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previous: Vince Hi,What if use "tonestack" ... -- 1120580604 View Thread

Re: tonestack instead cathode-cap?

7/5/2005 12:44 PM
Dr. Photon
Re: tonestack instead cathode-cap?
I don't think that would work well. Now, driving a tone stack from a cathode follower (ala a Marshal amp) is a perfectly fine thing to do, but the problem is that a tone stack is a three terminal network, with an input, ground (common), and an output. It seems you are looking to add a tone control or adjustable cathode bypass cap to an existing gain stage. for that you need a two terminal network. It also has to be fairly low impedance (if the impedance of your network is much higher than the usual 1.5K cathode resistor, it won't have much effect).  
You can't really put an entire tone stack into the cathode circuit of a simple common cathode gain stage, but you can make it adjustable. What you could do is use a multi-position rotary switch to select different bypass capacitors (and maybe coupling caps, or caps in series with the plate resistor in addition). You can also have a small value (not more than 5K) pot in series with the cathode capacitor. the different cathode caps will set how much bass will go through the stage (big 25+ uF will boost everything, a small one will only boost high mids and treble). The pot will set how much this boost is. It might be interesting to play with some inductors, too; that way you could boost the bass and roll off the treble.  
If you want a more "precise" bax stack, you could try putting it in a feedback loop around a gain stage. some old HiFI amps worked like this (I have a schematic of a Bell Sound unit that sounds good if anyone wants). The Bell Sounds sounded like a more "acurate" and "powerfull" (more boost/cut it seems)version of a baxandal tone stack, which does sound more "acurate" than the typical FMV stack. I have one in a "VOX - Route 66" style amp with an EF86, LTP, ultralinear EL84's, and no NFB. I like its versatility, but it took a bit of time to get used to. If you use HiFi designs, you might want to consider changin the corner frequencies to more "guitar" ones - values lifted from Ampegs seem to work great!

Albert Kreuzer Bell schematic -- 7/6/2005 2:53 AM
Rob Mercure Re: tonestack instead cathode-cap? -- 7/7/2005 6:47 AM