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custom tubescreamer

8/30/1999 11:10 PM
custom tubescreamer
I want to build a custom tubescreamer and I'm just wondering if anyone can help with a mod.  
I want to use one of RG's screamer boards in a large hammond box with 2 footswitches. One for standard operation and the second to switch to the Turbo setting that the TS9DX has.  
So, my question is: does anyone know what this Turbo setting changes from the standard ts9 setting. I've played one in a shop but don't want to buy it to trace out the circut, so any help would be appreciated, thanks.
8/30/1999 11:46 PM
Dave Chun

The "turbo" mode switches in a larger tone cap (10uf?) and different clipping diode for more volume, midrange, and bass. Check out RG's TS-x schematic for the labelling of the tone caps (the 0.22u caps around the tone control.)
8/31/1999 3:52 AM
Mark Hammer
The nice thing about op-amp-based gain stages is that the gain can be changed by altering the value of a single resistor, in this case either the feedback resistor between the output and - input, or the resistance between the - input and ground. By having two resistors in series, and wiring up a SPST footswitch in parallel with one of them, you can instantly switch between two gain levels. So, for example, having two 3k3 resistors in series in the path to ground would give a maximum gain in the stock unit of:  
(500k + 51k + 6.6k)/(6.6k) = 84  
Step on the footswitch to bypass one of the 3.3k resistors, and you instantly go to: 554.3k/3.3k = 168  
Which is pretty "turbo" in most books.  
Although in theory, the feedback and inv-input-to-ground resistors are functionally equivalent, in practice they aren't. This is because such gain stages will almost invariably include a hi-frequency limiting cap in parallel with the feedback resistor to stop oscillations and dump any signal that might produce more noise than sound, and a frequency-limiting cap in series with the resistor going to ground to block DC and take out non-useful low-end. The frequency at which either of these things happens is given by the standard 1/(2piRC) formula.  
As (I hope) you are aware, the unique tone of the TS-x series is partly produced by shaping of the low and high ends to keep all clipping nicely controlled. The implication is that adjusting the gain by either means will result in a change in either high or low-end rolloff which may deviate from the characteristics desired in the pedal.  
Some rules of thumb:  
1) As you increase the resistor to ground, the gain of a non-inverting op-amp drops, and so does the low-end rolloff. More bass, less brawn.  
2) As you increase the feedback resistor (such as you might with the gain pot), gain increases, but high end rolls off lower. More brawn, less bite.  
As Dave has noted, the stock Turbo TS-9 does a bit more than this, but it uses a rotary switch to do it if I'm not mistaken. If you want a stompswitch to do something equivalent, there is a bit more involved. The boost switch is simpler.  
Personally, I find that I like to trim some of the boost off at later stages. That is, when goosing the clipping stage harder, compensate at the output volume control. Fortunately, a standard DPDT stompswitch can do this. By putting a well-chosen resistor in series with input lug to the volume control, you can fake turning the volume down (since the output level is a function of those series resistances up to the wiper, divided by the resistance between wiper and ground.  
So, if you were to use the resistor-to-ground method for gain boost, you would wire up the stompswitch so that when you shunt one of the gain-setting resistors in the gain+clipping stage (thus raising the gain), you would simultaneously lift the shunt in parallel with the added resistor before the volume control (thus reducing the volume by a measured amount). Ideally, you would want to stick in a trimpot for each engaged/bypassed resistor, and season to taste (after all, some people like a bit of audible boost when they hit boost, and other just want the tonal change without the volume change).
9/2/1999 12:28 PM
Dai Hirokawa
Re: Inside the Turbo Tube Screamer
Hi, got a used one recently--anyway here goes:  
"TS9"(Normal) mode(for comparison): .047uF mylar w/two small signal Si diodes  
"+" mode: 0.22uF polarized electrolytic w/four small signal Si diodes--two for the top of the waveform, two for bottom (these look the same as the ones used for the normal setting)  
"Hot" mode: 1uF polarized electrolytic w/pair of LEDs(w/a 470k in parallel w/the LEDs)  
"Turbo" mode: 2.2uF polarized electrolytic w/no diodes(seemed odd but it does look open)  
-the 4.7k and 51k are unchanged for all modes  
(Should be right, but any corrections welcome)  
Dai Hirokawa

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