8 years ago, under my old Talkbass username Duff2, I started a thread to share an SWR amp mod that originated from Walter Harley (@cafewalter) of Cafe Walter Audio.
SWR Aural Enhancer Bypass
As shown in the schematic below, the mod involved removing the 100nF capacitor (C10) from the low pass filter and then installing it across the 1 nF capacitor (C12) of the high pass filter in the fixed value Fender tone stack. This effectively defeated the low mid-scoop and treble boost that define the SWR sound, and resulted in a flatter frequency response. I originally referred to this mod as the 'SWR Aural Enhancer Bypass', however since the Aural Enhancer knob still functioned as a gain control/pad for the signal entering the second half of the 12AX7 tube, a better descriptor would have been the "Harley SWR Preshape Bypass Mod'.
A subsequent contribution to the original thread by Talkbass user @the V, detailed how to incorporate the mod onto a DPDT switch so that one could toggle back and forth between the stock voicing and the bypass option. I had this version of the mod installed on my SWR SM-500 soon after. I was quite pleased with the result, particularly if I used the active input (which bypassed the first half of the 12AX7) and then dialed in the amount of 12AX7 coloration I desired using the Aural Enhancer knob.
This whole endeavor jump-started a serious electronics hobby for me. In the subsequent years I tackled a variety of projects as I got better with the soldering iron and audio circuit design theory. My most recent project was the restoration of an SWR SM-900 that had been almost trashed to the point of no return. After rebuilding the blown (and burnt!) power amps, replacing the power supply capacitors and swapping the preamp power supply cement resistors for chassis-mounted, aluminum-cased wirewounds, I turned my attention to incorporating the Harley Bypass Mod into the preamp.
Later in the original SWR AE bypass thread, @KeithFTW had suggested changing the values of the low and high pass capacitors to achieve more of an Ampeg 'ultra low switch engaged' mid-scoop vibe. Since I never used the stock preamp voicing, I decided to follow KeithFTW's advice and change the cap values such that the mid-scoop dip moved from 200Hz to 500Hz while filling in the 100-200Hz gap and attenuating the boost in the 1-3kHz range of the original preshape EQ curve. After working out the details (C10=47nF, C12=330pF), I realized that this mod could be taken one step further. Since the new cap values were smaller than the original values, it would also be possible to add a second switch to toggle between the original AE200 voicing (black curve) and the new AE500 option (yellow curve). More importantly, the Harley Bypass mod (blue curve) would still function for both voicings!
If you are interested in trying this out for yourself, here are the wiring diagrams:
Important: use capacitors with maximum DC voltage ratings that match the specs of the original SWR parts you are replacing.
Two switch version (SW1 = preshape in/out (bypass), SW2 = AE200/AE500):
Single switch versions:
SWR Aural Enhancer Bypass
As shown in the schematic below, the mod involved removing the 100nF capacitor (C10) from the low pass filter and then installing it across the 1 nF capacitor (C12) of the high pass filter in the fixed value Fender tone stack. This effectively defeated the low mid-scoop and treble boost that define the SWR sound, and resulted in a flatter frequency response. I originally referred to this mod as the 'SWR Aural Enhancer Bypass', however since the Aural Enhancer knob still functioned as a gain control/pad for the signal entering the second half of the 12AX7 tube, a better descriptor would have been the "Harley SWR Preshape Bypass Mod'.
A subsequent contribution to the original thread by Talkbass user @the V, detailed how to incorporate the mod onto a DPDT switch so that one could toggle back and forth between the stock voicing and the bypass option. I had this version of the mod installed on my SWR SM-500 soon after. I was quite pleased with the result, particularly if I used the active input (which bypassed the first half of the 12AX7) and then dialed in the amount of 12AX7 coloration I desired using the Aural Enhancer knob.
This whole endeavor jump-started a serious electronics hobby for me. In the subsequent years I tackled a variety of projects as I got better with the soldering iron and audio circuit design theory. My most recent project was the restoration of an SWR SM-900 that had been almost trashed to the point of no return. After rebuilding the blown (and burnt!) power amps, replacing the power supply capacitors and swapping the preamp power supply cement resistors for chassis-mounted, aluminum-cased wirewounds, I turned my attention to incorporating the Harley Bypass Mod into the preamp.
Later in the original SWR AE bypass thread, @KeithFTW had suggested changing the values of the low and high pass capacitors to achieve more of an Ampeg 'ultra low switch engaged' mid-scoop vibe. Since I never used the stock preamp voicing, I decided to follow KeithFTW's advice and change the cap values such that the mid-scoop dip moved from 200Hz to 500Hz while filling in the 100-200Hz gap and attenuating the boost in the 1-3kHz range of the original preshape EQ curve. After working out the details (C10=47nF, C12=330pF), I realized that this mod could be taken one step further. Since the new cap values were smaller than the original values, it would also be possible to add a second switch to toggle between the original AE200 voicing (black curve) and the new AE500 option (yellow curve). More importantly, the Harley Bypass mod (blue curve) would still function for both voicings!
If you are interested in trying this out for yourself, here are the wiring diagrams:
Important: use capacitors with maximum DC voltage ratings that match the specs of the original SWR parts you are replacing.
Two switch version (SW1 = preshape in/out (bypass), SW2 = AE200/AE500):
Single switch versions:
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