Cathode Follower vs non Cathode Follower

[shocki]

I recently built this preamp into a 6L6 power section which does not have a cathode follower.
http://rh-tech.org/public/18-Watt/18..._schematic.gif

This is a similar version with the cathode follower
http://www.unimind.us/images/Trinity..._Schematic.jpg

As far as I understand the follower acts as an impedance buffer so the signal is not loaded down too much in the filter section that comes after. I think for this reason this would also give me more gain?
Is this correct? What would be the sonic difference when using the cathode follower ?

Thanks alot

[Zozobra]

Rather than having more gain, it will attenuate less signal. It will also change the response of the tonestack too.
A DC coupled CF also causes compression which is explained nicely by Merlin:

The Valve Wizard

[jazbo8]

The cathode follower by itself has no gain, you will also note that the part values for the two tonestacks are quite different, and can not be swap'd between the two different configurations. Which one do you prefer?

[shocki]

Which one do you prefer?

Don't know yet. I think I will have to test it

[Gingertube]

Shocki - I posted this to an Aussie Guitar Amp Forum where a similar question regarding teh cathode follower ahead of the tone stack was asked - haven't edited for applicability etc.

COPY OF POST
The zout of the cathode follower is approximately 1/gm in parallel with the cathode load resistor, say about 600 Ohms for a typical 12AX7 triode.
The zout of a typical gain stage (for an anode connected tone stack) with a bypassed cathode resistor is the tube internal rp parallel the load anode load resistor (approx 62K || 100k) say 38K

The popular wisdom around the various forums and reference books is that there is more to it than the drive impedance - to site 2 opposing attitudes....

Kevin O'Connor (London Power Press TUT Series author) prefers anode driven tone stacks, as the B M T tone stack adjustments are made the tone stack's own impedance vs frequency changes. This impedance is effectively in parallel with that 38K and forms part of the stage load impedaance. The change in (load) impedance with frequency actually changes the gain vs frequency of the driving stage, that is, the tone stack is more interactive/reactive because the tone stack settings also change the drive into the tone stack. He is quite "down" on the cathode follower driven stack saying that it is just a waste of a triode stage which could be better employed as another gain stage somewhere else in the design.

Merlin Blencowe (The Valve Wizard) is a fan of the cathode follower driven tone stack as long as that cathode follower is direct coupled from the previous gain stage. He suggests that this is because that cathode follower is running at higher currents and is therefore in the "drawing grid current" operating area. He says that this imparts soft compression on one side of the signal ONLY, thus introducing 2nd harmonic distortion (Asymmetrical distortions introduce even harmonics and symmetrical distortion introduce odd harmonics). His preamp book actually has oscilloscope traces showing this. So his claim is that the cathode follower driving the tone stack actually is important in forming the basic tone/sound of the amplifier, quite aside from its function of providing a low impedance drive to the tone stack.
I just noted he has some of this on his web pages (worth a look);
The Valve Wizard
Aside: Ignore the bit about bootstrapping for more gain, you get a bit more gain but it kills the tone, making it harsh and abrubt in its onset of overdrive (this is typical of many circuits which use feedback, when staturation happens then there is no gain and any and all feedback effects suddenly cease)

If you look at a Marshall 30th Anniversary Lead Channel you can see 3 cascaded gain stages, each consisting of a standard triode gain stage with a direct coupled cathode follower. This is an example of using that cathode follower arrangement for tone generation ONLY.

Of-course the other thing which Merlin says is that the "standard" FVM (Fender Vox Marshall) Tone Stack with the bass middle and treble is pretty awful and there are much better options. We continue to use it from lethargy or laziness.

Looking at 100's of circuit examples we can draw the following conclusion:
Most Clean preamp channels use an anode tone driven stack.
Most Medium to High Gain preamp channels use a cathode follower driven tone stack BUT the reason for this is quite possibly because of the tone contribution of that cathode follower not because of its less interactive drive of the tone stack.

Cheers,
Ian

PS You will also (usually) note a difference in the tone stack circuit values. The anode driven tone stack needs to keep its impedance fairly high so as to not dump excessive amount of gain from the driving stage. The "slope" resistor which feeds the middle and bass controls is therefore nearly always 100K. With cathode follower driven tone stacks we have more flexibility to mess with things and you will see "slope" resistors as low as 33K which gives more control over mids and bass. This is good in higher gain preamps - limiting bass limits "muddiness" and more mid control allows us to get that high gain mid gutted sound (although there are better ways).

[ric]

Practical overview not buried in formulas I don't understand. Thanks Ian.