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previous: T.B. Couple of questions? 1) I'm... -- 7/29/1999 3:09 AM View Thread

Re: 10K or 25K Midrange Control Opinions?

7/29/1999 5:05 AM
Steve A.
Re: 10K or 25K Midrange Control Opinions?
:tone stack caps  
When I was building my first project amp a few years ago (a butchered 65 Pro with a Torres SuperTexan board), I had this "great idea" of adding mini-toggle switches to the rear panel to select between different values for the tone stack caps (that way I could emulate the tone stacks of many different amps). For the Normal channel (almost stock BF but with a few tweaks), what sounded head-and-shoulders above the other choices was the combination used in the BFSR: 0.047uF bass and a 0.022uF mid cap (both Sprague Type 418P Orange Drops) along with a 250pF mica treble cap. This combination really sparkled while the other choices were either too dark or too bright.  
While that combination worked pretty good in a fairly stock BF preamp, for higher gain OD channels its hard to beat a matched pair of 0.022uF's for the bass and mid caps. It was Bruce Collins of Mission Amps who pointed out to me that the slope resistor is every bit as important as the tone caps- and possibly even more so. So for a typical BF preamp, try replacing the 100k slope resistor with an 82k or even 68k resistor to allow for more bass and mid response (you can always back off on the bass or mid controls if you find yourself getting too much bass or mids). If you are looking for cleaner BF sounds, try the 0.047uF/0.022uF caps; if you want to crank it up for some natural overdrive try the 0.022uF caps.  
For the typical 5F6A/Marshall tone stack, stick with the tried and true 0.022uF bass and mid caps, but play around with the treble cap and slope resistor to adjust the overall range of tones. For many of my amps I've settled on a 330pF mica treble cap and a 56k slope resistor (Bruce Collins' recommendation- thanks, Bruce!)- with a switchable 390pF mica cap across the treble cap for a midrange boost (it lowers the overall curve of the tone stack about 7 half-tones; a 750pF mid boost cap lowers the curve about an octave). For my Pignose G40V I wired in a center-off 4PDT mini-toggle switch to choose between 250pF/68k, 330pF/56k and the stock Pignose values of 1000pF/33k (mondo boost there!). I hardly ever use the 250pF/68k combination (it sounds "wimpy" to me compared to the 330pF/56k combo) so I'll probably rewire it for 330pF/56k, 720pF/56k and 1000pF/33k one of these days... (Look for the article on the Pignose G40V on my site for the details of wiring in this switch.)  
These are all just my own very non-expert observations!  
Steve Ahola  
P.S. In replacing a 10k mid pot with a 25k mid pot, can't you just set the control lower if it is too "thumpy" for you? Of course you don't have as much control as you would with a 10k pot... (maybe that is why I usually end up setting a 25k mid control to 4 while I have the bass and treble controls set to 7- if I want more mids I'll switch a cap in parallel with the treble cap rather than turn up the mid pot.)

Mark Hammer Re: Midrange Control Taper -- 7/30/1999 1:04 AM