JCM 800 Effects Loop Layout: Please Verify

[RadioComm]

Hello,

Just finished implementing the layout for Kurt Stewart's JCM800 Effects Loop. Used DIY Layout Creator for the first time. It was fun!
Would like some expert verification of the layout for issues, completeness, and optimization.
Attached:
    * Kurt's schematic
    * PNG of my layout
    * DIY of my layout
    * JCM800 schematic/layout that I build.

Not too sure which is pin 1 of the level potentiometers, so cw'ness may need correcting.

Thanks so much.

[sluckey]

I moved a green wire from pin 6 to the board. I deleted a cyan wire from pin 2 and moved one end of the .022 cap.

[RadioComm]

I moved a green wire from pin 6 to the board. I deleted a cyan wire from pin 2 and moved one end of the .022 cap.

Awesome. Thanks so much @sluckey for making those corrections. Looking at it now, I'm wondering why they didn't jump out and bite me. I've attached the updated diy file.

I'll be adding the layout to my build. I'll post pictures when done. Thanks again @sluckey.

ttfn

[d95err]

I would put the loop before the master volume. If it’s after the master, you will get bad signal-to noise ratio at low volumes and you’ll have to adjust the send level every time you change the master.

If it’s before, you get a fairly constant and predictable level going into the loop. The send level becomes set and forget (probably don’t even need a pot for it).

[RadioComm]

I would put the loop before the master volume. If it’s after the master, you will get bad signal-to noise ratio at low volumes and you’ll have to adjust the send level every time you change the master.

If it’s before, you get a fairly constant and predictable level going into the loop. The send level becomes set and forget (probably don’t even need a pot for it).

I started looking at JCM800 schematics for effects loops about 15 years ago when I just had finished building my JCM800, and the only two that I found *both* inserted the effects loop post master volume, which to me didn't make sense. A master volume is intended to push a hot signal into the power amp as to drive the pentodes into compression mode. Certainly not intended for effect pedals expecting an input signal from an instrument. Like you, I figured that if the effects loop was inserted before the master volume, one could take advantage of both the tone shaping of the effects pedal *and* driving that power amp to its edge.

But... since I rarely (and I mean rarely) use an effects loop, I'm not going to question the design and just build it with the designed/intended values. And for the very few times that I might use the effects loop, I'm just going to set the MV for the most neutral signal, and let the effect pedals color the sound. So most of the time (like almost always), the DPDT switch wil bypass the loop.

But that's the beauty in building your own amps: you can tweak, experiment, and reverse if not satisfied. So maybe one day I'll give it a shot. But for right now, just too happy with JCM800 tone that I currently have.

[RadioComm]

Finished the build. The effects loop is quiet, and takes a little tweaking since post MV. But works as it should. Bypass works great  (my preferred mode)

Attached is a picture of the finished build, plus updated diy file and layout.

ttfn.