Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

February 05, 2023, 02:10:22 AM

Login with username, password and session length

Recent Posts


Adding an Effects Loop to a Marshall CODE25/CODE50

Started by dpaterson, October 01, 2018, 07:35:52 AM

Previous topic - Next topic



Interesting site (forums) and just came across it by sheer luck (and sure does seem like the right place to be asking for help).

I need to add an effects loop to my Marshall CODE25 (x3) and my Marshall CODE50 (x2).

I've spent a good while trying to source schematics for these amps. and nothing (and Marshall will not budge).

I guess I just need to know where to splice into the circuitry/wiring (just before the power amp. stage) (I at least know that there is indeed a standalone power amp. stage/board in these amps.) to add the effects loop.

One thing I need mention: the CODE100/CODE100H do indeed have effects loops and given the design of these amps. I'm almost sure that if I could just get pics. of the inside of a CODE100/CODE100H showing where the effects loop board/jacks are connected to the main board(s) then I'd be good to go but to date I've not been able to find any such pics.  Also: for some or the other obscure reason the headphone out/line out on the CODE100/CODE100H disables the effects loop on these amps. so there's a clue right there (in my opinion) maybe???

Any ideas , tips, tricks, pics., schematics anybody???




I have indeed managed to create a "pseudo" effects loop using multiple amps. (details here: http://www.marshallforum.com/threads/creating-an-effects-loop-for-code25-code50-using-multiple-amps.103913/) but it's hardly an elegant solution although it does work.  There's also some pics. on that thread of the inside of a CODE25 (taken from others) but I cannot make head nor tail of them to be honest.


I do not know what does the circuitry of CODE amps exactly consist of. However, it's a good idea to try finding a  line-level signal around the amp chip using a VU meter or a pair of computer speakers with probe made of alligator clips, 1/8'' TRS lines and a 3.3uf cap(in order to block out the DC bias which would fry your speakers) . Once you've probed out the signal successfully, you can then find the connection of it to the preamp, cut it, and put the LOOP circuit between the gap. Opamp buffers might be added in order to deal with the impedance problems. Good luck!


Hey 66cccfff.

Thank you very much for your input.  It is really very much appreciated.

Since my original post I have indeed managed to accomplish what it is that I set out to do and it was a lot easier than I thought too.  I've yet to test with a pedal in the loop though (simply because I didn't have any 1/4" jacks on hand at the time) but I've at least managed to trace the audio signal path from the pre amp. to the power amp. and splice into the circuit.  It's a little confusing because it would appear that I have no less than four wires (common plus three) all carrying the same mono signal to the power amp. but stereo to the headphone/line out jack.  And because I've not tested with a pedal as yet: I've no idea if I'm going to run into impedance or level problems.