What is “Sag”? from aikenamps.com

What is “Sag”?

General

One of the terms you often hear in discussions about tube guitar
amplifiers is “sag”.  Sag refers to the drooping of the power supply
voltage in response to large transient signals, which lends a certain dynamic
“feel” to the tube amplifier that is not generally found in solid-state
amplifiers.

What causes it?

There are three main places where sag occurs in a tube amplifier:

  • The rectifier:  If a vacuum tube
    rectifier is used, sag is generated because of the internal resistance
    of the tube.  Unlike a solid-state rectifier, a tube rectifier exhibits
    a fair amount of voltage drop which varies with the amount of current passing
    through the tube.  In a class AB amplifier, the current drawn from
    the power supply is much greater at full power output than it is at idle.
    This large change in current demand causes the voltage drop across the
    tube rectifier to increase, which lowers the available plate supply voltage
    to the output tubes.  This lowering of the supply voltage lowers the
    output power slightly in opposition to the larger input signal, making
    it act like a compressor.  The lowered supply voltage also tends to
    decrease the available headroom, increasing clipping and changing the operating
    point of the tube dynamically.  This type of sag can be emulated artificially
    in an amplifier with a solid-state rectifier by adding a series resistance,
    typically around 100 ohms or so..
  • The transformers: The resistance
    of the high-voltage secondary winding also creates sag.  From Ohm’s
    Law, the voltage drop across a resistance is equal to the resistance multiplied
    by the current flowing through it.  This means that there is no voltage
    drop if there is no current, and the amount of voltage drop goes up linearly
    with increases in current draw.  A typical power transformer B+ winding
    might have a resistance of 50 ohms – 300 ohms, depending upon the current
    rating and regulation of the transformer.  For example, if the current
    draw in a push-pull class AB output stage at idle is 70mA total, and it
    increases to 170mA at full power, there is a change of 100mA in the current
    drawn through the secondary windings.  If the winding resistance of
    the secondary is 200 ohms, there is a voltage drop of 100mA*200 ohms =
    20V in the plate voltage to the output tubes.  Likewise, the resistance
    of the primary winding of an output transformer varies as well, typically
    80 ohms – 200 ohms plate-to-plate, depending upon the primary inductance,
    the transformer power rating, and the rated impedance.  This resistance
    also creates a voltage drop, but the amount of sag introduced is minimal
    in pentode mode, because the plate voltage doesn’t have near as much effect
    on the plate current as does the screen voltage.  In triode mode,
    there is more sag because the plate voltage has more of an effect on plate
    current in a triode. The supply sag created by the power transformer resistance
    lowers not only the plate voltage, but the screen voltage as well, since
    the screen is nearly always a filtered version of the supply going to the
    plate.  The amount of sag induced by the power transformer winding
    can be offset if there is a large filter capacitor reservoir to hold the
    voltage constant during current peaks.
  • The filter capacitors:  The size
    of the filter capacitors in relation to the amount of current drawn from
    the power supply also creates sag.  The filter caps charge up during
    the peaks of the AC input cycles, and hold the voltage constant during
    the “valleys”.  If the ratio of peak to idle current is high, and
    the peak current demands are high in relation to the capacitance size,
    the voltage will sag appreciably during the valleys, creating a lower average
    voltage.  If there is no further filtering, there will also be a 120Hz
    sawtooth ripple riding on the B+ supply.  This normally doesn’t induce
    much hum into the output stage because of the inherent power supply rejection
    afforded by the push-pull output stage, and the screen supply is usually
    filtered further with a choke and another capacitor.  However,
    insufficient filtering can induce ripple into the amplifier if the output
    stage is not well balanced, or if the screen and preamp supplies aren’t
    well filtered.

The downside of sag

Sag creates a certain amount of compression, but it also can
have bad side effects, the worst of which is “ghost” notes.  Ghost
notes are notes that are “riding on top” of the guitar note, not generally
harmonically related to the note being played.  This can be a bit
disconcerting when it is mixed in with the guitar tone.  The ghost
notes are usually caused by inadequate power supply filtering, which allows
a 120Hz component to modulate the guitar signal. (The frequency is 120Hz
because of the full-wave rectification of the 60Hz supply. It would be
100Hz in countries that use 50Hz mains power.).

Note that sag effectively only occurs in class AB amplifier output stages.
A true class A amplifier has no sag because the current draw at full power
is the same as the current draw at idle.  However, most class A amplifiers
aren’t biased exactly at the midpoint of the range, and will tend to clip
asymmetrically, especially when going into grid clamp on the output tubes,
so there will be an offset current component, but it will be much smaller
than in a class AB output stage.

Another term: “squish”

An effect similar to sag is produced by cathode-biased amplifiers
because the increased current through the power tubes increases the voltage
drop across the bias resistor, which in turn decreases the current through
the tube, with a time constant dependent upon the size of the bypass capacitor
in relation to the cathode resistance.  This compression effect is
commonly referred to as “squish” to differentiate it from “sag”, which
applies only to compression induced by power supply droop.  “Squish”
also is used when referring to the effect produced by the transient recovery
time of AC-coupled amplifier stages.  A sharp transient temporarily
increases the effective negative bias, and the time constant of the RC
coupling determines the recovery time following the transient.  Long
time constants caused by large coupling capacitors in conjunction with
large grid resistor values will increase the amount of squish in an amplifier.
Too long a recovery time leads to “blocking” distortion.  This is
why it is not a good idea to use overly large coupling capacitors in the
output stage of a guitar amplifier.


Copyright 1999,  Randall Aiken.
May not be reproduced in any form without written approval from Aiken Amplification.

Revised 11/16/99

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