This is a great contribution by Retrobob!: -
Here is a way of installing the FX loop mod that should work with all of the 15/30/50 amps universally regardless of which model (VT, AD, XL).
This design puts the headphone jack after the loop, so what is in the loop goes out through the headphones/line out. Plus it adds a speaker mute switch so you can use the headphone jack as a line out with the amp's speaker working for monitoring, yet still be able to switch the speaker off for headphone use.
You’ll be adding 2 new jacks and a mini switch.
*Read through the entire project to make sure you are capable of performing this modification before you start.
USE AT YOUR OWN RISK!
You will need;
3 - 2’ lengths of 22 gauge strand wire in the following insulation colors, red, blue, orange.
1 - 4’ length of 22 gauge strand wire in green.
2 - 47? 1 watt resistors
2 - 100uf/25 volt radial electrolytic capacitors.
2 - Neutrik NMJ4HC-S (or equivalent).
1 - mini switch, single pole single throw (on/off).
And of course your tools and skills.
BEFORE YOU START, DISCONNECT THE POWER CORD (A/C power source) FROM YOUR AMP!
We will assume you have the proper supplies, tools, skills and know how to take your amp apart correctly. If you do not know how, search this forum for posts that go over this:
1). Before removing the amp back, take a pencil and draw a light line around the cover’s opening at the chassis, marking the chassis exposed area for easier placement of components.
2). Remove the amp chassis from the cabinet (mark any wires for their placement and connections that will need to be disconnected). Locate and position the two new jacks (send and return jacks should be horizontal and next to each other) and mini switch at the rear of the chassis near the headphone jack inside the pencil lines and where they will not interfere with anything inside or outside the amp. Once the proper location is determined drill holes for the jacks and mini switch.
3). Remove the circuit board that has the headphone jack attached to it and turn the board over exposing the solder connections. Find the six (6) headphone jack connections on the circuit board, these connections should look similar to picture (A) below. Completely remove the solder from all six of these connections (use a solder sucker or de-soldering braid) and remove the headphone jack. Completely remove the solder from the chassis jack grounding washer and remove the washer. Your circuit board should now look similar to picture (B).
4). Cut your new wires as follows, red wire = 1 - one foot long, orange wire = 1 - one foot long, blue wire = 1 - one foot long, green wire = 3 (three) - one foot long, green wire = 1 - three inches long, orange wire = 1 - three inches long.
5). Strip one end of your one foot long wires and both ends of your three inch long new wires approximately 1/8” and tin the bare ends neatly with a very small amount of solder.
6). Push the stripped end of your one foot long wires through the circuit board holes where the headphone jack was removed (one wire color at a time) and solder them to the connections as marked in picture (C).
7). Pair the wires you soldered to the circuit board together as follows “red and green”, “orange and green”, “blue and green”. Neatly twist the wire pairs together their entire length, as shown in picture (D). Position the wires so they lay flat on the circuit board and (do not cross each other) so they will not interfere with headphone jack when it’s reinstalled. If there is any insulated wires showing, cut and place electrical tape neatly over them so they can not make a connection with the headphone jack when reinstalled (the headphone jack will be turn over with its terminals up).
8 ). Reinstall the circuit board in amp. Install the headphone jack (new or old one, doesn’t matter) with its terminals facing up away from the board. Install the new send jack, return jack and mini switch in chassis (looking at the rear of the amp the send jack should be to the left of the return jack). Label the send jack, return jack and speaker mute switch.
9). Run the wire pairs (one pair at a time) up to their proper jack and switch connections (E), cut the wires for appropriate lengths (allow extra length, do not make wires tight). Strip one end of each wire approximately 1/8” and tin the bare ends neatly with a very small amount of solder, as shown in picture (D).
10). Solder each wire to its appropriate jack/switch connections as marked in picture (E) and FX Loop circuit drawing. Solder the two three inch long green and orange jumper wires at their appropriate jack connections as marked in picture (E) and FX Loop circuit drawing.
11). Assemble the headphone dual mono signal split circuit as follows, attach one leg of the 1 watt 47? resistor to the positive leg (the negative side of the capacitor is marked by an arrow for direction) of the 100uf capacitor, solder and trim excess leads at this connection (do this for both resistor/capacitor sets), attach both resistor/capacitor sets together at the resister end forming a Y, solder and trim, insulate both resistor/capacitor sets with heat shrink tubing. All steps are shown in picture (F)
12). Solder the dual mono headphone circuit connections to the headphone jack as follows, the split Y end of circuit (negative side of capacitors) attaches to the headphone jack terminals, as per marked colors shown in picture (E) & (F). The single end (resistor end) of the circuit attaches to the return jack (this end of the circuit may require a short jumper wire to enable it to reach the return jack), as per marked colors shown in picture (E) & (F).
13). Check all connections to be sure they soldered and insulated properly, and they are in their correct places per the FX circuit drawing. When you are sure all connections are correct, put your amp back together, you’re done.
(Q). The amp does not work?
(A). Flip the position of speaker mute switch (mini switch), if it still does not work check all of your connections for proper location (and shorts) per FX loop drawing.
(Q). The amp does not work when the FX loop is used?
(A). You have confused one or more of the switching terminals on the send and return jacks, check and reverse the connections at jack terminals as need to match the FX loop drawing.
*Note; the levers in the jacks that contact to your cable plug when inserted must connect to the long red wire on the send jack and the long orange wire on the return jack.
(Q). When I use the headphones the speaker still works?
(A). Flip the position of speaker mute switch (mini switch), if the speaker still works check all of your connections for proper location (and shorts) per FX loop drawing.