normally it's mounted directly between the low input and the preamp gain pot but it looks like someone has messed with the input jacks and wiring to/from V1.

True. Previous owner said he moved the input jacks from rear to front panel. I'll clean up that lead dress.

[Decko]

Okay, I have narrowed down the tone.

I would like to implement the Jose A Mod.

How many Jose mods are there?

Which mod would you recommend?

Schematic?

Thanks all!!!!

[Colossal]

Okay, I have narrowed down the tone.

I would like to implement the Jose A Mod.

How many Jose mods are there?

Which mod would you recommend?

Schematic?

Thanks all!!!!

Here is a Jose Modded schematic which is based on a 2203. I don't know who the author of this drawing was. There were other amps modded by Jose A. so there isn't necessarily one specific spec, but the schematic here is what is typically referred to. Of note is the cascaded preamp with 2k7/0.68uF with a 220k plate load resistor on the front end. One side cascades into either another 2k7/0.68uF which is the bright channel of a plexi or the bass channel of a plexi (depending on the input), then into the standard 820R third stage (which can be, and often is, bypassed with 0.68uF for a nice midrange boost). All very doable with the amp you have on hand Decko.

The Jose Master Volume mod is the addition of a) a clipping circuit just after the cathode follower and b) the placement of a master volume before the tone stack in addition to the standard master volume post tone stack which is the normal configuration on a 2203 master volume amp. The clipping circuit is composed of a 10k resistor and a 0.22uF (to keep DC) off the pair of zener diodes (which face opposite direction for symmetrical clipping). The zeners can be 12V, 14V, 16V, 18V or 20V. The lower the zener value, the greater the compression, but there is also a significant volume drop in the overall volume (which is not a bad thing with a 100W amp) with low values. 20V zeners for instance result in a classic plexi sound (very loud), 16V (a little volume drop) a more 80s tone and 12V (significant drop in volume...compensate by opening up the master volume) is quite "modern" and smooth, in my experience.

Nik of Ceriatone developed the Chupacabra which is very similar to this schematic except that it has three 820R/0.68uF cascaded stages in series followed by the Jose Master and clipping circuit. The first plate load in the Chupa is 430k (lots of compression). I have built an amp very similar to this spec and it sounds very good indeed. I have to say that I really like the diode clipping circuit. It adds a lot of compression if that's your thing but I found the amp to have plenty of gain without being excessive while maintaining good note-to-note separation. The diode clipping circuit did not sound "phony" at all but it does add a particular smoothness which some might find a bit too modern for their taste. I think it sounds very, very good however and I've been asked to build an amp for a friend based on a similar tone.

Notice in the power amp that the output coupling caps are 0.1uF. The effect of this is subtle and while it does shift the bias excursion time constant, it does seem to give the amp a bit more wider tone, perhaps also contributing to the smoothness.

The icing on the cake is a resonance circuit (not shown in the Jose schematic but is included on many high gain amps) which acts very similarly to a presence control but selects for bass frequencies. With resonance, you can dial in an absolutely massive bass response in your cabinet, which really brings in a lot of punch assuming you have speakers up for the task. With the aforementioned amp and a seven string, I was able to get huge, tight low end and harmonically rich wide interval chords.

As was noted in an earlier post by John, a PPIMV will reduce the effectiveness of the feedback loop, so as you dial down the PPIMV, you will lose the bandwidth flattening effects of negative feedback and your presence and resonance controls will be less effective and have less influence. That's just the way it is, the price to be paid for using a PPIMV.

You might consider increasing the size of the feedback resistor which will make the amp more aggressive, but it's important to get a nice balance here so the amp isn't too unruly. Splawn uses a 220k feedback resistor and this is about as good as it gets; any higher and it's a bit too over the top (IMHO).

[Decko]

Hi Colossal,

A few questions on the JA mod schematic.

According to the schematic, it looks like there are 3 tubes plus one PI tube.

The 2203 schematic shows 2 preamp tubes before the PI.

Does this mod require another 12AX7? What am I missing here?

Can you guide me on the signal path from V1?

Also what is LOOP1 and LOOP 2? Effects?

Thanks,
Danny

[Colossal]

Hi Colossal,

A few questions on the JA mod schematic.

According to the schematic, it looks like there are 3 tubes plus one PI tube.

The 2203 schematic shows 2 preamp tubes before the PI.

Does this mod require another 12AX7? What am I missing here?

Can you guide me on the signal path from V1?

Also what is LOOP1 and LOOP 2? Effects?

Thanks,
Danny

Danny,

The JA mod is really more of a modded Plexi. You can see that in the second and third tube which have the typical Plexi values. The added tube in this case is the one in front with both cathodes at 2k7/0.68. Note that one has a 200k plate load and the other a 100k. The JA mod is higher gain due to that added gain stage in front (1) which cascades into (2) and (3). The amp does take some cues from the 2003 but they are subtle. The tone stack is unique in that it has a switchable pre-stack master volume or the 2203's post-tone stack master volume, and of course the Jose clipping circuit.

As for the signal path, the switches you've highlighted in yellow allow you to bypass the added gain stage (1) for a traditional plexi amp. The amp is derived from a 4-input Marshall so that's why all the options. Yes, Loops 1 & 2 are for effects.

Does this mod require another 12AX7? What am I missing here?

The first tube is the added gain stage(s) and yes you'd have to add that. So you'd start with a classic 1987 or 1959 Plexi, add the extra tube, and configure as shown (if that's the sound you are looking for). You would not need to add the Loops or Slave out if you didn't want them. Just pick the parts you want.

The other route is to start with a 2203 which is a cascaded preamp and then tweak values to really dial it into a modern metal amp, including an added gain stage in front.

I think the choice of amp topology will depend on what you are after. A real Jose modded Marshall sounds very Van Haleny. The bright voiced added gain stage pushes the front end a bit harder and you just get more of that Van Halen I tone. The post tone stack master can be useful and the clipping circuit can give a bit more compression and hair to the preamp which is useful when not running the amp at 10! The 0.1uF output coupling caps are definitely part of the formula so don't overlook those.

[M Fowler]

Dave,

Very good post thank you. I hope to get my #39 done soon so I can continue on to #34, #35, #36, and the Chup down the road.

Marshall amps just seem bring out the tone I am looking for.

If I remember right on your build you were using 12v zener for a more modern tone?

Mark

[husky]

I'm going to comment that I have never heard a Jose that sounded EVHish and I have worked on 10~15 of them. I also owned one Jose did for me on a Plexi and I specifically asked for Ed's mod. Years later while talking to Ed I found out Jose didn't do much to his amps except for retubing and a slave out. I just rebuilt 3 a few months ago for Peter Frampton which were water logged at Nashville, his were all identical Now the others I have seen are a little different same basic idea but I have only seen a few with clipping diodes in them and I have worked on Ed's main amp which really never had a major mod in it. It was pretty close to stock and the critical parts were original and unsoldered. Usually think not so much gain and warm and dark. That can be cured but on a standard Jose after the first gain stage and coupling cap there is 34K or more to ground which kills most of the gain boost. Off the plate of the first stage prior to that is usually a 2n2 and another one going to ground. So there goes most the highs and the bass from the boost. It is an interesting mod because it can be close to stock as well. Most people are luke warm about the tones from them though. I would start with a JCM master 2203, start playing around with that. The jose is a lot of work for something that will make you say meh. I have seen this schematic above but I have never seen a real Jose that looks like that. Granted he didn't exactly do most of them the same way as each other either.

[Colossal]

Dave,

Very good post thank you. I hope to get my #39 done soon so I can continue on to #34, #35, #36, and the Chup down the road.

Man, I can't keep all those #3x Marshalls straight! Is there anything that clearly states what they were? The Chupa is a nice amp. Straight up Plexi on steroids tone.

Marshall amps just seem bring out the tone I am looking for.

I agree! I love 'em!

If I remember right on your build you were using 12v zener for a more modern tone?

Yes, on that Chupa-ish build I made the zeners switchable. 20V adds a little something but still maintains most (probably 90%+) of the amp's volume. 12V zeners add a lot of smoothness, crunch, and compression but they do drop a significant amount of volume. That's not bad though and you can then open up the PPIMV (if you use one) and get the power amp working. I found the 12V zeners to give the most modern sound. And that is with the extra gain stage added in. FWIW, I used 820R/0.68uF on the added stage with a 330k plate load. Dial in the resonance with the 12V zeners and the extra stage and with a 7 string I was able to get some really killer tone through my test 2x12 closed back with EVs. Just huge low end and very tight attack.

[M Fowler]

John, thanks for the information it's alway nice to hear from pro builders what they have experienced and help us filter out the internet digs.

Dave (colossal) and I had discussed the use of zener's in the Marshall amps before and that we preferred without or at least I was opinionated enough that I have stayed away from building the Chupacabra with zeners.

I would suspect that guys like Peter Frampton probably doesn't go for that really gainy Marshall sound others are wanting. I like to try my version of shredding for about 2 minutes and then call it good, get back to getting some subtle sustain, mid range and low end not finger blistering gain.

Mark

[Colossal]

I'm going to comment that I have never heard a Jose that sounded EVHish and I have worked on 10~15 of them....The jose is a lot of work for something that will make you say meh. I have seen this schematic above but I have never seen a real Jose that looks like that. Granted he didn't exactly do most of them the same way as each other either.

Hi John,

I probably should have clarified that my generalization was based on the (one) Jose that I have heard in person as well as a couple of clips here and there. It was about 20 years ago when I helped haul a friend's gear to an audition at a practice space in North Hollywood. I was the friend with a pickup :P . Anyway, I wasn't so much of a gear head back then but I do recall that one of the guys had a modded Marshall and turns out it was a Jose. One of the inputs had been converted to a pot (master?). The amp sounded very much 80s metal but also somewhat bright in the way Van Halen I does, that kind of congested sputtering Superlead-about-to-blow kind of tone. Of course it was turned up quite loud. I wouldn't say a lot of gain by today's standards, but harmonically rich and a full, spongey sound. I can't tell you (or remember much) more than that however. I have heard an amp built close to the specs shown in the above schematic (2k7-2k7-820R(no cap)-CF-PI) and if that's actually a Jose, I would agree that the tone wasn't particularly mind-blowing...sounded good, but like a Plexi really.

[Decko]

Hi Dave, thanks for dissecting the schematic. Helped me to understand the high gain thing...cascading gain stages and such. I will continue studying and planing before I delve into this old Marshall.

Here is the tone I am after:

High Gain Shootout 6

Updated link:

http://www.youtube.com/watch?v=NzrgBF7v9fo

Push slider to 6:42

There were a ton of contenders these guys sampled Mesa, Bogner, Marsha, Soldano, to name a few. All sounded bad ass...but the one that sounded awesome to my ear was the Marshall 100 watt JA modded.

Nice low end, tons of gain but good separation and that midrange shimmer. That is what I am after that midrange shimmer! My design would attempt to inject a bit more low end.

[Structo]

That is not a valid YouTube link.