I often get lots of questions about master volume controls. Can I add one in? Will the amp sound the same? What’s the difference between non-master and master volume amps? What about Post Phase Inverter and Pre Phase Inverter Master Volumes? Here are few answers to common questions about them:
What is a master volume?
A master volume are basically allows the user to turn up the pre-amp section while controlling the signal going to the power amp controlling the overall volume of the amp. In a non-master volume amp this control does not exist. The volume on a non-master volume amp is the same control as the gain, drive or pre level control on an amp with a master volume.
What affect does this have on the sound?
In general non-master volume amps are more dynamic and responsive to the player‘s touch since the pre-amp is directly interacting with the power amp. The overdrive is usually power tube breakup or combination of pre-amp and power amp breakup which sounds different. The clean and semi-clean tones are one of the places these amps really shine. A player can control breakup by varying their pick attack and adjusting the knobs on the guitar. However this is not really very practical in many locations a player would use the amp.
For overdrive and distortion in master volume amps the pre-amp is high gain. The pre-amp overdrive tends to be at bit buzzier and more of a square wave. Some players like it and some do not but it is practical way to get distorted tones without high volume. There are many ways to shape a distorted pre-amp to fit a player's needs. The advent of hi-gain amps gave players distortion tones without ear bleeding volumes but changed the tone too.
Can I add a master volume to a non master volume amp ?
Some vintage amp designs can have master volumes added but most behave very differently with a master volume installed. In others adding a master volume simply has no real benefit so this must be evaluated on case by case basis.
One amp that master volumes are commonly added to are Marshall Plexi amps. Some Plexi amps are ridiculously loud and the classic plexi crunch sound depends on power tube breakup and speakers being pushed hard. A master volume may help quiet things down but you will still have to play loud enough to get the sound people typically associate with those amps. The sound is simply not the same with the master as without. That is not to say it won't sound good with a master---just different. A combination of a master volume and an attenuator is often needed for such amp. A better solution for many players is to use a lower wattage amp or install power scaling.
Post Phase Inverter vs. Pre-Phase Inverter Master Volumes?
These two types of masters simply place the master volume in a different places in the amp. Many Post-Phase Inverter Master Volumes have the advantage that when turned all the way up they are out the circuit. Thus the amp retains it’s function as a non-master volume amp. However I find that Post-Phase Inverter Master Volumes need the amp to be turned up more to sound good. No matter what all master volumes affect the tone--- they sound different when in use and should be selected for a player‘s needs.
Have questions about putting one or removing one from your amp? Contact me for service and advice!