This is reprinted from Premier Guitar Magazine:
"Jeff,
My issue: why is it that when I turn up the FX mix volume on my Mesa Boogie F-50 to anywhere near 12 o’clock I get feedback like crazy? My effects loop consists of: FX Send >Cry Baby pedal >OC3 Octave pedal >MT2 Metal Zone pedal >Return
With only the MT2 Metal Zone pedal on, with the volume set at 11 o’clock on the pedal, little to no distortion is heard. But if you run the pedals straight through the amp, and not through the effects loop, they sound perfect. What’s the deal?
Thanks,
Juan
"Hi Juan,
I have a theory regarding your effects loop problem, which I ran past Dave Barber of Barber Electronics. Since he actually manufactures effects pedals, I wanted to get his take on this, and he seems to concur. I will attempt to explain it as clearly as possible; I hope it doesn’t become too convoluted.
The effects loop in your amp is a form of passive parallel loop. The “mix” control is actually blending in the wet (effected) signal into the main (dry) signal path. The problem is that the dry signal is always present at the mix point, and since there is no isolation between the dry and wet signal at that point, the dry signal can wind up appearing back at the output of the effects. What is also happening is that the wet signal can appear back at the input of the effects. While this may not be as much of an issue with timebased effects (reverb, delay, etc.), gain-based effects may react very differently.
When a dry signal appears back at the output of an effect, the signals can, depending on the effect, possibly be out of phase. This could cause a substantial reduction in amplitude, which may be why your MT-2 Metal Zone pedal has little to no level when used in the loop. Also, when the wet signal appears back at the input of an effect, it may possibly start to oscillate, giving you the feedback symptom that you are experiencing.
There are two possible fixes for your problem. The first is to run the non-time-based effects in the traditional manner, which is through the guitar input of your amp. The second would be to find a competent tech to modify the effects loop, so that it’s a standard series loop, as opposed to its current parallel status. That would completely isolate the send from the return, and it should clear up the issues you’re having.
So much for the ins and outs of your send and return."
Of course, the Mark III loop is series, but still... Hope this helps.
