From: info@turneraudio.com.au   
      
   Hey Patrick,   
      
   Was thinking about your musings on a tone stack. On the LC 400Hz mid circuit.   
   A small inductor 1.6mH, only has a reactance of around 5 ohms at 400Hz. The   
   cathode Z is in the hundreds of ohms. Since Q in a series circuit is Q=X/R,   
   you can't develop enough    
   effective resonant Q to be useful. Might save you some disappointing results.   
   I do like the concept of the cathode series tank, but note that will give you   
   a boost only, not boost cut, unless you do a little switching. What might be   
   cool is to implement    
   all the tone stack that way. It might keep the interaction down too.   
      
   Also note that significant boost is difficult to achieve with RC only with   
   your proviso that 2 octave away has essentially no change in response. You'll   
   get at least a dB or so change that close. (Thinking of the 100Hz boost vs   
   400Hz no-boost.   
      
   Cheers,   
   Stephie    
      
   You are perhaps correct. Maybe make a larger value L of 0.16H, with C = 1uF so   
   Fo = 398Hz. I have not tried the idea.   
   But a larger L value is a Royal Pain In Arse for most ppl farnarkling around   
   for novel ways of making what is a single band graphic eq. The L is prone to   
   hum pick up, and would use lots of turns of say 0.15dia Cu wire on a bar type   
   of core, so size is a    
   bother.....   
   Then comes the idea of a gyrator and adherence to normal graphic eq circuit   
   methods, only you use tubes instead of an opamp and a transistor plus R&C for   
   a "fake" L.    
      
   Simpler, possibly, is the inclusion of a bridged T between CF buffer after   
   gain control after tone stack and before input to gain tube before power amp.   
   Only a few R&C and a pot are required, but you only get a notch of lowish Q.   
   But I cannot see where anyone would want a peaked mid. This would make sound   
   strange where bassy notes go missing, and just a small range of upper notes   
   are way too loud.   
      
   Bridged T should be driven with low Rsource, then has R in series with R to   
   high Z grid input of gain tube. R + R is shunted by a C value that is much   
   lower than the pylon C between join of 2 R and 0V. Examples are to be found   
   online if Googled, and see    
   images.    
   The Fo of the minimum Vo at the dip can be changed with a dual gang pot for   
   each of the R spans of the bridge.   
   Insertion effect can be governed by a pot somewhere, not sure where/how, but   
   there must be a way for between flat and no dip to full dip.   
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
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