Marshall JMP -78 Master model 100w Lead

[niilopi]

Hey,
Can you guys help me spot the wrongs in my amp. Mods in the circuit, preamp stage sounding electric and bad. New tubes and bias done but no help.

Main wrongs i spotted two of the main tubes have 3W 1KJ resistor instead of 5KW 1KJ. Input jacks, preamp pot and circuit have some wrong resistors and caps.

pictures :
https://postimg.org/gallery/n816md36/

[silverfox]

Not sure which of these would be the amp in question. If you can identify the schematic and indicate the incorrect component values, it should be a good start in troubleshooting the amp. Repost the correct schematic, should it not be this one; if you can identify it.

http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_mastervol_100w_2203u.pdf

silverfox.

[niilopi]

Yes this is the right schematic alltough its with 6550s when mine with el34. I heard the change is just a few components tough.

[niilopi]

https://postimg.org/gallery/1glq17vzo/

Heres a link to measurements when i went trough the circuit board with a multimeter. Sorry about the writings they are in finnish.
Mitattu = Measured

The two first pics are of the circuit board from left to right. And third pic is in/output and preamp pot.

[birt]

well since you have the boards in front of you and you know the connections and values you can write them on the schematic. it will make everything much clearer for you and for us to answer any questions. i suggest you use the schematic you have and keep that layout so it is easy to compare.


my '77 JMP master volume 100W is a 2203 model.
i like this schematic for clarity: http://prowessamplifiers.com/schematics/thumbs/MarshallJcm_800_2203.gif

[silverfox]

Agree with the Prowess schematic being a good one.  What are your soldering and rework skill like? i.e. removing components and replacing with new values? If there are only a few changes it may be expedient to simply put the original component values back in and see what it sounds like. Then, you could make any changes one at a time. Unclear if the power amp was modded to run different output tubes but if so I would just leave that part alone for now.

silverfox.

[niilopi]

Just changed major components to my marshall to get it back to original spec.

Found some cold joints under the circuit board and tried my best to fix anything that looked suspicious. Ground cable from the pot grid was missing.

Replaced :
-Input jacks hi and lo
-bunch of Resistors
-470pF / 500V ceramic cap
-47pF / 500V Silver mica
-F+T 10uF / 450V ELKO
-ERO / Vishay / Roederstein MKT 1813 680nF (0.68uF) / 250V polycap
I bought this Ero for now but ordered also a SoZo Next Gen 680 nF = 0.68 µF - 160 V because i read a lot of good about this cap.

Im not an expert with this yet so correct me if i got some wrong components before i blow up my amp :D

Heres the schematic i made before my changes with the wrong values written in red. Theres one thing bothering me when i measure the negative feedback resistor 100k the multimeter shows me 4.5k ohms like in the presence cap. Is this normal? Also the huge diodes in the rectifier worry me as i dont know if they will work normally it says LT 40 6A07 on the diodes and they are 6A. Should i just change back to 1N4007 with 1A?

https://postimg.org/gallery/okfc6daw/

Tomorrow gonna get some shielded wire and put the last wires together. Then i can do some voltage measuring and finally some testing after bias.

[silverfox]

Judging from some of the component values used to mod the circuit I'm going to suspect you were at least getting some grid blocking; An over loading condition where the input to a tube gets flooded with electrons. I say this due to the bypass cap at V1b having been changed to a 1 uf and the attenuator resistor being reduced to 320k ohms in conjunction with the increase to 2M of the grid leak resistor on V1b; (some one will likely correct me if that is an off base assumption). I believe the change in the plate resistor from 100k to 150k would have changed the headroom-gain of V1a and with the cutting, (it looks like), the cutting of the 470pf bypass cap, again this would favor low frequencies passing onto the next stage in Hi gain and also low gain. I'm going to guess this at times sounded like it was cutting out and the overall sound was bass muddy and fuzzy. (Okay guys, fire away). The change of value in the slope resistor would shift the frequency response in the tone stack, a little, but I don't know which way. There was a discussion of this very topic on a thread I read last night.

So far, the changes you made to return the circuit to it's original design would not cause any blow up. None of the, "return to stock" changes would do that.

As for the change in value of bias feed resistor, I believe that would have raised the voltage in the bias and caused the power amp tubes to turn on at a higher signal level which means: The distortion from the preamp would be at a higher level as the amp got louder. As usual but in this case, the bad distortion would really become prominent. If the bypass cap in the phase inverter was doubled, lower frequencies would be shunted out of the power tubes. A bass cut.

What mod recipe was the basis for the changes? Was this someones tried and true modification of did the tech just wing it? I don't know myself without going out and looking for Marshall mods to compare it to. Is this possibly some sort of Cammeron Mod?

As for capacitor brands and tone influences, one cap type and brand being the secrete sauce: It really depends more on the application of a particular capacitor technology rather than a brand name. While brand name can have an impact on reliability it is more the construction details and ingredients used that determine the frequency response and noise levels associated with a particular capacitor type; mylar, polyester, electrolytic have different tonal and electrical properties associated with each technology.

The Diodes are fine. They are very heavy duty for this application and appear to be silicone diodes but I couldn't find an exact datasheet quickly. I wouldn't change them.

Have you tried it yet?

silverfox.

[niilopi]

I got to say i havent found a good answer to someone making these mods but from what i see as an amateur there seems to be a lot going on for high gain and lows. As for the components.. some of the cap changes had been done with very good components but possibly someone else has altered another part of the amp at some point as there were some really bad caps also and very bad soldering at points.

The amp has sounded alrite with a single coil stratocaster but im mostly playing a -76 humbucked les paul so the overall sound has been very muddy with these pickups. Mostly ive just been playing without the preamp stage using pedals because of the lousy preamp sounds.

Tomorrow im picking up some shielded wire to finish the wiring off and then give it a go after bias to see how it sounds.

This cap i ordered SoZo should be made to be like the original mustard caps and especially for guitar amp purposes. And it was only 10euros with postage so its still a long way down from the NOS caps available.

[PRR]

> i measure the negative feedback resistor 100k the multimeter shows me 4.5k ohms like in the presence

One end goes to the OT speaker winding, near-zero Ohms.

The other end goes to the 5K presence pot/resistor, right?

Just like that, you will be measuring 5K+0, in parallel with the 100K. Will read a bit under the 5K in the presence leg.

Assume it is right. If you HAVE to know, lift one end.

Usually, resistors in-circuit with a lot of other resistors, you measure all the resistors together, and "none of them measure right!" That's normal. Tube circuits are a semi-exception: "most" resistors go to tube pins, a cold tube is an infinite resistance, you get the resistor value directly. But there's always some networks with several resistors in some kind of loop that won't "measure right" in-circuit.

[niilopi]

Finished the wiring. Also the shielded wires were regular ones before so i changed them to proper. Now the amp is all stock. And no noticable hums.

Bias went now way up to 37ma and would have gone even further than before the parts were changed top bias was 32ma.

Did a little test drive and the amp has changed dramatically from what it was before. No more mud fuzz shit but clear even when breaking up. My amp and guitar really came alive. Its a huge win for me as ive been playing this amp for 8years like this.

I also made myself an attenuator and tested that for first time. After the surprise that my amp is now killer sounding i plugged it in and set the masters to a place where the main tubes start driving and there it is. The sound ive been hearing on my favourite records and never been able to achieve. The amp is really driving the guitar for the first time. Huge sound and great tone.

Theres still a few questionmarks.
-preamp pot has a Blindspot from 0 to about 1or2. After this pot works normally. If i turn the preamp all the way down theres no sound at all even if master is up is this normal? The pot has been changed at some point not original. Value is original.

-Effects. Most of my effects sound ok before the amp on my pedalboard. But is there a way to get in the delays and fx that should be on a loop without a loop and without losing tone?

[silverfox]

Great to hear it's sounding better. If it were me, I'd just play it for a few days and let everything settle out. Sometimes the initial sound can be misleading to some degree- Good or bad... The pot may be the wrong value; Has that been changed? It is also possible it has a dead spot in it or a manufacturer's defect.

In regards to Effects placement, there are some general rules I've seen. Here is something that came up with a first search: http://www.rolandus.com/blog/2013/03/06/how-to-chain-your-guitar-effects-pedals/. Most of the effects work best in the Loop while, as I recall, placing the Delay in front of the Distortion is not the best choice. As for the Loop: That is a point in the amp circuitry, between the preamp and the power amp.

Congratulations on the repair.

silverfox.