NAD!: Marshall Lead 12

pjoliver182

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Hey folks - first post on MLP!

I have a later Reverb 12 with the headphone & line out sockets and have replaced all the pots and electro caps with correct values to get the amp working well again.

I found that when cranked it still doesn't have 'quite' enough distortion for my tastes so I began to tinker. I upgraded the first opamp to an RC4558 and the second two to LM4562 (as I happened to have some). These are not present in the Lead 12 as they are only used in the Reverb circuit. For this reason I wanted them to be as transparent as possible.

Anyway...I'd read Dr Distortions posts elsewhere on the forum about the early circuit having diodes for a 'fuzz' like circuit. I also did some reading online about diode clipping circuits. Having the diodes after the opamp is considered 'hard' clipping and having them in the feedback loop is considered 'soft' clipping.

I have tried both: using the diodes in the feedback loop definitely sound more like an overdriven tube amp - particularly palm mutes, where as the diodes after the opamp (as it seems the original lead 12 had them) sound like a fuzz pedal.

In both cases however I only added a back to back pair of diodes (I also tried red LEDs) without any extra resistors. The result was great overdrive or fuzz tones but with a HUGE drop in volume...

I only just found this particular thread today which shows LP59's reverse engineered schematic and his recommendation of using a 40k resistor for an easy Gibbonizer fix...so I won't bother asking about that now...but in the case of using diodes in the opamp feedback loop, is the solution the same? Add some series resistance?

Many thanks,

PJ
 

truetone6

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I know that there are 3 versions of the 5005 Lead 12 but could there also be a 4th version?
A guy on another forum posted this photo of his Lead 12 combo . He said it has a Z serial number on the back which would date it to 1991-92. also He said it has Marshall S301 speaker. The cabinet appears to be a wider than the other 3 versions of the 5005 Lead 12.



Below is my version one made in 1982



a version 2 made circa 1985 I used to own




and a version 3 made circa 1988

 
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Matlevo12

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Hi,

If anyone's interested I got my hand on a early Lead 12 combo, a mk 1 5005 (same as the mk1 3005, without any line out/headphone outputs), and traced it on order to correct the schems I could find online (including one in this very topic).
Here it is :
 

rt1mx

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Hey gang! I have 2 of these little fire breathers! They’re from ‘86 and ‘87. I guess they’re the 2nd versions. I have them in my shop as test amps with a bunch of old Boss pedals. Every time I fire them up I’m 14 again. I’d really like to put effects loops and XLR cab sim outs on them but, I don’t know how exactly. Any gurus out there that have done this?
 

Matlevo12

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Hi.
IMHO effects loop is between preamp and power amp, so just after the volume pot. You can do that by unsoldering and bending lug 2 or with the cap that's right after it. You can have a look at a 5213 schem to see what I'm talking about. Try and see if that owrks, and don't forget to use amp style jack sockets that connect pins together when no jack is inserted in the loop.
About XLR out / cab sim, there are modules you can add to do that, have a look on your favourite gadget website :)
 


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