The Master Volume Control

This month will be the first in a series about guitar amps. Over and over I see people misunderstand the features of their guitar amps and how to use them to get the sound they are looking for. We will start with a discussion of the Master Volume Control as it applies to distortion.

Before we introduce the Master Volume let's look at amps from a historical perspective. The early guitar amps of the forties and fifties were pretty low in power. The majority of players certainly weren't looking for distortion and those who were, such as blues and early rock players discovered that pushing their amp to the limit yielded some degree of distortion. As the sixties arrived amp circuits improved and amps became louder. The Fender Twin Reverb became an industry standard and could output a very loud clean tone. But as music progressed to heavier in the latter half of the decade players began taking their 100 watt Marshalls and playing them loud and distorted. Not a problem if you were Ritchie Blackmore playing in front of 10,000 people, but it left the rest of us playing smaller venues to get fired for playing too loud. (I know I did)

Enter the Master Volume Control.

Manufacturers began adding this as a feature to allow distortion at lower volumes. Unfortunately it didn't work well. The reason for this being all they did was add the Master Volume control into the existing circuit. Since the engineers of amps at that time designed amps to sound as loud and clean as possible (and never thought anyone would want to do otherwise) it was only when you pushed the output stage power tubes and transformer past their designed limits that distortion was achieved. The preamplifier tube circuits were designed only to output a signal large enough to drive the output tubes to full power.When the Master Volume got dropped into the circuit it was placed between the output of the pre amp and input to the power amp. When the control was set to it's lower ranges it allowed you to max out the input volume and push the pre amp tubes into distortion. But since the design of the preamplifier did not have much gain you didn't get very much distortion and what you did get didn't sound like the output stage distortion you got when the amp was cranked. Good examples of this are any Marshalls with masters pre JCM-800 and any Fender Silverface Twin Reverb with a Master Volume that pulled out for a "Distortion" circuit. Ever try using that? Pretty sad As time went on Randall Smith at Mesa Boogie designed a Master Volume that came after a preamplifier with enough gain and the correct EQ to sound good at lower volumes. This began the modern era of master volume distortion amps. Designers began working on amps with this in mind and now it is common to get usable distortion at a lower volume.

We can now divide tube guitar amps into three classes

Any amp either vintage or current production with no master that you can play clean or turn up for distortion if the situation allows.

Any vintage or reissue amp that has a master volume but a low gain pre amp.

Modern amps where the distortion is built in to sound good at lower volumes.

Let's move on and address the most misunderstood concept about the Master Volume. Simply put, It does NOT make the amp louder, it makes the amp lower.

It is not ON when it is on 10, it is OFF. It is on when it is on zero. This is certainly a counter intuitive concept to most people but that IS how it works. Time and time again I see people try and set up their amps for a clean sound with the Master Volume down. They become frustrated that it is not clean. If you look at the Master Volume as off and out of the circuit on 10, then especially on the older amps, where the Master Volume was just dropped into the existing circuit, you are now using the amp as per the original design.

This brings up the subject of boost pedals.

If you have a vintage Fender Super Reverb and you can get away playing loud, by all means boosting your signal will take you places the amp can't go by itself. Same for some of the early Master Volume circuits. If the master is down and you want to boost the feeble amount of gain for low volume playing all the better. But where it goes wrong is using a boost/ distortion pedal with a modern Hi gain amp. Every time I see a player with an amp like a Marshall DSL -100 and they have a pedal hooked to it I just roll my eyes. An amp like that has more gain than you will need in your next three lifetimes. If you have what you believe is a special sounding pedal and want to play it in the clean channel for it own unique sonic signature, OK, but invariably the pedal gets added to a Hi gain channel creating noise, mud and disguising the character of the guitar being played. I've always liked to say that to a guitar player Gain is like a heroin addiction. The more you get the more you want. It is easy to get used to the amount of Gain you have and think you need more, but every time you add Gain you add noise and hum along with it. I prefer to look for what is the minimum Gain I can get away with rather than how much can I get. You get a purer signal not modulated by noise and hum and let some of the character of the guitar itself be retained.

Lastly I would like to touch on different types of distortion.

This is a large and complicated subject but there are two main concepts that should be understood. The distortion that comes from an amp pushed past its power amp limits can be very different in sound from a well designed master volume amp that sounds good at a low volume. When an amp is played loud and the power stage is pushed the resulting sound is one of sag and compression of the power stage adding sustain to the guitar. The harmonic content is increased also adding to the sustain and timbre of the sound. It is possible to control the amount of distortion dynamically from either your picking dynamics or adjustment of the guitar's volume control. Therefore it is possible to go from a clean tone to semi distorted to full distortion all by varying your picking intensity or adjusting the guitar's volume. In contrast, when pre amp distortion is used the resulting distortion sound is basically due to the generation of harmonics created when the pre amp tube stages are overdriven. Since a pre amp stage does not output power, there is no sag or compression effect. Blues or classic rock players used to output style distortion will usually find that the dynamic control over the distortion is absent from this style of distortion. It is more suited to the modern guitar sound of of either very harmonically complex power chords bordering on white noise or to achieve infinite sustain for solo playing. Of course there are many different designs of pre amps and some are more agreeable to dynamic control than others. It should be noted again that this is a place where the the less Gain used means more possible dynamics to the distortion. If you run the pre amp gain on 10 you are less likely to hear the sound clean up than if you run the gain lower on this type of pre amp.

It should also be mentioned that there can be a difference in the amount of distortion on the same amp played at different levels. If you are home and playing at bedroom volume levels and have your amp set up with a lot of pre amp gain, then if you leave it set that way when you take it on stage and turn up to stage volume it is now possible (based upon the power capability of your amp, volume of the other musicians and size of venue) you will be overdriving the power amp and adding sag and compression to your sound. This additional distortion can turn your sound to mush. The solution is too find the appropriate settings on the amp to use on stage which means adjustment of the gain control to the situation. Unless you are in a national act where the stages are always large and similar in acoustic environment it is necessary to adjust the amp to the situation you are in.

That's all for this month. Feel free to send in your questions and I'll try to get to them in future articles.