One Way to Add an FX Loop to an Amp

syscokid

syscokid

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Earlier today I mention to Robert Von Herndon that I added effects loop circuits to a couple of my amps that originally didn't have one. Both amps are JCM800's. One is a '81 Marshall 2x12 combo, the other is a clone I put together about 8 years ago. These effect loops will also work for the old Plexis, JTMs, JMPs, and others as long as you can get close to 300 volts going thru the circuit.

This particular effects loop is a kit that was purchased from Metropoulos Amps. The board is already assembled. You would need to drill 3 to 4 holes, connect 3 to 4 wires, and a resistor. My kit looked like this:

A.jpg


Here's the chassis to my clone built, and look at all that open real estate at the left side before any mods were done:

B.JPG


Three new holes drilled. The fourth hole for the grounding tab has not been drilled yet:

C.JPG


Left to right: fx out; fx in; bypass switch:

D.JPG


The long purple shrink tubing is covering two shielded cables. One cable is connected from the master volume pot to the board. The other cable is connected from the board to the treble pot to complete the loop. That resistor is used to drop the voltage to the circuit. The range is anywhere from 250vdc to 320vdc. 300vdc is optimum:

E.JPG


Now you can see where that fourth hole was drilled for the ground point of the cable's shield and the board itself. The trim pot is adjusted to your preference. I have mine set for just the slightest boost when the FX loop is activated:

F.JPG


Looks like it was always meant to be there:

G.JPG


This is the current version of the Metroamps Zero Loss FX Loop that is available right now. This newer one has been updated with an "On board switch (that) allows selection of pedal level or line level (-20db or +4bd) to accommodate anything from pedals to high end rack gear at the flip of a switch":

H.jpg

This is one of the best add-on effects loops out there today... (y)

Linky , linky, linky:
Zero Loss FX loop kit
 
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JohnnyGoo said:
Wow exellent work there Sysco. That amp is awesome.
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Thanks, Johnny. That amp sounds and performs very awesomely. Both amps that I'm referring to are 50-watt JCM800s. The old Marshall combo, which was not pictured, is set up with EL34s. The clone that's pictured is set up with 6550s! Circuit voltages have been intentionally set up differently from each other, too. Both amps sound a bit different from each other, and when you get them to make those speakers move a lot of air, it's just glorious... :dood:
 
smitty_p said:
This only works with master volume amps, right?
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No. Trust me... the link at the end of my first post has all kinds of info. But this should help your question:
"We ( Metro Amps ) officially support this loop when installed in Marshall 1959, 1987, 2203 and 2204 models, and authentic replicas of these amps. The installation instructions are specific to this style of amp, with the long tailed phase inverter circuit. The loop will also work great in many, many other tube amp circuits, but we do not supply specific instructions. Please download the instructions from the link below to determine if the loop is compatible with your amp. Also, any good tube amp tech/designer can tweak this loop to work in most applications. It has been used successfully in tube amps from 18 to 200 watts."
 
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Si;;y detail, but where did you get the labels for the jacks and bypass switch. They look like an exact match for the rest of the text!
 
RVA said:
Si;;y detail, but where did you get the labels for the jacks and bypass switch. They look like an exact match for the rest of the text!
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Not silly at all...

On the amp that you see at my first post, the graphics was done on clear waterslide decal paper. The "image" was transferred from a pdf template provided by Metro Amps:

fx loop template.png


However, on the other amp (Marshall JCM800 Combo) I took a much easier route:

IMG_4252.JPG

For this amp, I used my label maker:

Label Maker.jpg
 
What brand and model was the base kit for the amplifier? I'm feeling like doing some point to point soldering...
 
Sp8ctre said:
Yes.
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This amp was built 9 years ago, and is not a kit. All the parts were sourced from a few different vendors and manufacturers like MojoTone, Mouser, Magnetic Components (Classic Tone). This was my first attempt at a "scratch" build.

If you're in to JCM800s , I'm pretty sure that kits from Valvestorm would be a premier suggestion. Mojotone's stuff is real good, too. StewMac got into the game recently and at least on paper, their Marshall 18 watt kit looks solid, along what looks to be a comprehensive builders experience with all the parts very organized and an instruction manual that is very professional. However... If your incline to a Fendery amp build.... look no further than to Allen Amps... hands down... mic drop...
 
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