Hi all. I just registered and thought I would pitch in a "how-to" for the subject mod I made recently.
Many (all?) of the Mark Bass amps use a parallel effects loop, which essentially mixes the effects loop signal with the uneffected signal. This has a positive side effect: if something happens to your effects pedals or rack, your amp will still have signal. On the other hand, if you are using a multi-effects pedal on this loop as your tuner, any built-in mute function will be useless. Even if the effects are muted during tuning, you won't be silent on stage.
Normally Mark Bass requires you to have a "qualified technician" do a modification to change the effects loop from parallel to serial. But with minimal effort and common sense, you can do it yourself. It's quite simple, and also reversible in case you want to change back to a parallel effects loop. Caveat emptor
-- Follow this guide at your own risk. While I'm confident this can be a very safe mod, I'm not responsible for any consequences you or your amp might suffer as a result of your attempting this mod. Now with that out of the way... What you need:
A slim pair of needle nose pliers, a small Philips head screwdriver, a flashlight, a clean working space, and a steady hand. You may also want a small cup or foam pad where you can place loose screws and jumpers without losing them. Before you start:
Turn off power to the amp, and remove the power cord and all other connections from the amp! There is no reason you should be messing around inside your amp with the power connected for this mod. Use common sense and be safe. Don't poke around any components other than the ones listed and photographed here as part of this mod.
1. To open your Mark Bass Little Tube 800, simply remove each of the 12 screws on the top of the amp housing using a small Philips head screwdriver. Put the screws in a cup or on a soft cloth or foam pad so they don't get lost or roll around your workspace. You'll now be able to see something like this picture:
2. Locate the black jumper at the rear of the amp, next to a gray ribbon cable. It may be difficult to spot, so use a flashlight for extra light. The jumper may be partly covered with epoxy to keep it from being dislodged. One side is labeled "PARALL," the other "SERIAL." Use a pair of needle nose pliers to detach and reattach the jumper so it spans the center pin and the pin on the "SERIAL" side. You may need to pull a bit to dislodge it from the epoxy. You should now see something like this:
3. Locate the black jumper at the front of the amp, behind the mini-tube and the row of three purple capacitors. Again, use your flashlight to help spot the jumper since it is located under a small board for the piezo input and mix controls. Use a pair of needle nose pliers to detach the jumper. You may need to pull a bit to dislodge it from the epoxy, but be careful
not to harm the tube or capacitors. Come at the jumper from the side near the small gray ribbon connector, to avoid getting pliers near the other components. You may want to reattach the jumper on just one side, leaving the other pin open, which effectively stores the jumper for later in case you decide to reverse the mod. You should now see something like this (note, the jumper is completely removed in this picture for clarity):
4. Before you put the cover back on, restore all the amp's connections including power, speaker cabinet, and instrument. Put your effects on the effects loop and turn them on. Then power the amp on. If your effects include a mute or volume control, you should now be able to hear them working properly without a dry signal -- if you mute, your amp should go silent.
5. Replace the amp housing cover using the 12 screws.
To undo the modification, remove all connections to the amp (including power!), then reverse the steps above, restoring the jumpers to their original locations. This will bring your amp's effects loop back to a parallel configuration. Enjoy!