Amplifier Sag, What Is It?


Well we are not talking about what some of us seasoned veterans see when we look in the mirror or what the ladies detest, we are talking about the sound of an expanding or blooming note on a tube amplifier, on high volume, when attacked hard. This effect is most prevalent in vintage tube amplifiers with rectifier tube power supplies. Often sag is felt more than heard, a guitarist digs into an amp, but it doesn’t really respond crisply; the volume lags and comes up slower than it should. This results in some compression (clean) at the beginning of the note and then the subsequent rise in volume and the loaded power supply catches up.


Sag occurs when there is a sudden drop in power supply voltage due to the sudden increase in current draw resulting from a hard attack on a note. The voltage does quickly recover and in doing so results in a swelling or blooming note. In order for the load on the power supply to be sufficient to produce sag, the amplifier’s volume will need to be set very high.

Before the invention of the solid state diode, amplifiers had vacuum tube rectifiers to convert AC to DC voltage. Vacuum tube rectifiers are not very efficient and do not respond well to a sudden increase in current draw. The effect can be minimized by utilizing a more robust tube rectifier, but the amps of the day were designed as economically as possible, and in addition to this, they were not really designed for the style of playing that resulted in sag, i.e. cranked volume and hard attacks; can you say Rock and Roll?


Is your amp a single ended amp? If the answer is yes, then sag is not a factor. A single ended amp is a class A amp; this type of amp is drawing maximum current while at idle. Initially, when you strike a note, the voltage and current both drop and the load on the power supply is reduced. In a single ended amplifier there can be no sag; it is not possible. It makes no difference if you have a tube or a solid state rectifier; it is impossible to produce sag. A single ended amp will typically have one power tube—a 6V6, 6L6, or maybe an EL84—and produces 5 to 12 watts.

Is your amp a push-pull amp? If it is over 15 watts and has two of the aforementioned tubes, then it probably is push-pull. Next question is what type of rectifier does it have, solid state? If so, then there should be no sag. If it has a tube rectifier, then yes it probably does produce sag. Examples of vacuum tube rectifiers are 5Y3 and 5U4 types. At idle a push-pull amp is drawing about 70% of maximum current; this is a class AB amplifier, and by far the overwhelming majority of push-pull amps are class AB. With a hard attack and at high volume the power supply voltage will drop slightly (sag) and then catch up producing the swell or blooming note that we discussed earlier.


This is interesting, and there are solid state rectifiers on the market that will plug directly into a tube socket. The thing to remember is the solid state rectifier will have a much higher output voltage than the tube rectifier, and this will change the operating parameters of your amplifier. The result being: higher plate voltages and bias voltages throughout the amp as well as an increase in power dissipation in the power section. This will shorten the life of your power tubes, unless you re-bias the power tubes and lower the current draw to result in the correct power dissipation. Your mileage may vary on this option.


Disclaimer: this is my opinion and others may disagree. With that in mind, I do not believe sag is a desirable aspect of an amplifier for the harp player, and here is why. Harp players strive for a sharp articulate attack on their notes. Our instrument is inherently soft, producing more rounded notes, and to counter this we articulate our notes with syllables like “ta”, “ka”, “tukka” and so on. An amplifier with sag is counter-intuitive to this desire. A good harp amp needs to be punchy, to have drive to cut though a mix, and to be heard by the audience, and an amp with a solid state rectifier is conducive to this goal.

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