Cathode Follower- Good-Bad-Ugly

[SilverFox]

In the process of building an amp here it was suggested the cathode follower driving the TS in the JTM-45 is a detriment to the tone. How would this be the case? I was under the impression a CF stage is for matching purposes and adds little to nothing to the tone of the preamp?

It was further suggested it could be removed and that would improve the tone. I don't know enough about this to know either way so if someone has some insight I would like to hear it.

Regards,

silverfox

[Malcolm Irving]

The way the cathode follower operates in this type of amp, and its predecessor the Fender Bassman, does produce significant distortion
(possibly not what the original designer intended). It is also used in many modern amps for this distortion characteristic.

Merlin gives a technical description here:

http://www.valvewizard.co.uk/dccf.html

and also in his book (which is highly recommended).

I've not heard of the suggestion to remove it before. It seems to be part of the recognized (and sought after) tone of these amps. Removing it would make a difference to the tone, but 'improvement' or 'detriment' would depend on individual preference.

[pdf64]

The dc coupled cathode follower will affect the tone and may be seen to be part of the amp's character.
But tonal preference is an individual thing and some may prefer the amp without it. The load of the tone stack on the preceding common cathode stage will reduce the gain though, and its higher output impedance will affect the control range.
It shouldn't be difficult if you want to try it both ways.
See http://www.valvewizard.co.uk/dccf.html

[Littlewyan]

The Cathode Follower affects this amp in a very big way as others have said. Only once have I heard someone say to change it to a normal gain stage, but I've never actually seen it done.

[jelle]

Look here:

[SilverFox]

Pondering process....

silverfox.

[roberto]

I think that if someonesays that something is detrimental, he has simply not found yet a way to make it work correctly.

[gingertube]

I have cut and pasted this from a post I did to another forum coz, it sort of relates.
Cheers,
Ian

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 impedance. 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);
http://www.valvewizard.co.uk/dccf.html
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.

[teemuk]

In the process of building an amp here it was suggested the cathode follower driving the TS in the JTM-45 is a detriment to the tone.

IMO, this "detriment to the tone" -concept needs a lot more serious evaluation than what it gets.

Anyway, cathode follower and common cathode circuits inherently react somewhat differently to reactive loads, such as a highly capacitive "tonestack".

Cathode follower's output impedance is lower, which makes it behave more like an ideal voltage source, and it can often also source more current to the load. In practice this mean that the dynamic reactance won't significantly change gain of the stage at various frequencies and complex loads can be handled without weird shifts "in the loadline". Due to high degenerative NFB via cathode transfer characteristics are remarkably linear up to the point of clipping, which will be abrupt.
Cathode follower's voltage gain is also less than unity, a characteristic that plays a significant role in this gain stage's interaction with other surrounding stages.

Common cathode amp is somewhat an opposite: Output impedance is traditionally high so stage gain varies in interaction with dynamic reactance of the load. Similarly, reactive characteristic of the loading will dynamically shift "loadline" of the tube. Harmonic distortion is higher than cathode follower's and with complex loads (e.g. "tonestack") likely also varies much more across the effective bandwidth. Clipping characteristics aren't neccessarily any "softer" or "less asymmetric" than those of cathode follower's, and similarly to cathode follower are also highly circuit architecture/design -dependent so only broad generalisations can be made of these topologies "as is".

But in practical sense one circuit at least seemingly offers more "ideal" (ideal in the sense of linear and "high fidelity") performance than the other.

In practical sense both also clipping distort at least beyound certain points of their "designed" operating areas and in context of signal processing this can be considered a virtue instead of a drawback. Hence may opinion to seriously evaluate what signal degradation means and in what context. Especially in signal processing circuits (e.g. guitar preamps) it is traditional to deliberately design and employ circuits that actually operate in rather "low fidelity" manner.

Both circuits inherently have somewhat different clipping characteristics (further enhanced by additional circuitry (e.g. a direct coupled common cathode amp driving the cathode follower). Much of this has been pointed out already in this thread and provided links.

Different means to different ends; none of the ends being "good" or "bad", or "better" or "worse" until evaluated in a specific context.

Anyway, IMO, it's wortwhile to not forget the bigger picture: If we would just isolate any of these stages "as is", sans pre and post signal processing, sans significant EQ'ing, the tonal outcome probably wouldn't be anything to write home about; they just "distort". There may be fine differences in "tone" and dynamic characteristics of that distortion but a chance is that the rest of the circuit enhances these towards a much more "pleasing" direction. A good recipe is hardly ever dependent on single ingredient.

[pdf64]

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

It seems fine to me, I'm happy to use it if a bit more gain is called for.
Are you sure that there wasn't something else going on?

[cxx]

Hey I like fendery cleans and overdrive, which covers a lot of amps, but I also really like the characteristic tones of early Marshall amps both cleans and overdrive. I can't imagine changing the cf in that amp except as a goof. It wouldn't be the same amp. After all the cranked Marshall is the yardstick tone that all the others are compared with.