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Marshall Code 25 - FX Loop
Marshall Code 25 - FX Loop
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Old 24th January 2019, 10:10 AM   #1
Pontiac51 is offline Pontiac51  Austria
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Default Marshall Code 25 - FX Loop

Dear DIY Audio Gods! Sorry to say that, but I am still thrilled how you all brought me forward on my last project. Cannot say it often enough, you guys rock!

My newest project is to relocate a Marshall Code 25 from the combo to a head housing (again). If you are interessted, read the full story here:
DIY Code 25 Rebuild

Now, a fellow reader (thx to dpaterson) has pointed out that the Code 25 misses an FX loop in contrast to its bigger brother, the Code 100. He has found the right points on the PCB by trial and error (and without exploding something) and I want to build on that.

So I have scratched up the simplest wiring diagramm I could think of and would like to request you to have a look at it. I am trying to stay as minimalstic as possible (as you have teached me) and then enhance further once basic functionality is confirmed.

Please do not hesitate to point out weaknesses or errors in the wiring.

Pin1 is ground, pin3 is signal. Pin2 and 4 are NC.
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File Type: png Schematic_Code-25-FX-Loop_Sheet-1_20190124110730.png (26.5 KB, 61 views)
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Old 24th January 2019, 06:46 PM   #2
PRR is online now PRR  United States
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The switching should be on the Return, see snip stolen from
Bass Guitar Amp 2

Your level control as-shown will be troubled. Full down it shorts-out whatever is trying to drive it. If you use it to drive the power stage while ignoring the preamp (aside from using a different preamp, VERY valuable for trouble-shooting), half-up the 500k half-pot will make hardly any difference to the presumably low-Z signal from Send. Full up Return and Send fight each other unless you brought a dummy plug/cord for the Send jack.

This is not a new invention. Plagiarize.
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Old 25th January 2019, 06:55 AM   #3
Pontiac51 is offline Pontiac51  Austria
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I see. Thank you for pointing this out.

How would you realize a level control, if at all possible? And how to size it properly?

I have changed my first design accordingly and did another which should be a classic too. But again, how to dimension the pot? I found values ranging from 22k to 1M. Does it matter?
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Old 26th January 2019, 08:37 AM   #4
Pontiac51 is offline Pontiac51  Austria
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I combined the circuits with a switch. Is this still correct? Any input on the pot I should use?
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File Type: png Schematic_Code-25-FX-Loop_Sheet-1_20190125215830.png (14.4 KB, 25 views)
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Old 27th January 2019, 05:13 AM   #5
PRR is online now PRR  United States
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The pot depends on SO many factors I can't guess from here.

100K seems a likely value.
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Old 27th January 2019, 04:29 PM   #6
Pontiac51 is offline Pontiac51  Austria
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Thank you. I will start from there.
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