[Patrick Turner]

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.