#1 · Aug 28, 2008 16:57 UTC
The Peavey JSX amp is a modern-day amp that a lot of folks have given a lot of good press towards. I thought I'd spend some time discussing the magic behind this new "myth". I have one of these bad boys and it's a gain monster.
The basic structure of the amp is much like the MESA architecture. All the tubes, in the amp, are biased right-center of the load line for maximum grid voltage swing. As per Fender's topology, the tubes sport a 100K plate resistor (RP), and a 1.5K cathode resistor (RK) bypassed with 22uF capacitors - the exception to this is the 2nd gain stage and the final "driver" stage. This makes each stage, essentially, a flat gain stage, with maximum gain involved. There are 4 gain stages prior to the EQ section, in the "hi-gain" section. The EQ, on this amp, is an active EQ with cut/boost functionality. All controls at the center of their sweep = flat response from the EQ. One 12AX7 is dedicated to just dealing with the EQ controls.
The output of the tone control circuit is fed into a final driver stage with an approximate 8kHz high-pass filter. This puts the final "sheen" on the output and pushes the signal to the levels required by the power amp section.
The power amp is the power amp from the Peavey XXX amplifier with two basic changes - the addition of resonance and presence controls.
So, let's look at the amp, overall. The input circuit has a lower input impedance (470K), in parallel with a 39pF capacitor. This combination produces a slight rolloff (3dB) of high frequencies "warming" the guitar signal up, just a bit. This is, truly, to attenuate RF noise entering the amp on the guitar cable (think "Spinal Tap" and the police radio incident).
Stage 1 is a flat gain stage. The output of stage 1 is fed into the interstage coupling. This is where the "FAT" switch exists. When the FAT switch is out, this section forms a 720Hz (recognize that) high pass filter. When the FAT switch is in, the section forms a 385Hz high pass filter adding a bit more "girth" to the final result. This is then fed into either the "Crunch" or the "Ultra" gain control. The Crunch pot is much smaller than the ULTRA pot, dropping less of the voltage across it. This is why the gain structure changes - there's significantly less signal fed to the rest of the amp when it's in "Crunch".
Stage 2 has a 470nF capacitor across the cathode resistor. This "shelves" this stage at about 225Hz cleaning up some of the "mud" that could potentially show up. Signal input, to this stage, is still low enough that distortion is just starting to rise. Nearly all of the signal output, from the first stage, is actually "dropped", or absorbed" by the interstage coupling - nearly 31dB worth.
Stage 2 amplifies again, pushing the signal through a 470pF/470K (720Hz) high pass filter (just like Marshall) and this is fed into stage 3 with a bit more level/volume and stage 3 is where much of the distortion begins to occur.
Stage 3 is a flat gain stage, again. It's output is sent to a divider network, lowering the voltage entering stage 4 which, too, is flat response. The output of stage 4 feeds a final divider network and the channel's "Volume" control. Across the volume control is a low pass (high bleed) filter network, centered at 328Hz. It's pretty high impedance, as compared to the volume control, so some of the high frequencies are bled off, but not a lot of them. It has the direct effect of "softening" the relatively distorted output feeding the tone stage.
The tone stage, as per before, is an active gain stage with a low pass filter, a high pass filter and a midrange bandpass/bandcut filter. The ranges generally overlap, to some extent, in this amp. Satch has the midrange bandwidth extended quite a bit, from the original XXX design to provide a more "gradual" increase/decrease in mid frequencies. The boost/cut frequencies on the Ultra and Crunch channel are slightly different.
Out of the active EQ stage, then, the amp feeds into the final gain stage which actively shelves the output above about 8kHz - emphasizing that range of frequencies over everything below it. This adds that "sizzle" and "sparkle" to the amp channel that Satch is kind of known for.
The power amp stage is pretty much a standard EL34/6L6 output stage (EL34 standard). The "difference" here is the "Resonance" and "Presence" controls implemented into the feedback loop which decouple the feedback at low and high frequencies and leave the midrange alone. This has the effect of "loosening" the output some, when the Resonance is turned up, giving it a "looser" or a "fuller" sound. This, coupled with the active bass control, is probably the reason why that final driver stage is shelved at 8kHz. To account for the increased mid/bass boost that you wouldn't normally get with a standard Fender tone stack.
That is, pretty much, the JSX. Distortion is created, in this amp, in multiple successive stages with each stage contributing, overall, to the final amount of distortion. The "Gain" control is placed between the first and second stages allowing the gain structure of the entire amp to be controlled from the front-end. Because each stage is biased at the center of the load-line, and made to run "full-out" with the large value bypass capacitors, the distortion generated by this amp is somewhat more symmetrical than that generated by a standard Marshall, wherein much of the distortion is generated in the final stage of the amp, just before the tone stack. This has the effect of making the JSX, potentially, a bit more "buzzy". Peavey accounts for this by providing several "low pass" filters which bleed high frequency content off to ground, softening the sound quite a bit.
The main difference, between the Crunch and the Ultra channel resides in two things: 1) The "Gain" pot used to capture the signal from stage 1 and feed into Stage 2. The Crunch channel "Gain" pot is 20 times smaller than the Ultra channel (50K versus 1M) - that SIGNIFICANTLY reduces the amount of signal the gain pot will capture versus how much is "absorbed" or "dropped" by the circuitry preceding it. 2) The tone stacks, on each channel, vary in terms of how much "cut" and "boost" the mid/bass controls create. The "Ultra" is a bit "deeper" in terms of effect, but are slanted in favor of treble, whereas the Crunch channel is slightly more balanced.
I didn't create a block diagram, on this one, but I can. I believe I have JSX schemats for those that might be interested in having a look see at them.
Dar
The basic structure of the amp is much like the MESA architecture. All the tubes, in the amp, are biased right-center of the load line for maximum grid voltage swing. As per Fender's topology, the tubes sport a 100K plate resistor (RP), and a 1.5K cathode resistor (RK) bypassed with 22uF capacitors - the exception to this is the 2nd gain stage and the final "driver" stage. This makes each stage, essentially, a flat gain stage, with maximum gain involved. There are 4 gain stages prior to the EQ section, in the "hi-gain" section. The EQ, on this amp, is an active EQ with cut/boost functionality. All controls at the center of their sweep = flat response from the EQ. One 12AX7 is dedicated to just dealing with the EQ controls.
The output of the tone control circuit is fed into a final driver stage with an approximate 8kHz high-pass filter. This puts the final "sheen" on the output and pushes the signal to the levels required by the power amp section.
The power amp is the power amp from the Peavey XXX amplifier with two basic changes - the addition of resonance and presence controls.
So, let's look at the amp, overall. The input circuit has a lower input impedance (470K), in parallel with a 39pF capacitor. This combination produces a slight rolloff (3dB) of high frequencies "warming" the guitar signal up, just a bit. This is, truly, to attenuate RF noise entering the amp on the guitar cable (think "Spinal Tap" and the police radio incident).
Stage 1 is a flat gain stage. The output of stage 1 is fed into the interstage coupling. This is where the "FAT" switch exists. When the FAT switch is out, this section forms a 720Hz (recognize that) high pass filter. When the FAT switch is in, the section forms a 385Hz high pass filter adding a bit more "girth" to the final result. This is then fed into either the "Crunch" or the "Ultra" gain control. The Crunch pot is much smaller than the ULTRA pot, dropping less of the voltage across it. This is why the gain structure changes - there's significantly less signal fed to the rest of the amp when it's in "Crunch".
Stage 2 has a 470nF capacitor across the cathode resistor. This "shelves" this stage at about 225Hz cleaning up some of the "mud" that could potentially show up. Signal input, to this stage, is still low enough that distortion is just starting to rise. Nearly all of the signal output, from the first stage, is actually "dropped", or absorbed" by the interstage coupling - nearly 31dB worth.
Stage 2 amplifies again, pushing the signal through a 470pF/470K (720Hz) high pass filter (just like Marshall) and this is fed into stage 3 with a bit more level/volume and stage 3 is where much of the distortion begins to occur.
Stage 3 is a flat gain stage, again. It's output is sent to a divider network, lowering the voltage entering stage 4 which, too, is flat response. The output of stage 4 feeds a final divider network and the channel's "Volume" control. Across the volume control is a low pass (high bleed) filter network, centered at 328Hz. It's pretty high impedance, as compared to the volume control, so some of the high frequencies are bled off, but not a lot of them. It has the direct effect of "softening" the relatively distorted output feeding the tone stage.
The tone stage, as per before, is an active gain stage with a low pass filter, a high pass filter and a midrange bandpass/bandcut filter. The ranges generally overlap, to some extent, in this amp. Satch has the midrange bandwidth extended quite a bit, from the original XXX design to provide a more "gradual" increase/decrease in mid frequencies. The boost/cut frequencies on the Ultra and Crunch channel are slightly different.
Out of the active EQ stage, then, the amp feeds into the final gain stage which actively shelves the output above about 8kHz - emphasizing that range of frequencies over everything below it. This adds that "sizzle" and "sparkle" to the amp channel that Satch is kind of known for.
The power amp stage is pretty much a standard EL34/6L6 output stage (EL34 standard). The "difference" here is the "Resonance" and "Presence" controls implemented into the feedback loop which decouple the feedback at low and high frequencies and leave the midrange alone. This has the effect of "loosening" the output some, when the Resonance is turned up, giving it a "looser" or a "fuller" sound. This, coupled with the active bass control, is probably the reason why that final driver stage is shelved at 8kHz. To account for the increased mid/bass boost that you wouldn't normally get with a standard Fender tone stack.
That is, pretty much, the JSX. Distortion is created, in this amp, in multiple successive stages with each stage contributing, overall, to the final amount of distortion. The "Gain" control is placed between the first and second stages allowing the gain structure of the entire amp to be controlled from the front-end. Because each stage is biased at the center of the load-line, and made to run "full-out" with the large value bypass capacitors, the distortion generated by this amp is somewhat more symmetrical than that generated by a standard Marshall, wherein much of the distortion is generated in the final stage of the amp, just before the tone stack. This has the effect of making the JSX, potentially, a bit more "buzzy". Peavey accounts for this by providing several "low pass" filters which bleed high frequency content off to ground, softening the sound quite a bit.
The main difference, between the Crunch and the Ultra channel resides in two things: 1) The "Gain" pot used to capture the signal from stage 1 and feed into Stage 2. The Crunch channel "Gain" pot is 20 times smaller than the Ultra channel (50K versus 1M) - that SIGNIFICANTLY reduces the amount of signal the gain pot will capture versus how much is "absorbed" or "dropped" by the circuitry preceding it. 2) The tone stacks, on each channel, vary in terms of how much "cut" and "boost" the mid/bass controls create. The "Ultra" is a bit "deeper" in terms of effect, but are slanted in favor of treble, whereas the Crunch channel is slightly more balanced.
I didn't create a block diagram, on this one, but I can. I believe I have JSX schemats for those that might be interested in having a look see at them.
Dar
