FX Loop Questions

[PerFusionist]

Hi all,

I have a couple of questions regarding the Zero Loss FX Loop instructions:

Question #1:
Step 11:

I have a 1987 Plexi clone but have the Lar/Mar PPIMV kit installed. Sooo, do I connect the shielded wire (from the left middle lug of the on/off switch) to the PI input on the board or to the left lug of the PPIMV? If the PPIMV, would it be the lug of the inner or outer pot?

Question #2...
Step 15:

I have two 8.2k (2 watt) resistors in series here. This is not mentioned in the options. What do you think I should do here?

Apologies for the noobishness of these questions Any help is appreciated!

[SDM]

On question 1:

A Larmar PPIMV and the loop are installed in different parts of the circuit, loop is before the PI, PPIMV is after it. So the PI input cap on the board is where you must hook up that wire, exactly as the instructions show.

A PPIMV (Post Phase Inverter Master Volume) in the amp has no bearing on the loop install procedure, and nothing from the loop should ever connect directly to a PPIMV pot.

You may be a bit thrown here by the traditional Pre-PI master volume hookup shown for a traditional master volume amp. In those amps, the master volume is different than a PPIMV and is located before the PI now, same location as the loop. Hence in amps with a traditional NON-PPIMV master volume, the loop will connect to it.

Q2:
If you are feeding the loop from the screen node, nothing needs to be changed concerning your two series 8.2K resistors. Can be confusing in the 45 or 50 watt amps as often the connections to those series resistors in question are wired opposite of how a 100 watt amp is wired. Screen B+ nodes and PI B+ nodes are at opposite ends of the board, opposite ends of the series resistors.

If unsure which node you are hooking the loop up to, just measure the DC voltage at one end/turret of the series 8.2K's, then measure at the other end /turret. One turret will read notably higher DC voltage than the other, note which is higher. Be sure all preamp tubes are in at least when you do this by the way.

Now if you have the loop hooked up the turret where you read the highest amount of DC, then your are hooking the loop up to the screen node and no changes to your series 8.2K resistors are needed. If you are hooking the loop up to the lower DC voltage of the two turrets you measured, then you can change one 8.2K to a 4.7K to get voltages back to where they were before loop was installed.

Hope that all made sense.

[PerFusionist]

That absolutely made sense and cleared everything up for me. I'm on this first thing tomorrow.

Thanks, you rock!

[PerFusionist]

Okay, loop installed...it works, but I have an issue...the noise/hum is ridiculous.

So I'm running:
Guitar -> dirt pedals -> front of amp
Delays, trem and reverb in the loop.

-When I switch the loop off, the noise floor goes back to where it should be
-Running all of the effects straight into the front of the amp, the noise disappears
-Running one effect with its own wall-wart through the loop, the noise isn't bad



Any ideas?

Thanks!

[SDM]

Ground loops. Having effects plugged into the wall up front, plugged in to the wall in the loop (or sharing a wall wart with stuff up front for power), and of course having the amp chassis grounded easily can lead to ground loop troubles with any amp. So need to pay attention to how things are powered, where ground loops may be occurring.

Try running all effects -in the loop itself- only on battery power for a test to see what happens to the noise. Goes away, you know it's very likely a ground loop issue.

[908ssp]

I did a bunch of testing of the loop and with a jumper cable from send to return it was near impossible to say whether the loop was in or out. Truly a fantastic loop circuit.

[PerFusionist]

Thanks for the replies.

There definitely does seem to be a ground loop here....I'm just not sure there is much I can do about it with my setup I'm pretty unwilling to use batteries long-term (I use a good amount of fx velcro'd on a board).

Its just weird that all of my effects going into the front of the amp there is absolutely no noise issue, but half up front and half through the loop = HUGE noise

I'll keep messing with different options it but might just have to live with it...?

[908ssp]

Try independent power supplies for the effects. At least make sure the effects in the loop aren't running from the same power supply as the effects going in the front. Get a heavy duty AC plug box, plug everything in that one box. I have four amps, effects in the front, effects in the loop and no ground loops, it can be done.