A Comprehensive SCR mod picture!

[mth5044]

I whiped up a picture that included the mods I could find of the stage center reverb, most of which from Mark Hammer.



GGG layout for reference: http://www.generalguitargadgets.com/diagrams/stage_center_reverb_lo1.gif

In the picture, the mods in red are those outlined by Mark in this post:

As a mechanical device, a spring reverb probably offers up more variation by adjusting the tone going into the springs than coming out of it.  Doesn't mean that a post-spring tone control will be of absolutely no use, but you may find that simply altering the bass response going into the springs will have a much more robust effect on tone than a subtle treble-trim control like the SWTC will have.

I've recommended it before and will recommend it again here:
1) Make the 470k feedback resistance in the op-amp feeding the springs variable.  Something like a 500k pot in series with a 220k fixed resistor is probably about right.  This will vary how hard you drive the springs, and elicit varying amounts of "sproing" out of them.  Note that with a 220pf feedback cap, a max resistance of 220k+500k will roll off the treble around 1khz, so you may want to drop the 220pf to 120pf or so if you use a variable drive-gain mod such as described. (Incidentally, in traditional Fender circuits, "Dwell" is actually how hard the springs are driven, NOT how much spring signal is mixed in with clean as the GGG schematic suggests).

2) The amount of "sproing" will depends on how much of that higher-amplitude bass you feed the springs.  The 22k/.02uf combo on the driver stage gives a bass rolloff starting around 360hz.  You can make it more sproingy by dropping that rolloff down with a .047uf cap, or make it more "air-ey" by raising the rolloff with a .015 or .012uf cap.  Note that by trimming off any bass signal fed to the springs, you will need a bit more drive.  Yet another reason to incorporate variable drive into the basic schematic.

3) Since the amount of pre- and recovery drive is bound to acquire some hiss, a variable treble cut control is probably a reasonable idea.  Here's how to incorporate an SWTC.  The mixer stage (IC1c in the GGG schematic) uses almost-equal input resistances (10k vs 4k7+4k7) to match the dry and wet signal levels.  Based on the roughly 10k input resistance, a 33k feedback resistor gives an approximate gain of a little over 3x.  The wet path is divided into two 4k7 resistors, with a .01uf cap to ground from their junction.  This provides one pole of lowpass filtering at roughly 3.4khz whilst still maintaining an input resistance similar to the dry path.  Readers of my posts will know that I often suggest something like this when people ask how to "warm up" the delay path in a delay unit.  Of course, what Anderton has done here is really the same thing as an SWTC, except its fixed rather than variable.  So let's unlock the variability.
     If the input resistance to IC1c is increased to 15k and the feedback resistance is increased from 33k to 47k, we end up with close to the same amount of gain in the final op-amp stage, but benefit by having an input resistance we can now use productively.  Replace R3 (10k) with a 15k resistor.  Replace the two 4k7 resistors from the wet level pot (R1, R2) to the mixer stage with a 2k2 resistor, 10k pot, and 2k7 resistor (in that order).  The wiper of the pot goes to a .015uf cap and then to ground.  With the 2k2 resistor on the wet-level side of the tone pot, the treble rolloff will go from roughly 880hz when the wiper is closest to the mixer-stage input (i.e., the junction between the 10k pot and 2k7 resistor), to roughly 4.8khz when the wiper is moved closest to the wet-level pot (the side where the 2k2 resistor is).

Inclusion of a bass-rolloff switch, a dwell/drive control, and a post-spring variable treble rolloff, in addition to the usual wet/dry level pots should

Also included is a splice in before the spring tank for effects like compression. Mark recommened putting in an orange squeezer, but I remember reading somewhere by someone (Wild Zebra maybe) that it didn't make a difference at all. Oh well.

The other blue ones are some ideas I had. They could be totally wrong, but from what I understand about the circuit (uh oh), they could be right. The first, bottom switch is to cut out the dry signal, leaving only the wet reverb. Could be a novelty sound, or it could get interesting with the other mod. An 'effects loops' on the wet signal after it comes back from the spring tank. With delay, it could get pretty fun with pitchshifting, oscillating fun.

Anyway, I hope someone would get some use out of it. I'm welcome to corrections. Infact, I'm ready for them...still have the Illustrator file open 

[mth5044]

No takers? 

[Harry]

not sure if anyone wants to see a pic of something you 'whiped up'.