Adding a bypass capacitor... effect on the NFB resistor?

[tubedood]

I'm going to try this first as Chuck has kindly drawn it up and then tinker and experiment some ideas from there. I can use my tone potentiometer push/pull DPDT switch to make the connection for .15uf capacitor in the second gain stage and at the same time open the NFB 33k (corrected value I believe with the 8ohm tap being utilized or is it still 22K?). We will have to try it and see if the 3db is a boost worth the efforts! I agree with you all saying too much of a good thing can become a "one trick pony" treble boost that's overly bright.

Thanks and as always any input and suggestions are welcomed (and needed!)

Dale

[Chuck H]

Well the 22k in my last schematic is an arbitrary value. It only serves to keep the feedback loop closed so the switch won't pop. 22k or 33k (or 100k for that matter) shouldn't make any noticeable difference. Your actual NFB resistor in that diagram would be the 2.2k. Notice that when the switch is closed ther 2.2k is the series resistor to the 100r shunt resistor. That would be the NFB loop. So with the 22k (or any other relatively large value) in series (switch open) the NFB divider ratio becomes so large as to virtually moot the NFB.

[eschertron]

Are we discussing the schem in post #8? If so, I'd suggest using 47k (or higher) to significantly damp the NFB. I have a SF champ - stock uses a 47R NFB 'tail' resistor - and added a 25k audio taper pot in the NFB loop. For comparison, 25k/47R is just a little larger than 47k/100R.

There's an audible gain difference in the last fraction of a turn with the 25k pot, I'd suspect that you could hear a difference between 22k + 2.2k and 47k + 2.2k in the feedback loop.

Of course, experimentation is encouraged: we are all offering likely values as starting values. Find the circuit you like best.

[tubedood]

Hi Eschertron yep folks are offering ideas about my adding a bypass capacitor for a treble boost onto the second stage cathode which already happens to have the negative feedback loop on it. This will surely end up a day of clipping various values to get the end result. I'm hoping to have a very stock sound with the "Brilliance" turned off and a nice treble boost (But not overpowering) tone when its switched on.

[tubeswell]

Once you've finished experimenting with a cap in parallel with the NFB resistor, here's some other ideas for frequency shaping the boost of the gain stage where the NFB goes to (these are adaptations from a BFPR, but these will work for a 5F2A, altho' you may want to experiment a bit with the circuit values.)

[tubedood]

Thanks tubeswell that is some great information to work with! I chose 10uf on my 1st stage bypass cap is that still considered fully boosted? (is fully bypassed the same meaning?) I am also curious on the use of the limited boost bypass with that small resistor in series with the capacitor, that is sure to be one of my many days of tinkering when I get to that stage.

Dale