Name:    Steve Kennedy
Email:   steve@pacair.com  
Subject: reverb tank./ground switch
Thread:  49
Time:    Wed, 9 Jan 2002 20:23:51 UTC

First, the "ground" switch mystery (it is actual a noise filter switch).


If you look at a Music Man (or Fender for that matter) schematic, you will find the ground switch next to the power switch at the primary (input) side of the power transformer.


The ground switch connects a .047uF capacitor to ground from either side of the incoming AC line (or neither side).


Noise from lighting, motors, faders, dimmers, etc. all travel down AC power lines. Most of the time BOTH lines are subject to the same (induced) noise. If this is so, the noise is cancelled at the primary input to the power transformer (called common-mode rejection, this is the same basic concept behind a humbucking guitar pickup or a balanced microphone line).


However, some noises are primarily or exclusively present on one side of the AC line or the other. This energy is a much higher frequency than the 60Hz AC power. So, by connecting a small cap to ground from the offending AC line we can direct the noise to ground without effecting the 60Hz AC energy.


Because the most optimum noise cancellation configuration cannot be known ahead of time (because every AC power & noise situation can be slightly different), the "ground" switch can select either or neither of the AC lines for this noise filtering.


The ground switch has 3 positions. The middle position is no connection, the two sides are the two different sides of the AC line.


Now, your Reverb Tank..... you have may a bad cable, corroded connector, a cold solder joint or a flakey tank assembly. It seems that just the right connecting, vibrating or wiggling can cause it to work or not.


This sounds like one of those times where everything in the Reverb cirsuit should be checked and all connections and cables should be cleaned, treated and tested (or at the very least BYPASSED with another cable) to see what might be at fault.


Steve