JCM900 DR100w mods

[Opry Audio]

Hey guys,

I’m modding a JCM900 for a friend. After checking out some recommendations, I lifted R7 to disconnect one of the fixed tone stacks and bridged D10 with a wire which supposedly changes the clipping characteristics. It sounds much better even after these small mods but I know some of the solid state stages can become unstable when modded. Has anyone else done this successfully? I’m not an expert on these amps and would like to know if anyone has had problems, or can foresee anything that I should be considering before returning it. Thanks!

[Opry Audio]

Like I said, I’m not an expert on these but took some suggestions from the Marshall forum, which i know is not always vetted. Maybe someone here can shed some light on what this mod actually did. I understand disconnecting the tonestack, but not the diode bypass exactly

[2deaf]

Dang I hate it when nobody responds to a thread.  One of my functions on this forum, and possibly my only function, is to get people going.  So maybe if I respond, it will stimulate others to join in.  It worked before.

D10 and BR2 puts three diodes in series to ground in both directions to clip both the positive and negative phases of the audio signal.  D10 does double duty by conducting during both phases.  If you short out D10, there will only be two diodes in series going to ground in both directions. 

Let's say that these diodes don't conduct until there is a difference of 0.6V across them.  One diode will then clip when the signal reaches 0.6V, two in series will clip at 1.2V, and three in series will clip at 1.8V.  The stock circuit will then clip at 1.8V and the modded circuit with D10 shorted will clip at 1.2V.

The tonestack in question has a midrange "scoop" and causes a substantial loss in gain.  Disconnecting it flattens out the frequency response and alleviates the gain loss.  The three diode to two diode clipping mod results is a lower amplitude signal during clipping.  The larger signal from the tonestack mod and the smaller signal from the diode mod pretty much wash resulting is a very similar maximum signal going into IC something or other (possibly IC4, but it isn't particularly evident on the schematic). 

 

[pdf64]

I think that the thread needs the schematic applicable to the amp in question. Things can get changed over the years of production, the relevant issue level is required, as it can be counterproductive to take a guess and hope for the best.

[shooter]

the thread needs the schematic applicable to the amp in question.

naw, 2Deaf nailed what he was given, why mess with a good thing 

[pdf64]

..2Deaf nailed what he was given, why mess with a good thing 

Probably so, but, just in case not, more info would be a good idea.
Referring to component # with only minimal amp model info seems a potential source of error.

[sluckey]

..2Deaf nailed what he was given, why mess with a good thing 

Probably so, but, just in case not, more info would be a good idea.
Referring to component # with only minimal amp model info seems a potential source of error.

The subject line gives you all the info you need to get to the right schematic.

[2deaf]

Always better with a schematic.

[mresistor]

And here's one you can actually read..     DrTube has em in pdf format


https://drtube.com/schematics/marshall/cd0189-iss7.pdf

[sluckey]

The schematic in Hoffman's library is readable too.

[2deaf]

In addition to the JCM900 dual rev 100W 4100 schematics, check out dual reverb jmp 52b and 52c in Hoffman's library.

[Opry Audio]

Sorry! I just saw these replies. Thanks 2deaf!! That was exactly the explanation I was looking for. It makes total sense! It seems like a perfectly reasonable thing to do as a modification, and low risk in terms of long term functionality.

I just don’t like to trust forums I’m unfamiliar with without some confirmation that it’s good info, especially with “easy mods”. Anyway, if anyone has a 900 lying around, this should be tried. I’m wondering what others think. Huge improvement tone wise, a lot more bottom end, and the breakup is a lot smoother.

[pdf64]

..2Deaf nailed what he was given, why mess with a good thing 

Probably so, but, just in case not, more info would be a good idea.
Referring to component # with only minimal amp model info seems a potential source of error.

The subject line gives you all the info you need to get to the right schematic.

In this case it seems so.
And that's probably the norm.
But a manufacturer is free to change component ref #s over time.
I've happened across a Marshall model where I'd noticed they'd done just that, eg so R10 of one schematic is a different thing to R10 of the other schematic.
I'm not claiming that's typical / common, just that if possible, to avoid confusion / errors, it's beneficial to provide a schematic that matches the amp in question, especially so if making reference to component part #s.
https://el34world.com/charts/Schematics/files/Marshall/Marshall_slp_reissue_100w_1959.pdf