Amplifiers [1]: Fender Master Volume Fiasco [2]

Posted by : Rick on Dec 13, 2007 - 07:38 AM
[3]
Ever wonder why Fender bothered with a Master Volume?


Few innovations in tube amps were as ill conceived and as misunderstood as the Master Volume control found on Fender amps of the 70's. To most people this control signifies the end of an era. Pre master volume amps are more sought after and command higher prices. This master volume just sounds too thin when you turn it down and just doesn't seem to be right. And what about the pull switch, what exactly does it do?

Well we have to back-track here a little to understand what is going on. The fender design utilizes a 1 megohm pot inserted between the preamp and the power amp. This is pretty much the standard convention and is utilized in Marshall circuits and most other designs as well. The 1 megohm value is chosen because it is high enough that it does not load down the circuit and alter the sound when it is set full up to 10.

The problem is that as you lower the control you loose high frequencies and the sound gets muddy. At low setting to 1 or 2 the high end loss is not so much of a problem but at mid settings of 4 to 8 it is extremely apparent. In order to counter this effect Fender utilizes a special pot that has a tap at about 70% of its operating range, a small capacitor is connected from this tap to the top of the control which acts like a bright cap and prevents the loss of highs. This is fine if you are playing clean and with no distortion but most people don't do that. The whole idea of having a Master volume in the first place is to make the sound dirtier and to crank up the preamp gain. When this happens in a Fender the sound is too shrill due to this bright cap.

Another problem that arises with high gain settings is that the reverb circuit, which is before the master volume, is easily overloaded and sounds just terrible. Fender tried to resolve these problems by means of the "pull switch" connected to the Master volume. When you pull the switch out it lowers the drive to the reverb to prevent it from overloading while at the same time connecting a treble cut capacitor from the top of the master volume to ground. Neither of these ideas really work very well and in modern designs the reverb circuit is almost always after the master volume to prevent this overload.




Links
  [1] http://rickonslow.com/index.php?name=News&catid=1
  [2] http://rickonslow.com/index.php?name=News&file=article&sid=4
  [3] http://rickonslow.com/