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

[Decko]

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...

I have a DANA VVR2 (not installed) Not sure if scaling the whole amp is the way to go or just the power tubes.

That said, with these two ideas what is the best route to go?

Regards,
Danny

[Colossal]

Hey Danny,

Let's start with your JMP. Do you have the schematic for the model or is this the one you have:

http://www.drtube.com/schematics/marshall/1987pljp.gif

[Decko]

Hi,

It is a 50 watt ( 4 holer) Lead running 6550's.
It is PCB board layout.

Serial Number is 2270H and MFG 29-3-1976.

I do not have a schemtaic.
And I do not thin this is the right schemtaic:
http://www.drtube.com/schematics/marshall/1987pljp.gif

Thanks for your response!

Danny

[plexitone]

Hi.
I'm very familiar with Marshalls and would be happy to help.

Depending on what you consider "high gain", you may not get there without cascading V1 similar to a 2204. However, there are several easy mods you can do to increase gain in a stock 1987.

I recommend a PPIMV for a master volume, which can also increase gain because of its effect on the PI.

schematics
The first one should match your amp but it's hard to read. The second has a very similar circuit.

http://www.drtube.com/schematics/marshall/1987mk2u.gif
http://www.drtube.com/schematics/marshall/1987u.gif

[Decko]

Hi Gents,

Thanks for the recommendation.

PPIMV is installed. Please see picture. Any comments on the installation? Wiring? I based it of Ken Fisher PPIMV...

Works great!

What can I do to tighten up the low end?
Sounds a bit muddy.

Looking for the "chunk, chunk" effect.

Cabinet is a vintage 4X12 with g12m rolas.

Would you recommend EL34's over 6550's for a heavy rock sound?

Danny

[Colossal]

Hi Gents,

Thanks for the recommendation.

PPIMV is installed.

Good call. Works well to get the tone at less than dimed volume levels.

What can I do to tighten up the low end?
Sounds a bit muddy.

Definitely get rid of that 330uF cap on V1a. Drop it to 2.2uF, even 0.68uF. Use a film cap here, not an electrolytic.

Looking for the "chunk, chunk" effect.

After changing V1a's cathode bypass cap, you might consider changing the tone stack's slope resistor from 33k to 47k. This will bring the mid focus up slightly and should have a nice attack. Also, check to see if V2a's cathode resistor (should be 820R or 1k) is bypassed. If not, then after trying out the V1a cathode cap mod and the slope resistor mod, consider bypassing the 820R with 0.47 to 0.68uF. This will bring up the midrange Plexi sizzle a bit and a small bump in gain.

To get more of the hot-rodded JCM800 tone, if that's what you're after, will likely require that you cascade the preamp as Plexitone mentioned. This can be done with the "One Wire" mod or just permanently wired that way; depends if you want it switchable however. First though, try the aforementioned mods changing one variable at a time and see how you like the changes. It's very important to control the changes so you build on the tone incrementally before really altering the amp.

If you are after a much heavier and tighter modern metal sound, it's in there, but it will take a cascaded preamp and some additional component mods to get it. Once you cascade the preamp the gain is going to increase significantly and you'll need to be particularly attentive to your lead dress and using shielded wire for several runs, otherwise you might end up with a squealing mess. Go through your PCB and make sure parts are lined up nicely and not leaning over or leads squashed. Make sure wires running from the board to the tube sockets obey good dress rules such as crossing at right angles to one another. Some production Marshalls got by with production standards, but may not always lend themselves to becoming a fire breathing beast without some rework.

Would you recommend EL34's over 6550's for a heavy rock sound?

Not necessarily, you'll have to see how you like it. Some really think the 6550s are a killer sound and they should sound pretty killer with a modded preamp. But if you do want to switch to EL34s, the 150k/150k grid resistors would need to be changed to 220k/220k and the amp rebiased. The dropping resistor at the entrance to the bias circuit would need to be changed. Also, you'd have to keep in mind that the OT may be best suited for 6550s but I think they will run EL34s too. You might want to measure the voltages in your amp and make (and post) a chart.

Let us know what mods you decide to try and how the tone changes when you do them. For starters, I would most definitely reduce that first 330uF cathode cap way down. That is where the mud is. If you have polypropylene film caps on hand, that is what you would want to use. You want a fast transient response here. Part of getting the ultra-tight chunk chunk is component choices.

[EDIT] You might want to strap a pair of 2M2 across the PPIMV pot. This will bring the total resistance down to about 224k which is a closer fit and if the pot fails will keep your tubes from running away to meltdown[/EDIT]

[plexitone]

One popular PPIMV is the "Lar/Mar" (based on Fischer's), which I have in a couple of Marshalls:
http://metroamp.com/wiki/index.php/Lar/Mar_PPI-MV

It's similar to how you wired yours but usually has shielded cables and the resistors Colossal mentioned (1.5M to 2.2M is fine).

If you don't use shielded cables, you may want to twist the signal wires from the board to the MV and the ones from the MV to the tubes. Add 5k6 grid stoppers on each power tube pin 5 if you don't already have them.

Regarding the V1a bypass cap, I agree that reducing it to around 2uF will help reduce flubby bass but this will only affect the normal channel, not the bright channel. Obviously, If you jumper the inputs to use both channels it will still help.

The stock transformers are designed for EL34's. Marshall put 6550's in amps shipped to USA for a while because of poor quality EL34's not surviving shipping and warranty period. 6550's have more clean headroom and more bass than EL34's. EL34's will overdrive earlier and have more mids.

If you keep 6550's in the amp, I recommend biasing them as if they were 25w tubes to help reduce strain on the PT and OT. Personally, I prefer EL34's but I go for a classic rock tone. If you want a tight, "chunky" metal tone, I'd keep the 6550's.

[Reeltarded]

Definitely get rid of that 330uF cap on V1a. Drop it to 2.2uF, even 0.68uF. Use a film cap here, not an electrolytic.

Looking for the "chunk, chunk" effect.

After changing V1a's cathode bypass cap, you might consider changing the tone stack's slope resistor from 33k to 47k.

Hay! This is all great stuff, and I would only add that changing that slope resistor to a pot so you can change it on the fly. Worthy.

NFB sweepable off the 8 ohm from zero to about 56k although you'll never go that high. I have no objective example of what this does, but you will like it, thumper.

You probably want to go ahead and cascade those channels too.. but I am not sure which way would be best for your amp.

I never cared for 6550s. We used to think they made rock and roll suck, but then we realized that was mostly Gibson's fault, and singer/songwriters. Then the great cloud that was disco, I mean sure I'll dance, but sheesh, don't you people ever sleep? I saw a friend's mother jump naked once into an empty pool. I swear..



Oh, but the NFB thing and the other stuff mentioned. For sure. These features are on the screaming Marshalls I like best.

[John_P_WI]

The ppimv will mess with the NFB and will cause things to loosen as you "lower the volume". Great for taking the edge off or getting a little crunch out of a plexi where the added gain from the pi is needed but may not do it for your high gain quest.

Look at cascading gain stages, pay attention to inter stage attenuation between stages. Some guys will put 100 k on a switch to ground after the 1st stage for crunch.

You can always change the cathode follower to tone stack to a plate loaded tone stack for more gain ala Splawn.

If you give some examples of tones we can give more specifics.

Of course there always is Jose master mod using diodes, check this build interpretation out:

http://ampgarage.com/forum/viewtopic.ph ... om/ddailey

and a sound clip several posts down with Great tones and playing.

There always is the Friedman Marsha or Niks take the Chupa....

Also study the SLO, Bogner, Engl or Egnator schemes.

Finally you may need to play with filtering etc.

Good luck

[Decko]

Very pleased with what I have read and will start to implement the each change incrementally! I will be sure to post the results!

Anything that I should know when working with PCB boards?

Quick question on "stock" lead dress.

I noticed most of the wire used is stranded. Once I started playing with the suggested mods, I would like to clean up the lead dress. What wire would you recommend? Stay with stranded or move to solid core?

I have attached some chassis shots and gut shots for your viewing.

Thanks again for your input....this is going to be fun!

[Colossal]

As John asked, it would be helpful if you could post an example of what kind of high gain tone you are going for. There are a fair amount of differences between the myriad of high gain amps out there. Are you going for 80's Dokken type tone or more modern metal? You didn't say if you plan to cascade the amp but if you are after modern high gain tone, that's probably going to be the route you'll need to take. If you're just trying to tighten up your Plexi and want to keep it mostly stock, then the aforementioned basic mods will help, but it will still be very much a classic rock machine.

I would go with teflon stranded copper wire if you're going to rewire. Far easier to work with, stays put, and you won't melt it. If you start modding your PCB, be careful not to overheat as the traces can lift off the board and that's it. I would use an alligator clip to heat sink your parts when you remove them from the board. Also, you will probably want to have a plan so you are not subbing parts on and off the board endlessly trying combinations. That will definitely stress the board and could even cause a failure or short.

[Decko]

Hi Colossal,

I am bending toward a more modern metal tone.. I will explore that a bit more and give a more definitive answer on the comparable tone as it applies to the varies amps out there.

Most of my experience has been with Trainwrecks and the like so this is new territory.

Danny

[xtian]

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?

[img441]http://www.monkeymatic.com/pub/ebay/marshall800a.jpg[/img]

[img422]http://www.monkeymatic.com/pub/ebay/marshall800b.jpg[/img]

[img374]http://www.monkeymatic.com/pub/ebay/marshall800c.jpg[/img]

[img618]http://www.monkeymatic.com/pub/ebay/marshall800d.jpg[/img]

[img597]http://www.monkeymatic.com/pub/ebay/marshall800e.jpg[/img]

[Structo]

Nice score!

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

I circled it in yellow.

Also, isn't it strange that the heater wires are not twisted?

[plexitone]

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.

A lot of late 70's and 80's Marshalls don't have the heater wires twisted. Lazy I guess.

Speakers jacks have been replaced. Overall, it looks good though.