Volumen pot position in cathode follower design

[chocopower]

Hello!

I,m gonna mod the Normal channel of my AB763 clon adding a cathode follower tonestack.

My design is like this:


imput---cathode follower---tone stack---volumen---gain stage---gain stage---P.I.


Could this work, or should i move the volumen pot and insert it betwen the cathode follower and the tone stack?


imput---cathode follower---volumen----tone stack----gain stage---gain stage---P.I.

regards from Spain!

[pdf64]

Either configuration will work, but the first will give a more consistant tone control operation across the range of the volume control. The output impedance of a 1 M ohm volume control will vary from 0 - 500k ohm, which will alter the turnover frequencies of any RC filters (ie tone contols) which follow it.
Better for the low output impedance of the cathode follower to feed the tone stack.
You have the potential for a lot of gain in your design (assuming you're using 12AX7 tubes) , you might need to trim it back. Peter.

[chocopower]

Thanks!

I,m testing a mod i saw time ago in Blueguitar.org.
In this mod the 12at7 reverb driver is de-paralelled and the free half is use as gain stage. This mod is explained in 2 channels, with reverb, Fender amps.
The 2 first gain stages are wired as a cathode follower and the 1/2 12at7 push the signal ater the tone stack.

regards from spain!

[mooreamps]

Originally Posted by chocopower

Hello!

I,m gonna mod the Normal channel of my AB763 clon adding a cathode follower tonestack.

My design is like this:


imput---cathode follower---tone stack---volumen---gain stage---gain stage---P.I.


Could this work, or should i move the volumen pot and insert it betwen the cathode follower and the tone stack?


imput---cathode follower---volumen----tone stack----gain stage---gain stage---P.I.

regards from Spain!

I would favor the first configuration as well. Not so much for any changes in the respoces curves, but more importantly the reason one would use a CF to drive a tone stack. A CF is a current amplifier, used to source current to charge the capacitance inside the stack, thus improving the slew rate. If you insert a pot inbetween the CF and the stack, you just nulled out your source current. Does that make sense ???

-g

[Bruce / Mission Amps]

Originally Posted by mooreamps

... used to source current to charge the capacitance inside the stack, thus improving the slew rate. If you insert a pot inbetween the CF and the stack, you just nulled out your source current. Does that make sense ???

-g

I don't know about that. Doesn't that term "CHARGE" imply that the caps (I assume you mean the three tone coupling caps) are "charged" with a DC source? Which of course does not happen.
I think the big difference is that the tone stack's natural total impedance is 25 to 35 times higher then the cathode follower so the stack has nearly no loading effect on the CF.

[mooreamps]

Originally Posted by Bruce / Mission Amps

I don't know about that. Doesn't that term "CHARGE" imply that the caps (I assume you mean the three tone coupling caps) are "charged" with a DC source? .

No Bruce, charged from an AC current source, from a lower source impedance.

-g