What is the function of "Cathode Follower"?

[psychepool]

I want to know the exact function of cathode follower.
It dosen't amplify, only lower the impedence. It's only things that I know about it

But many guitar amp circuit has no cathode follower and has no problem.
So then, why they waste a half of tube with add a non-amplified stage?

The reason that I wonder about it is I agonize circuits for make a diy amp.

I like the clean channel of Orange Rockerverb very much, so I considered to make a portable one-channeled clean amp.
By the way, when I look at the circuit, I found the Rockerverb clean channel is very similar with Vox Top-boost circuit with no cathode follower.

The schematics are like this.

Rockerverb :
http://www.orangefieldguide.com/OFG_SCH ... sheet1.jpg

AC30 Top Boost :
http://www.blueguitar.org/new/schem/vox/ac30_pre.gif

They are almost same circuit except the cathode follower.

What is the benefit with add a cathode follower on Rockerverb clean channel?

I don't want to waste a tube if there's no particular benefit.
But if it has attractive benefit, It will be better to make Vox.
(I also likes the clean/chrunch of Vox. I likes the clear and straight tone character of it.)

I want to know about the role of cathode follower in detail.
In particular, I hope that this explanation based on these circuits.

Thanks.

[martin manning]

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

[Theashe]

I'd like to add to Martin's post that that cathode followers can be AC coupled or DC coupled to the previous gain stage. AC coupled cathode followers (hopefully) serve as a "transparent" impedance buffer in the circuit - that is, they have a high input impedance and a low output impedance due to their 100% negative feedback (which is the same reason their gain is slightly less than 1).

Using either small signal models or load lines on tube transfer curves for a tube arranged as a gain stage, the gain depends on the load resistance. This is true for transistors, too. As the load resistance decreases, so does the gain. So if you connect a circuit with a low input impedance to a gain stage, you are placing a small resistance in parallel with its plate load - which reduces its gain. Since a cathode follower has such a high input impedance, it has very little effect on a gain stage, while its low output impedance means it can drive loads of low impedance.

A DC coupled cathode follower, on the other hand, creates a distortion of its own by compressing the positive peaks of a preceding gain stage. The article Martin linked to elaborates on this far better than I could. Basically, the AC30 isn't wasting a tube by using a DC coupled cathode follower. It acts both as a source of distortion and as an impedance buffer that protects the gain in the gain stage from the frequency-dependent load of the tonestack following it.

[tubeswell]

Have you ever heard a fender tweed bassman or a marshall jtm45 withe the volume above about 7 (out of 10?). they get a really nice growl. that's the DC coupled cathode follower working.

[psychepool]

Wow...very elaborate description.

To be frank, I can't speak english well, but even if it seems difficult that is written in my native language.
Because I'm not major in electronics.

The impressive content is the cathode follower that use in guitar amp is DC-coupled cathode follower.
And that is not only serve as impedence buffer, but also it creates the distortion with compress the peak.

And by the compression, the distortion sound is more smooth and growl, with no fizzy sound.

It is very difficult article that I can't understand perfectly, but it is enough to choose the circuit that I wonder about.
For more understanding, I will read the article repeatedly.


Thank you very much everyone!

[JMFahey]

And what would

my native language

be?

[gingertube]

The Merlin pages (see Martins link) say it all.

Just a caution: the bootstrapping scheme shown on the bottom of the linked page works but has a very nasty and aggressive overdrive. Merlin actually says this in his 2nd Edition of his tube preamp book and says that he abandoned it in at least one preamp design he tried.
Cheers,
Ian

[pdf64]

Yes, the bootstrapped DCCF overdrive doesn't seem as nice as the regular version, but if there's not a master volume after it, it may not be a tone sucker due to subsequent stages entering overdrive first.
Similar to the horrid distortion caused by the V3 reverb driver in a typical 2 channel BF Fender.