Is JCM900 using the same OT as the Plexi?

[Jazz P Bass]

What does clicking on the 'infarction' icon do?

[Enzo]

You know, I am a 60's kind if guy. I think we should all get along and be peaceful. we should give others some space. Without unduly patting myself on the back I offer my track record here as to how I treat people... overall. Now we have the strange pathetic case of SGM. As I have said before, when you have a party, you sometimes have a guest who acts up and misbehaves. And out of social grace, you tolerate him. But when it gets to the point the guy is pissing in the punch bowl, it is time to drop the polite act and deal with him at his own level.

How big a jerk does someone have to be before you let yourself react? How much bullshit do you let go by to inexperienced readers who have no reason to know it is bunk? How long do we listen to Cliff Claven dispense "little known facts"?

[Alan0354]

I don't mind heated debate, but stick with the facts, avoid getting personal like calling others liar or make up things. SGM does seems to do a lot of it. I guess I am still new here, I don't feel it's up to my neck like some of you guys......yet!!!!

This is my thread, but I won't be offended when someone goes off the subject, I won't call people hijacking it. Just let the thread flow whatever it leads too. Just cut the nonsense. But even if it is nonsense, it's still ok as long as it's not pointing at me!!!!

I don't want this thread to be closed yet, I am working on my Fender style amp, I yet to work on the Marshall!!!! So I am not done by a long stretch. And even if I start another thread, what make you think it'll end up differently? At the mean time, you guys can keep going, I don't mind.

[cyclone]

He called me a fathead ....

[cyclone]

I did no name calling. read the posts big T. I just called it

[cyclone]

i don't disagree at all with SGM on the fact that he constantly and righteously promotes, its music first , daz, u know, chuck, you other guys, steve a, not sure who performs here, anyone who has performed live, how your amp makes you feel is the only thing.

[km6xz]

Being an engineer, I am sure the first thing you would advise others before attempting to design or change an existing design, is to define precisely what the goal is. You already mentioned fuller bass as a goal. That is easy to accomplish so start with that and layout out a plan of attack after measuring your baseline of performance before any work is done. That way, you can measure your progress and judge when to stop. I would suggest you not take wholesale shotgun approach since very one of the listed mods is interactive with the others and soon, cause and effect no longer is isolated to an individual step you are working on. When listening to the results, remember our brains are really poor in remembering sounds or being objective. We remember our impressions of a sound but few people can remember a sound. Moving the amp/speaker orientation a small amount will change the sound in an acoustic environment far more than a minor eq or gain change will. The best way to test a modified amp is an A/B test against an unmodified amp, like is done at the factory in late design prototypes, and not by introducing the time element as when playing/ listening, disassembling, modding, reassembling and playing again. In that period of time between samples, everything changes. You mood, whether you got sleep or had coffee or had an argument with the SO all impact our perception a great deal. Another help setting the controls to measured nominal values, because a hair's width difference in knob displacement will result in more change than the mod in most cases.
When evaluating the problem sought to correct, weigh other options. For example the assumption is bass is too weak. Is it? Maybe the mid or highs are too strong, and the difference is good to know for stage gain planning. Other issues mask a perception of bass, like mismatched tubes in the output which changes the second harmonic, which is reproduced more than low fundamentals by the speaker. Our brains equate a strong second in a harmonic sequence to a stronger fundamental even if we can't hear it as well.
So my suggestion is to take small steps, check against goal, and base line measurements, record the results of each test and the base line to help cue your brain in remembering the sound character between tests that could be many hours apart. When measuring the baseline performance, include set control values, AC mains, a repeatable signal source, not your playing(gets you out of the feedback loop) by recording direct out of your guitar into a recorder, unequalized(make sure the record amp is very high Z or use an active pickup) and use that as the signal source for all tests so one greatly varying variable is eliminated. Re-amping is about the only way to get a reasonably consistent test condition for listening tests.
The one most often overlooked concept in modding, is one you as an engineer will appreciate: there is no free lunch. Any changes introduce side effects that can impact the overall usability of the item, its reliability, its overload handling etc. For example, increasing bass response has many potential side effects such as over taxing the power supply, or creating undesired inter-modulation in earlier stages, OP saturation, exceeding the excursion tolerances of the speaker and reducing overall perceived sound level. Any mod is a balancing act between what is acceptable to give up in exchange for a desired addition. As you know, very design is a balancing act of opposing goals and the best design is usually in the center of the options when total service of the item is viewed. If a specific goal is decided on, my design principle is "do not get married to one solution", when you come up with a plan that makes sense, stop and think of another approach, and maybe a 3rd and weigh the compromises needed with each before proceeding. For example, increasing bass response of an amp might stress the speaker, power supply and transformer, but is the speaker or amp the real source of the lack of lows? Increasing bass response would just make the speaker problem worse if the weak point is the speaker. What about replacing the speaker[s] or acoustic characteristics of the cabinet to get the bottom authority but not stressing the amp more, or generating unpleasant intermodulation due to driving an amp harder for any given SPL?.
Good luck, approach this project how you would your other engineering projects were approached. You have an advantage over the normal hobbyist who finds an internet "expert" whose tastes and problem solving traits are very different and just throwing in one mod after another until something works. Only you know what you mean when you are interested in changing the total system performance.

[Alan0354]

Good post. That's what I do. I spend a lot of money buying one of the best amp for clean sound.......73 Fender Pro Reverb just as a reference. This is a constant sound I use to A/B compare. I am not gifted to have an absolute ears that can tell the slightest difference. So I always have to have a constant reference. I cannot afford to have two identical amps all the time, but using a high quality sounding amp as a reference is about the next best thing.

I tried to be objective and look at SGM,s long list of mods. Some make sense, some doesn't. I am not going to wholesale modding. In fact, I likely fix it and listen to see whether I can use it as reference for high gain channel ( Pro Reverb does not have high gain, it's for clean reference). If it is not good enough, I just rip the guts out and put in a Plexi circuit!!!

[Alan0354]

I finally get to the Marshall today trying to fix it before going under the knife, I only found both the power fuse and the power tube cathode fuse blown!!! I checked through everything for shorts ( rectifiers, caps etc.), everything looked good. So finally I put in new fuses and powered up. It worked!!!!

I check every voltage of all the tubes, everything looks good. I checked the +/-15V supply, looked good. The amp is working and I am stumped!!!


BTW. The amp sounded a hell of a lot better with the cheap Utah speaker in my Pro Reverb. That Celestion G12H-100 really suck!!!

[Alan0354]

I finally get to the Marshall today trying to fix it before going under the knife, I only found both the power fuse and the power tube cathode fuse blown!!! I checked through everything for shorts ( rectifiers, caps etc.), everything looked good. So finally I put in new fuses and powered up. It worked!!!!

I check every voltage of all the tubes, everything looks good. I checked the +/-15V supply, looked good. The amp is working and I am stumped!!!


BTW. The amp sounded a hell of a lot better with the cheap Utah speaker in my Pro Reverb. That Celestion G12H-100 really suck!!!

Anything I need to look out? I am stumped, I had it on of at least 20 minutes and everything works.

I am sure I am not going to rip the amp out or mod anything, I only need a speaker!!! Maybe, it's time for an Eminence Red White and Blue!!! I got the response from Eminence, they said RWB is good for tamming a very bright amp, this is exactly what the doctor's order for this amp.

Please tell me if any of you think I should check. I am going for capo tunnel surgery and I really want to put this amp back together if there is nothing wrong as I won't be able to lift heavy stuff of a while afterwards.

Thanks

[Alan0354]

The amp seems to be fine, I even changed the speaker. I am using the Eminence GB128 type of speaker from the cheap Fender Rock Pro cabinet and it make a day and night difference.

The amp sounds decent right now, not that good, but it's not that bad. Good enough to serve as reference for distortion. I was told that G12H-100 is a bass guitar speaker and it is very scooped mid. The cone is very loose compare to the other guitar speakers I have. Never imagine Marshall put such a lousy speaker in the Dual reverb. I bet if they have even the Eminence GB128, the resale value will be a lot better.

I did put some minor mod before, I even removed all the mod and the amp is 100% stock and it's decent enough.


Now you can lock this thread. I think I am done.

[km6xz]

I would sure be wary of the power tubes, an intermittent short in the tubes would result in the symptoms you saw. Also, carefully inspect the tube sockets for visual signs of arcing some time in the past.

[Alan0354]

Well guys, I was working on the Marshall JCM900 that blow tubes. I cannot find the problem. It's working. I decided to take a look at SGM's suggestion on the mod. I know exactly what he is trying to do. It's to balance the clean and the OD channel as there is only one tone stack. I followed part of his mod to darken the clean channel and I followed his lead ( not exactly the same) in the OD channel. It sure improved the sound. The clean is a very British clean, not that bad. The OD channel improved. Inside, there is a fixed valued tone stack exactly like the regular tone stack. SGM change the value of the slope resistor which I did. I also replaced the set resistor and put in a treble trim pot, and low and behold, I balanced the sound. I increase the gain of the preamp some, it sound good enough now. I sure can use it as a reference for the OD channel!!!

The amp sure sounds a lot better than a few of the amps I tested in the stores.

He does know this amp. Maybe, I think some times he just drink a little too much when posting!!!! The orange drop and the silver mica thing......Ah.........No.


For the life of me, I cannot figure what's wrong with the amp!!!! It's not blowing the tube, it is acting normal. I put in both 5881 now, I measured 480V +B, that might be too high for the 6L6.

[daz]

I don't care for them but there are lots of people who love those JCM 900 combo amps.

I used to gig with JCM800's and JMP's, but i then got a 900, then 2 more. I can tell you that while i don't know why, one of them was stellar onstage. It sounded better than any of my 800's or jmps. Anyone who's gigged a lot knows that it's not how good an amp sounds or feels on it's own, it's about how it comes thru a band mix. 2 of the 90's i had were bad and worse. One was a slx model with way too much gain and even when i removed diode clipping it wasn't a great amp. the other 2 were the channel switching 1x12 combos. (dula reverb models) One sounded absolutely horrid, while the other, the exact same model and both el34 models, sounded better than any other marshall i ever used onstage. It cut thru flawlessly with harmonic content and meat intact, and was super consistent from room to room. The 800's sounded better on slow bal;lad type stuff where theres a ton of space so the tone wasn't being crippled by the rest of the band. But other than that, anything from middle of the road rock like petty or such on up to raging stuff, the 900 was superior. In both the channel switchers i used the same tube....i mean the SAME ones which i took out of the one i sold and put in the new one and biased it correctly. So it wasn't tubes. But i think the 900's very VERY inconsistent. Maybe lots of revisions? i dunno, but a good one is damn good if you can find one like that.

[Alan0354]

With some mod, the Dual Reverb sound pretty good on the OD channel. Clean is OK. I have the small combo 1X12. From the testing lately, the speaker and the cabinet is the big problem. The cabinet sounds a little boxy. I plug the amp into the Eminence Legend 1258 cabinet, it sounded better.

I won't be surprised if you use an extension cabinet with different speaker that it sounds good. I think I am going to keep the amp as is, not gutting it.