Gain, Headroom, Volume and Power
Let’s start with GAIN. There are several ways to measure gain but in guitar amplifiers, when gain is discussed, we usually mean voltage gain. In this case, gain is the ratio of input voltage to output voltage. For example, if a signal of 1 volt is connected to the input of an amplifier with a gain of 25, then the output voltage would be 25 volts (if the amp has enough power to produce 25 volts). Voltage gain can also be converted to dB gain but let’s not complicate this too much. If an amp doesn’t have very much gain, then gain can also be thought of as volume. Voltage gain can often mean an increase in volume, but not always.
In reality, most guitar amplifiers have much more gain than just 25. The input signal from a guitar pickup could be as high as 1 volt (1V) and this signal is multiplied by the total gain of the amp but the amplifier’s output limits how much voltage can be produced at the output jack. For example, a guitar amplifier has a total gain of 500 but because of it’s modest output power, it can only produce 15V at the output to drive the speakers. In this case the one volt input multiplied times the gain of 500 would be 500V so what happens? The amp overdrives or clips. Clipping is when the loudest parts of the audio signal are clipped off by the limitations of any of the various amplification stages in the amp. This causes the sound of the guitar to be distorted while having increased sustain. The clipping can occur in the preamp, phase inverter, power amp or in all three areas. All guitar amps have a volume control that adjusts the gain of the amp. Many amps also have additional gain, volume or master volume controls that allow adjustment of the gain in each section of the amp. These additional controls can allow the player to introduce clipping tones in various amp stages in order to get more variations of tone, touch sensitivity, sustain and volume.
Now we’ll move on to POWER. Power is the amount of signal available to drive the speakers. Output power is measured in watts. Tube type guitar amps can have power output ranging from less than one watt to more than 200 watts but most range from 5 watts to 100 watts. Amps are traditionally rated at maximum power before the output tubes clip (clean power). However, most tube amps can produce quite a few more watts beyond their rated output power if driven hard enough to allow the output tubes to clip. For example, the Andrews A-22 produces about 22 watts before clipping and more than 40 watts when driven to full clipping. The maximum available volume is proportional to the maximum available power (given identical speakers). However the relationship is not linear. We’ll discuss that in another article.
Speaking of VOLUME, this is where gain and power come together. As stated above, more power equals more volume. However, some amps of the same wattage ratings seem louder than others. Why is this? Often it is because of speaker efficiency but that is for another article too. For now, let’s assume the speakers are the same. There are a couple of reasons that one amp might sound louder than another. One reason is that the louder-sounding amp might produce more of the frequencies to which the human ear is more sensitive. The other reason is that the louder amp might have more gain. High gain amps can sound louder for various reasons. First of all, the amp will need enough gain to drive the power tubes to full clipping in order to achieve maxium output power. A very low gain amp might not be able to do so without some kind of signal booster between the guitar and the amp. Also, higher gain amps have more apparent volume. What I mean is that a higher gain amp will achieve maximum volume at lower settings of the volume or gain control. If it has the same output power, it really doesn’t get any louder, it just seems louder because of the extra sensitivity to the guitar and volume control settings. Even lightly picked or hammered notes come through at full volume.
Now let’s discuss HEADROOM. This one is a little more difficult. This is a term that’s almost always misused by guitar players. Customers often ask if I can give their amp more “clean headroom”. Headroom is actually defined as (I’ll paraphrase) the ability of an amp to produce transient or instantaneous volume beyond the nominal output capability. When customers ask for more headroom, what they usually want is more clean volume, but they often don’t realize it. Actual headroom can be increased by replacing transformers with upgrades that incorporate more metal, adding a choke, converting to solid state rectifiers, or adding more filtering to the power supply. However, these changes will not produce much increase in clean volume, just actual headroom, and it will barely be noticeable to most guitarist. Adding more clean volume can be achieved by increasing the output power of the amp, changing to more efficient speakers or reducing distortion in the amp. When I explain this to customers, they often say, “I don’t want more volume, just more clean headroom”. The only way to achieve that goal is to reduce distortion in the amp without increasing power. One place to start is by substituting preamp tubes with lower gain tubes. By using lower gain tubes, the overall gain will be reduced and distortion MAY also be reduced slightly at the same time. If that doesn’t give the desired results, short of completely re-designing the amp, modifications to reduce gain can be tried. The results might or might not be satisfactory because some of the rich harmonics of the amp may be lost in the process. Reducing gain in an amp by any means will most likely only change the range of the volume and gain controls and rarely give you andy real boost in clean volume. An upgrade to a more effecient speaker can usually provide the most improvement in clean volume. Most of the time, if tube or speaker substitutions don’t achieve the desired result, (assuming your amp is working properly) it’s time to start shopping for a new cleaner-sounding, more powerful amp.
Preamp Tube Substitution – It is generally ok to substitute any tube number that begins with “12” and ends with “7” with any other that begins with “12” and ends with “7”. The exception being the reverb driver which should not be substituted. I prefer the sound of 12AX7s and 12AU7s in preamps. Compared to 12AX7, a 12AU7 will have much less gain and a sweet clean tone with more definition. The 12AX7 has more gain and a softer sound that overdrives more smoothly. 12AT7s sound fine for reverb drivers and phase inverters but not so great for any other preamp positions in my opinion. 12AY7 sounds good if you can find a good one but I really don’t care much for any of the ones being made these days. The 5751 and a few other industrial types can also be used but be sure to check substitution tables before using.