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 basically allows the user to turn up the pre-amp section while controlling the signal going to the power amp thus 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 but also more 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. One common problem is most distortion channels are made for very hi gain sounds which don’t appeal to all players. It is possible to make pre-amp distortion channels with bluesy more vintage sounds although few commercial amps attempt it.

Can I add a master volume to a non master volume amp ? 

Some vintage amp designs can have master volumes added but most behave differently with a master volume installed. In others adding a master volume simply has no real benefit . 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 reasonably loud 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 who want real power tube breakup is to use a lower wattage amp or an amp with London Power Scaling like some of the models we build at Carl’s Custom Amps.

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.  All types ofmaster volumes affect the tone--- they sound different when in use and should be selected for a player‘s needs.

The standard master volume works well in amps designed for them. On phenomenon that an occurs is that the tone of the amp changes with the sweep of the control. Often the low setting does not sound as full. This is common on many amps with the Marshall JCM 800 being a prime example. A simple mod will correct this giving a more even tone throughout the range of the control.

If you have questions about putting one in your amp or removing one from your amp contact me for service and expert advice!