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February 04, 2023, 06:07:39 PM

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Add effect loop to jc120

Started by Psc, January 20, 2023, 11:25:13 AM

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Psc

Hello all, I'm looking for some help adding an effect loop to a jc120. I mostly want this so that I can use it as a power amp in for other preamps, not so much for adding stereo effects like delays. I have attached the schematic for the amp in question. I'm just not clear exactly where to split off.

Psc

I'm guessing I would put a jack in between where the two boards connect just after the volume, and just pick one of the two? Either use switching jacks or just run an in and out and jumper with a patch cable?

I'm assuming between the output of board 1and the input of board 2. Is there a better way to do this?

phatt

Yes output of VR10, cut before R19 and R121.
Should work and easy to reverse if it does not work.
Phil.

Psc

Thanks for the confirmation. I'll update when I get around to doing it.

Tassieviking

#4
You can also remove those resistors and use the holes for wires to run to the send/return jacks, just replace the resistor along the wires so it goes back in the circuit.
I would solder the resistor to the send jack and then add the wire to the resistor.
If it works ok then put a bit of heat-shrink tubing over the resistor.

I would have thought that the place to insert the send/return would be at C26/C126, or just before C26/C126.
I think it looks like the circuit between R19 and C26 is where the effects and distortion might enter into the circuit.
I might be wrong as I am not familiar with this amp.
There are no stupid questions.
There are only stupid mistakes.

Tassieviking

#5
I had a look through my files and found a 84 JC120 schematic with an effects loop,
if you compare the circuit after the return jack you will understand what I meant above.

Never mind, that's a single channel head only.  (Thanks Phil)
There are no stupid questions.
There are only stupid mistakes.

phatt

Hi Tassie,
Note;
Vr4 And VR10 is the mix point for the 2 power stages, trying to insert a loop at C26/C126 means you will only be able to use one power section. (I'm assuming a mono FX loop is being asked for?)
I think you will find the 84 schematic is a single channel circuit which makes your idea possible but this one is a different setup.
 I assume by the schematic the in/out wires are easy to lift on the unit posted so less chance of stuff up.

For a full run down on all the JC models try this page;
https://www.ssguitar.com/index.php?topic=2243.0
Thanks to Teemu ;)
Phil.

phatt

Also the Fet pair phase splitter Q1 and Q20 form a limiter circuit so inserting after that may (or may not) cause strange things to happen.

Teemu mentions this limiter but I'm still not sure how it works.
A note,
Sadly Teemu along with Enzo and J M Fhaey don't post much these days but they have a wealth of knowledge if you go searching for older posts. plus a few others, whose names I can't recall right now.
These wonderful members have helped me many times when I've been stuck. 8)
Phil.

Psc

Thanks for the input. Yes, I want to be able to have a mono input that would go to both power amps if possible.

Phatt, you're correct, the in/out between board 1 and 2 is exactly like that in the amp, very easy spot to access, very reversible. I think I'll try it there when I get a chance. Free time is hard to come by these days, but thinking at work I can manage!

Tassieviking

There are no stupid questions.
There are only stupid mistakes.

Tassieviking

Thanks Phil for pointing out the JC models page, I have just finished reading it for the second time and it was very informative.
I have also re-named the JC-100 84 file to head only single channel.

I would do what Phil said regarding the effects loop, ignore what I said. :) 
There are no stupid questions.
There are only stupid mistakes.

phatt

Hi Tassie,
          All good, like you I've not ever been inside a JC amp and just looking at Schematics tiss easy to miss things. I had to scratch my head for a bit to work it out. :-\

Thanks for the info on Enzo, yeah a great mind who was always helping us lesser minds learn from his wealth of knowledge. He will be missed.
Phil.

Tassieviking

Enzo was a great man, he was always helpful AND informative.

Most of the time when Enzo gave you an answer he also explained it so you learnt something.

He always made me think of the old saying:
Give a man a fish and you feed him for the day.
Teach a man to fish and he has food for a lifetime. (or something like that)
There are no stupid questions.
There are only stupid mistakes.

joecool85

Quote from: Tassieviking on January 21, 2023, 09:52:50 PMPhil, unfortunately Enzo passed away in August last year.

https://music-electronics-forum.com/forum/music-electronics/967156-rip-enzo

Thank you for sharing this, I had no idea.  Enzo will be missed across many forums - he was a gem of a man.
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