If your amp has a particularly bad sounding "cutting in and out" kind of distortion (commonly referred to as "farting out"), it is likely suffering from what is known as "blocking" distortion (not the Radiotron Designer's Handbook definition of blocking, but the more esoteric Norman Crowhurst definition). Blocking occurs when the output tube grids are driven to the point of conduction. Since the previous stage is a relatively high impedance source, and, to make matters worse, is AC-coupled, it cannot maintain enough drive to the output tube grids to maintain the signal once the output tube goes into grid conduction.Reasons
The grid of a tube normally presents a very high impedance load to the driving source when it is biased in the negative region. When the tube grid bias voltage nears zero, or goes positive with respect to the cathode, its input impedance drops drastically, and it starts drawing current from the source. It behaves similarly to a forward biased diode at that point. If the driving source is capable of providing this extra current that is needed, the tube will continue to amplify normally and put out additional power. This is commonly known as class AB2 operation, where the 2 suffix indicates grid current being drawn during a portion of the cycle, as opposed to class AB1 operation where no grid current is drawn.
Now, the problem with your standard resistance-capacitance coupled amplifier stage is that the onset of grid current makes this forward biased diode out of your previously high impedance grid circuit. This results not in diode clipping, as mentioned above, but rather diode _clamping_, which is the cause of both the transient distortion, and an increase in another type of distortion known as crossover distortion.Solutions
What happens is, the forward biased diode clamps the tops of the grid waveform to a relatively fixed point. Since the previous stage is AC coupled to the grid, the tops are fixed at the clamp point and the entire waveform then shifts downward as gain is increased, pushing the amp more into class B operation, with a resultant increase in crossover distortion.
Since each tube in a push-pull class AB or class B pair amplifies only part of the waveform, the two halves are summed back together in the output transformer secondary to produce the full waveform. If the amp is pushed farther into class B operation, the resultant waveform will have it's center "cored out", producing a dead zone, or flat spot, at the zero crossing of the waveform. This occurs because the center portion of each half has been pushed down into the cutoff portion of the tube's operation. This is known as crossover distortion, and too much of it can sound really bad.
In addition to an increase in crossover distortion, if the time constant of the AC coupling is large enough, and the transient waveform is of low enough frequency, a transient distortion known as blocking occurs. This happens when the transient signal quickly pushes the clamped grid waveform down far into the cutoff region, and there is a finite time that is required for the grid waveform to recover and float back up to the correct bias point once the transient signal is removed. Until the bias comes back to the correct point, the output stage is effectively cut off for a major portion of the signal. This results in a choppy, "farting" sounding distortion.
Fender amps are particularly susceptible to this because of the large values of coupling capacitors on the grids of the power tubes (0.1uF). Blackfacing your Super Reverb can actually make the problem worse, because you change the grid bias feed resistors from 100K to 220K, which increases the time constant of the AC coupling to the output tube grids. You will note that most Marshalls use 0.022uF coupling capacitors and 100K resistors, which gives a much faster time constant. In addition, the preamp stages have a much more rolled off low frequency response. This is why they sound tighter when played wide open.
Negative feedback can exacerbate this problem. When the output tube goes into cutoff, the negative feedback loop opens up, and the gain of the phase inverter stage increases by the amount of degenerative feedback that was formerly present. This increase in gain pushes the amp even farther into cutoff.
What can be done to fix this problem? There are several solutions, and any of them will change the sound of your amp to a certain degree. Fenders sound the way they do because of the design values, and anything you do to change them will take away from some of that characteristic tone. Having said that, here are the solutions:
Reduce the value of the coupling capacitors. Try progressively lower values until you find one that reduces the problem, but doesn't rob you of too much low end. Reduce the value of the grid bias feed resistors. This has the unfortunate side effect of attenuating the gain of the signal from the phase inverter, which may or may not be that noticeable. Increase the size of the so-called "grid stopper" resistor, which is usually a 1.5K - 5.6K resistor in series with the output tube grids, and is usually soldered directly to the grid pin as a parasitic oscillation prevention measure. Increasing the size of this resistor will limit the amount of grid current that can be drawn and reduce the clamping effect quite a bit. This modification has the unfortunate side effect of rolling off the high frequency content of the amp. Depending upon the type of output tube, you can probably go up to around 50K - 100K max before the high frequency loss becomes too noticeable. (Remember, most guitar speakers don't reproduce too much above 4kHz-5kHz anyway). Another potential problem with this method is that the output tube is rated for a maximum grid circuit resistance, beyond which it will become unstable due to changes in grid voltage caused by the voltage drop across the grid resistor from small grid currents. This value is published by the manufacturer typically as two values, one for fixed-bias operation and a higher value for cathode bias operation, since it is self-correcting to a certain extent. The sum of the bias feed resistor and the grid stopper resistor should not exceed the maximum stated value for the grid number 1 circuit resistance. Add a DC-coupled cathode follower between the phase inverter and the grid of the output tubes, with the cathode follower cathode resistor returned to a high negative voltage, and the grid bias applied to the grid of the cathode follower. This effectively isolates the output tube grid circuit from the phase inverter and its associated AC coupling, and provides a very low impedance source for the output stage. This will prevent the output stage from going into grid clamp, and will eliminate the long time constant of the AC coupling. This method has the unfortunate side effect of requiring an extra tube and completely ruining the value of your vintage amp, so it is best used only on new designs, but is highly recommended. You will also get more power out of the output stage because it is now running in class AB2 or class A2 (the "2" suffix indicates grid current flows for a portion of the cycle). Limit the signal level to the output stage at full volume by adding either a resistor across the phase inverter outputs (after the coupling caps) or by adding series resistance on each side between the coupling caps and the grid bias resistors, or by limiting the signal drive out of the phase inverter by increasing the value of the "tail" resistor to a large value. Adjust the attenuation so the phase inverter stage starts to clip just after the output stage starts to clip. If you can prevent the output stage from going too far into grid clamp, you can minimize the problem. This can also be accomplished by adding a negative-clipping zener circuit to limit the maximum negative swing of the output tube grid drive. If you limit the peak negative signal drive to a level equal to twice the bias voltage, the clipped waveform will not be "pushed down" into the class B region, because the signal is much more symmetrical, and the average level is centered around the bias voltage. Reduce the amount of negative feedback used, or remove the negative feedback loop entirely. Generally you will have too much gain in the output stage once you remove the feedback loop, so you will probably want to reduce the signal level as indicated in the fifth method outlined above. This method has the unfortunate effect of rendering any existing presence control inoperable. If you must have presence, add a "treble cut" pot across the phase inverter output as in the VOX AC30, and increase the treble in a previous stage to give you some extra treble to work with. Look for the problem in the preamp stages. Blocking can (and does) occur just as bad on RC-coupled preamp stages that are overdriven. The solution is similar. Add large value series grid resistors (100K - 470K), reduce coupling capacitor values to the minimum required for the desired low frequency response, add interstage attenuators to limit the amount of grid drive to the next tube, reduce the size of, or eliminate, the cathode bypass capacitors, as they also contribute to the blocking due to the long time constant associated with the recovery from a transient signal, or add a diode clipper bounding circuit to prevent the grid from being driven too far.
Copyright © 1999-2006, Randall Aiken. May not be reproduced in any form without written approval from Aiken Amplification.