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I was recently perusing some pedal building forums and came upon an interesting thread about Rory Gallagher’s Rangemaster, which many guitarists found appealing. Apparently, he or his techs modded his RM and deviated from the “standard” RM resistor values, achieving a generally warmer and smoother than normal treble booster. This is thought to be related to swapping out the bias network transistors, using a 180K resistor in place of the 470K and a ~39K in place of the 68K resistor. Apparently using lower value resistors shave off some of the shrill edge and even add a bit of a vowel like tone to the sound of the pedal. Maybe this was what Tonmann was aiming for with an R1 value of 200K in his design of the PURP. Using the new resistor values as above, the voltages and current values are changed. The Base voltage is greater around 1.6V, the Collector voltage is around 6V and the current on the Collector/Emitter side of the this common emitter amp is raised to 0.333mA, as below.
Using the trimpot (instead of a fixed resistor) on the PURP at R2, one could achieve the same Collector voltage as the RG mod by using a value around 43K as this would provide a base voltage of 1.6V and hence a Collector/Emitter side current of 0.333mA. The value of R2 should be around 29K for the standard RM Base voltage of around 1.1V. I have not built the PURP yet, although I have the parts, so I don’t know if there is any audible difference. The RM I built in the past does not have any shrill edge to my ears, but does incorporate an external bias and 5 input cap values so maybe I dialed it out.
Based on the research and experimentation I’ve done with the DRM circuit, I’ve found that filtering is the key to getting best sound out it. And that biasing is typically transistor dependent.
Analysis diagram . . . .
More detailed schematic of a modified DRM with tone and fuzz mods (courtesy of Peter Singfield – UK) . . . .
The Bias can be an internal trimpot or external control.
I love all the deep analysis.
In regards to: The RM I built in the past does not have any shrill edge to my ears, but does incorporate an external bias and 5 input cap values so maybe I dialed it out.
Rory to me was a master of pushing limits with all of the tools at his disposal. The AC30 amp he often used. He was a master of his Strat including pinch and pop harmonics which I think were enhanced by both his Rangemaster and preferred amplifier tone settings pushing the limits of biting tone which he seems to reign in using his guitar. Rory definitely had a tone all his own when compared to others using similar equipment.
I am not sure of what Tonmann’s intentions were but there is a lot of tonal variety in the PURP circuit.
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