marshall jcm900 mkIII mods?

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vic

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hi guys first post but does anyone know of some mods to the 2500 mkIII. any help would be very grateful thanks
 
thanks heaps man i dont think jerry mods the 900's anymore and im a long way from him unfortunately
 
the tgp page only has the dual reverb mods anyone else know of mods for the mkIII?
 
I know some had weak transformer designs so just replacing them with a quality unit is a start especially with reliability, my mk III was in the shop alot
 
yeah the transformers are really small but they arent the drake ones usually good?
 
vic":qvocrtyu said:
yeah the transformers are really small but they arent the drake ones usually good?
Click to expand...
yeah they were usually pretty good in the 800 but i was told by my tech that the early 900's had a design flaw with the transformers that often results in failure or low output
 
what exactly are you looking for tonewise?

do you intend to mod it yourself?
 
beef it up give it some balls it sounds thin but it does have a really good tone ive found this schematic dunno if its good or not but some people seem to like it



Uploaded with ImageShack.us
 
vic":2yvftgs9 said:
beef it up give it some balls it sounds thin but it does have a really good tone ive found this schematic dunno if its good or not but some people seem to like it



Uploaded with ImageShack.us
Click to expand...

there's no reason to replace R1 or to remove C2

the stock 10k on R1 sounds thicker, clearer, less fizzy and more dynamics-friendly than the usual signal sucking 68k resistor that most amps have

leaving C2 may be useful to avoid DC(?) noise
it doesn't suck tone

the rest of the mod basically involves taking the diodes out, which may remove some fizz
but it won't give you more balls at all

I'd replace R4 with 220k and R7 with 100k, remove C18, add a 4.7uf capacitor in parallel to R8 and replace D1 with a 1uf capacitor (instead of bypassing it)
this will thicken up the gain and sustain and give you a much fatter midrange
you can raise the phase inverter coupling caps (C11 and C22 on the power amp board) to 100n if you want more low end
 
thanks man the anode resistors r4 and r7 are 100k and 220k so i can just swap them around and r6 is the 2k7 acording to the preamp schematic below and with the 1uf capcitor and the c11 and c22 on the power amp board what type of capacitors should i use the same type? and what about the zener diodes do they have much impact on the sound?

http://www.drtube.com/schematics/marsha ... 9-iss9.pdf
 
On a similar note; normally I would just say that I blew some tubes last night, but since they went all 4th of July on me after I modded my JCM900 DR 4100 after those schematics posted earlier(something I've contemplated doing for ages) I'm kinda wondering if I fucked something up.

Basicly; one tube has a wierd, bright, light that blinks in time with some wierd crackling-noises from the speakers, and the other one just glows like hell. The last two doesn't seem to have any problems, though. And as I thought it might be the modding that was the fault, I swapped the tubes around to... "Check" if it was two of the sockets giving to much power or something, but it was still the same two tubes that went "bad".
 
vic":2xcpjirj said:
thanks man the anode resistors r4 and r7 are 100k and 220k so i can just swap them around and r6 is the 2k7 acording to the preamp schematic below and with the 1uf capcitor and the c11 and c22 on the power amp board what type of capacitors should i use the same type? and what about the zener diodes do they have much impact on the sound?

http://www.drtube.com/schematics/marsha ... 9-iss9.pdf
Click to expand...

sorry, I meant R8, not R6 (the last stage cathode resistor! my schematic is blurry)

you can remove the diodes shown in the first mod you posted, if you want to
it will probably reduce the fizz, but also take some bite out

for the phase inverter couplings you need 630v film capacitors
blue Epcos are pretty good and cheap
 
HeimBrent":2xte9pwn said:
On a similar note; normally I would just say that I blew some tubes last night, but since they went all 4th of July on me after I modded my JCM900 DR 4100 after those schematics posted earlier(something I've contemplated doing for ages) I'm kinda wondering if I fucked something up.

Basicly; one tube has a wierd, bright, light that blinks in time with some wierd crackling-noises from the speakers, and the other one just glows like hell. The last two doesn't seem to have any problems, though. And as I thought it might be the modding that was the fault, I swapped the tubes around to... "Check" if it was two of the sockets giving to much power or something, but it was still the same two tubes that went "bad".
Click to expand...

sounds like bad power tubes, but you should check if they didn't burn some resistor in the process
 
I did. Everything inside looks normal. Four new EL34's and one of those testprobes is next on the list in other words. A shame really, as I'm only home for a few weeks now, and was really looking forward to playing the living crap out of my amp. I guess I did that in a way.
 
I've never figured out the whole pulling-tubes wattdecrease-stuff. If someone knows; please tell.

The 50w/100W-switch on the back just messes with... Uhm. What is it called? Don't remember. Point is; that switch doesn't take two tubes out, it just changes the way the four tubes are run.
 
HeimBrent":kaksuwr1 said:
I've never figured out the whole pulling-tubes wattdecrease-stuff. If someone knows; please tell.

The 50w/100W-switch on the back just messes with... Uhm. What is it called? Don't remember. Point is; that switch doesn't take two tubes out, it just changes the way the four tubes are run.
Click to expand...
Pentode/Triode? I thought half power switches for 100 watters means cutting out the two outer tubes. For 50 watters, the only lower power option is Triode/Pentode since you can't cut out one of two tubes.
 
AndyK":f4iodysm said:
HeimBrent":f4iodysm said:
I've never figured out the whole pulling-tubes wattdecrease-stuff. If someone knows; please tell.

The 50w/100W-switch on the back just messes with... Uhm. What is it called? Don't remember. Point is; that switch doesn't take two tubes out, it just changes the way the four tubes are run.
Click to expand...
Pentode/Triode? I thought half power switches for 100 watters means cutting out the two outer tubes. For 50 watters, the only lower power option is Triode/Pentode since you can't cut out one of two tubes.
Click to expand...

This

(I run at half power)
 
I'm pretty sure it just makes it changes it between penthode and triode, as I've seen people talking about how they pull two tubes, and then run it at the low-setting to get an output of 25 watts... Anyway! I pull tube... 1 and 4, leave the two in the middle, set the amp at 8 ohms and connect it to an 16 ohm cab, right? I haven't missed anything crucial?
 
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