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Adding a rear effects loop to a GK 700RBII

Discussion in 'Amps and Cabs [BG]' started by yakmastermax, May 19, 2016.


  1. Hey TB!:D

    I never posted a NAD for this guy but a couple of weeks ago I scored this used GK 700RBII for $275! There were some stickers on it but nothing a little rubbing alcohol couldn't handle. I'm quite happy with the amp, but I was bummed that it didn't have the effects loop in the rear. I was planning on putting the amp in a rack case along with some EQ stuff and it seemed the only way to make that work was to add an effects loop in the rear. I made a short little write up for anyone else wanting to do something similar. I assume it would be a similar process for a GK 1001RBII.

    As with all amp modifications proceed with extreme caution. There are capacitors and other circuit elements inside the amp which can hold a charge even after the amp is unplugged. The stored charge can kill you. :dead:

    The name of the game is simple. We're wiring standard 1/4" mono jacks in parallel with the 1/4" jacks on the front. :thumbsup: All this takes is some soldering skill, some screwdrivers, and a drill.

    First you've got to take the case off of the chassis. There are several little screws so keep track of them. Once the case is off you need to remove the two wires that run from the power amp section to the front power switch. Once these wires are removed proceed to loosen and remove the 4-6 small nuts which hold the grill and "Gallien-Krueger" logo to the chassis. The main power switch is attached to the grill and needs to be removed to access the circuit board leads on the front 1/4" jacks. Once the main power switch and the grill are out of the way, use a spare instrument cable and multi-meter or circuit tester of some sort to determine which of the leads on the circuit board correspond to the tip and sleeve of the 1/4" jack. Once you know which leads are which, you can solder some wires to the leads. This is probably the trickiest part of the modification. It isn't easy to solder directly to a circuit board, but it can be done. I wrapped my wires in some left-over copper tape that I used for a shielding a P-Bass. If you want to skip the hassle simply buy shielded wires. This is important.:bookworm: If you don't use shielded wires you will introduce a whole lot of noise into your signal. The power amp transformer is right there and it does make a difference.

    20160501_151248_zpsb5qjs306.

    In the picture you can see how the main power switch gets in the way of accessing the leads on the circuit board. It is right above the leads you want to get at. You can also see the small nuts that need to be undone to remove the main power switch grill. You can also see my gross hobbyist electrical work. Go easy on me I'm a physicist. My roommate is the Electrical Engineer. :bag:

    Now simply run your two pairs of wires (tip and sleeve for both the send and return jacks) to the back of the amp.

    20160501_151326_zps0m9ndpsj.

    Now comes the second most difficult part of the modification, and it really is pretty easy! ;) You need to drill two holes for your new effects loop. The chassis steel turned out to be pretty soft so you might not even need to center punch. Even my cheap Harbor Freight drill bits went through quite easily, but you have to be careful! :eek: If you punch through too fast you can damage the circuit on the other side. Go slowly.

    Solder your pairs of wires to your new 1/4" jacks. I found mine on Amazon, but any guitar parts website has them. Make sure to solder the sleeve wire and the tip wire to the correct lead. Once they're soldered on you can install the new 1/4" jacks on the rear.


    20160501_151318_zpstitrrckg.

    20160501_151334_zpsvyeelicn.

    Almost done!

    Interestingly enough GK designed a safety feature that needs to be taken into account. The effects loop is not brought into play unless there is a 1/4" plug in the front "return" jack. Unless you "ground" out the return jack on the front, the normal signal path will not be bypassed and the effects loop is left out. I simply used one of the many 1/4" to 1/8"(3.5mm headphone) adapters that I have lying around. Plugging this guy into the return jack on the front completes some aspect of the safety circuit which then bypasses the normal signal path and routes things through the effects loop. I'm sure if you could read the circuit diagram and had the right type of 1/4" jack for the rear, you could wire this safety feature to the rear.

    Now you're done:bassist:

    I'm making use of my rear effects loop for a 1 rack space graphic EQ, a 1 rack space parametric EQ, and also a Markbass Compressore that I store in the back of the rack case.
     
  2. The effects return jack (or power amp in) have internal contacts that connect the preamp and power amp with no plug inserted. That is standard on any amp with a serial effects loop. Plug in and the preamp is disconnected from the power amp input.
     
    yakmastermax likes this.
  3. BadExample

    BadExample

    Jan 21, 2016
    Injiana
    Nice work! PITA but might be a good idea to move that safety feature to the back, or some dummy might borrow your dummy plug. I think it might be hard on your preamp section or effects stuff to apply a signal into the output of either via the effects loop with the safety defeated. Or just make it permanently open.
     

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