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Step 8The Preamp Stages
It might look like the preamp circuit was lifted directly from an existing design--excluding the tone control (see the next section.) But I really did design it from scratch. If it sounds good, it will likely be similar to other designs, of course. There's nothing new under the sun for (simple) 12AX7 preamps.
This is a standard two-stage preamp. In theory, more stages equals a thicker, smoother type of distortion--without the "hard" breakup common to transistor circuits. Two stages is generally considered minimum for a "modern" preamp (some older amps had a single pentode preamp stage.) Of course, that's one advantage of the 12AX7--it's two triodes in a single tube.
It was modified, too, during building. As I raised the coupling cap values (from .01 to .02), the amount of gain, "fatness" and distortion increased dramatically. Most of the modifications since have involved REDUCING gain from the original design. I had very high gain settings initially, as the preamp tube was running with a lower voltage than most production amps. However, the extra gain was unnecessary.
In fact, I might still reduce some of the gain for the preamp stage...But the sound is pretty edgy for a small amp.
One starting point for preamp design is the data sheets. Most include a helpful chart (see first image.) With this chart alone, a very workable triode stage can be constructed.
Some important concepts/components:
--The Plate Resistor (Rp)
Vacuum tubes are controlled by voltage, but amplify current. To make them output a voltage change, we must add a plate resistor. Good old "Ohms Law": I*R=E (Current * Resistance = Voltage.) So a larger value plate resistor increases the amplification (you can verify this on the chart.)
The value of the plate resistor also has a profound effect on the amount of 2nd order harmonic distortion the amp produces. A tube amplifier has an inherent peak-to-peak asymmetry, which can be lessened or increased by varying the slope of the "loadline." In preamp stages, the plate resistor determines the loadline slope.
2nd order harmonic distortion is a positive--and is considered one of the characteristics of a good guitar tube amp.
--The Cathode bias resistor (Rk)
All tubes require the grid (signal input) to be negative in respect to the cathode. A negative charged grid repels electrons, so no current flows. The simplest way to achieve this "negative bias" is to raise the cathode voltage sightly--that's the job of the cathode-bias resistor. Raising the cathode bias (increasing the resistance, or "cold bias") makes the grid more negative.
Together, these two resistors largely determine the gain (there are other considerations, also.) There are "sweet spots" for each, and poorly chosen, either the plate load or the bias resistors can result in some nasty (bad nasty) effects.
Rules of thumb
More gain: increase Plate Resistor
Less gain: decrease Plate Resistor
More harmonic content (2f): lower plate resistor
Less harmonic content (2f): higher plate resistor
The effect the Cathode Bias Resistor has on gain is a little more subtle. There's a maximum gain sweet spot for bias, which may or may not be the desired sound. Both raising and lowering the bias will cause clipping, but in different ways. Some clipping is often a good thing in the context of guitar amps.
As a general rule, a higher (cooler) bias voltage results in a harsher distortion. The amplified signal is clipping hard against the "rails" (the supply voltage.) But a hotter, lower bias can still clip. At this bias level, some clipping occurs (allegedly) due to "grid current limiting," which is somewhat softer. However, there's usually a range between high and low bias extremes that results in the most "natural" amplified guitar sound.
Although the rational for multi-stage preamps usually is that they create smoother distortion by only gently clipping in each stage, clearly another fundamental reason is that using lower plate resistors (lower gain) greatly increases the percentage of 2nd order distortion. More stages compensates for any gain losses.
More components
--Cathode Bypass Capacitor
This has a real effect on the overall output of the stage, and increasing capacitance will tend to boost the bass response.
--Coupling capacitors (and the grid-leak resistors following which constitute an RC filter.)
Labeled C and Rs, on the table below, together they have a huge effect on the frequency response of each stage.
The grid-leak resistors (in a cathode bias amp) are usually in the range of 220K - 470K. Oddly enough, the best-sounding value for the first stage was 120K. Surprising, since lower resistance here attenuates the signal somewhat. The specific frequency response overshadowed any signal loss. The second stage grid-leak resistor is a more typical 220K.
This is a standard two-stage preamp. In theory, more stages equals a thicker, smoother type of distortion--without the "hard" breakup common to transistor circuits. Two stages is generally considered minimum for a "modern" preamp (some older amps had a single pentode preamp stage.) Of course, that's one advantage of the 12AX7--it's two triodes in a single tube.
It was modified, too, during building. As I raised the coupling cap values (from .01 to .02), the amount of gain, "fatness" and distortion increased dramatically. Most of the modifications since have involved REDUCING gain from the original design. I had very high gain settings initially, as the preamp tube was running with a lower voltage than most production amps. However, the extra gain was unnecessary.
In fact, I might still reduce some of the gain for the preamp stage...But the sound is pretty edgy for a small amp.
One starting point for preamp design is the data sheets. Most include a helpful chart (see first image.) With this chart alone, a very workable triode stage can be constructed.
Some important concepts/components:
--The Plate Resistor (Rp)
Vacuum tubes are controlled by voltage, but amplify current. To make them output a voltage change, we must add a plate resistor. Good old "Ohms Law": I*R=E (Current * Resistance = Voltage.) So a larger value plate resistor increases the amplification (you can verify this on the chart.)
The value of the plate resistor also has a profound effect on the amount of 2nd order harmonic distortion the amp produces. A tube amplifier has an inherent peak-to-peak asymmetry, which can be lessened or increased by varying the slope of the "loadline." In preamp stages, the plate resistor determines the loadline slope.
2nd order harmonic distortion is a positive--and is considered one of the characteristics of a good guitar tube amp.
--The Cathode bias resistor (Rk)
All tubes require the grid (signal input) to be negative in respect to the cathode. A negative charged grid repels electrons, so no current flows. The simplest way to achieve this "negative bias" is to raise the cathode voltage sightly--that's the job of the cathode-bias resistor. Raising the cathode bias (increasing the resistance, or "cold bias") makes the grid more negative.
Together, these two resistors largely determine the gain (there are other considerations, also.) There are "sweet spots" for each, and poorly chosen, either the plate load or the bias resistors can result in some nasty (bad nasty) effects.
Rules of thumb
More gain: increase Plate Resistor
Less gain: decrease Plate Resistor
More harmonic content (2f): lower plate resistor
Less harmonic content (2f): higher plate resistor
The effect the Cathode Bias Resistor has on gain is a little more subtle. There's a maximum gain sweet spot for bias, which may or may not be the desired sound. Both raising and lowering the bias will cause clipping, but in different ways. Some clipping is often a good thing in the context of guitar amps.
As a general rule, a higher (cooler) bias voltage results in a harsher distortion. The amplified signal is clipping hard against the "rails" (the supply voltage.) But a hotter, lower bias can still clip. At this bias level, some clipping occurs (allegedly) due to "grid current limiting," which is somewhat softer. However, there's usually a range between high and low bias extremes that results in the most "natural" amplified guitar sound.
Although the rational for multi-stage preamps usually is that they create smoother distortion by only gently clipping in each stage, clearly another fundamental reason is that using lower plate resistors (lower gain) greatly increases the percentage of 2nd order distortion. More stages compensates for any gain losses.
More components
--Cathode Bypass Capacitor
This has a real effect on the overall output of the stage, and increasing capacitance will tend to boost the bass response.
--Coupling capacitors (and the grid-leak resistors following which constitute an RC filter.)
Labeled C and Rs, on the table below, together they have a huge effect on the frequency response of each stage.
The grid-leak resistors (in a cathode bias amp) are usually in the range of 220K - 470K. Oddly enough, the best-sounding value for the first stage was 120K. Surprising, since lower resistance here attenuates the signal somewhat. The specific frequency response overshadowed any signal loss. The second stage grid-leak resistor is a more typical 220K.
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However, my comment isn't about the missing bypass cap value, rather the fact that it isn't identified as a cathode bypass cap in the datasheet graphic.