Delicious alternate Rangemaster recipe

[Meowy]

You may have seen elsewhere about Rory Gallagher's Rangemaster, specifically that he (or his techs) deviated from the "standard" resistor values and achieved a generally warmer-and-smoother-than-the-norm treble booster. I've also read further about others duplicating Rory's tone by using a 180K resistor in place of the 470K and a ~39K in place of the 68K. This lowers the bias voltage into the 6-ish range and of course changes the current as well.

I have found that using these lower value resistors shave off some of the shrill edge and even add a bit of a vowel-ey shape to the tone. Also, even though one is generally not found on a treble booster I've tried a 100pf mica cap from base to collector to smooth it further, but I am still playing around with that

Why stop experimenting here? I am toying around with resistor values that are even lower and getting some more interesting results.

A Rangemaster is a great "what if I change ____ ?" circuits.

[eniacmike]

I love rory I am going to have to try this on the breadboard.

[Meowy]

The difference I am hearing between 470K/68K and 180K/33K is minimal but it seems to shave some of the grit off the top. I also tried 100K/17K and got just about the same audible results.

I understand that the change in resistor values I chose effects the circuit in at least 2 ways:

1) collector voltage is decreased since I did not maintain the same ratios in the voltage divider
2) current is increased since the resistance is lowered


Can anyone who has more than my minimal anecdotal knowledge try to explain why 470K / 68K were originally chosen rather than smaller, but similarly proportioned values?

What are the practical reasons for the choice of 470K/68K vs (for example) 47K and 6.8K? Is it to achive a certain current?

Even so, I've seen other vintage treble boosters such as the Apollo, Orange, EHX and Vox which use lower resistor values (and in some cases much lower) than the RM, but used what seem to be relatively similar trannys

[stecykmi]

The main difference has something to do with changing the response of the input filter.

i'm rusty with BJT's (particularly PNP), but I don't think changing the values of the bias resistors has much to do with changing the base current since that's more dependent on the voltages at the collector and emitter.

the gain characteristics will change... but not that much.

[jkokura]

The capacitors will have a much larger impact on the sound, as demonstrated by the input cap mod the current rangemaster project has. Change that input cap and you've got a drastically different booster on your hands.

Jacob

[Meowy]

Yes, I've been playing with this over many weeks as time allows. I have found the input caps are where the gross adjustments are made and I have tried quite a few variations to find those that are best to my ear. I am using a .0056uF (or .0068uF) as my "main" input cap with a switch to parallel in another .005 - .01uF. I am also considering a switchable full range input cap on the order of 0.1 - 1.0uF . I have not found a great deal of usable variation  on the output cap. So that leaves me at the point of trying to take a deeper dive towards understanding the interactions of the entire circuit to find other opportunities to fine tune even further.

I have found that reducing the bias resistor values lower thatn the "textbook" values (but in roughly the same proportions) does take some of the higher frequency edge off. I assume this unpleasant (to me) edge may be clipping of ther highest audible frequencies. I am also toying around with small value caps from input to ground, out to ground and betwen the base-collector and base-emitter. So far a mica cap from base to colector seems tom ake the most desirable difference to me. And so it goes on... and on...

Thanks Jacob for offering your guidance