Cathode follower question

[aro]

I understand what the cathode follower does, but I don't really understand how it does it.

In a regular gain stage with plate output, the plate gets all that voltage, which attracts the electrons from the cathode (with the grid acting as a controller).

How does this look like in a cathode follower? The plate still gets the voltage, but how come the electrons are attracted to the cathode (and go out through the cathode) ? Has the cathode a higher voltage with respect to the plate? If yes, how does that happen? Thanks.

[PRR]

> how come the electrons are attracted to the cathode (and go out through the cathode)

No.

[Willabe]

Hi aro,

Yes, the electrons are attracted to the plate because of the high  +voltage, from the cathode, which is pulling them (electrons) up from ground through the cathode. So since the current is flowing through the tube, up from ground,through the cathode, past the control grid, to the plate, we have a choice as to where to "tap" off our signal. We can tap it off the plate --or-- off the cathode, ---or--- sometimes both (plate being hi impedance and cathode being low impedance), take a look at a Fender PR or 5E3, look at the PI. The plate tap is feeding one OP tube and the cathode tap is feeding the other OP tube. Push/Pull OP stage, plate feeds "push" side , cathode feeds "pull" side.

In the CF, signal is being tapped off the cathode, which being a low impedance source, makes it easer to drive the tone stack and/or next tube stage. Theres more to it, but like anything else (seems like especialy with tubes) theres always more.  

Try  Aikins amps.com (?) and Merlins site, sorry I dont know his site address, but he post's here. Do a search for past post's from him, IIRC his site address is always on the bottom of his post's. They both have a ton of very good info, imho.          

I hope that helps you, otherwise there are many people here who can and I'm sure will give you a much better explanation.

   Brad      

[aro]

So since the current is flowing through the tube, up from ground,through the cathode, past the control grid, to the plate, we have a choice as to where to "tap" off our signal.

:icon_thumright: Wow, that was simple... Thanks. I know there's more to it, but at least I get the idea and I think I made it more complicated than it was.

I have Merlin's book on preamps, and it's absolutely great. I started at understanding about 10% from what I read, and I kept reading everything I could find. Now I understand about 90% of it. It's a fun learning process.

[HotBluePlates]

I know there's more to it, but at least I get the idea and I think I made it more complicated than it was.

The action is simple. Understanding could be complicated.

The key is learning that voltage is the force that makes electricity move, and that current is the actual moving electricity. Like the difference between water pressure (amount of push/pull, i.e. voltage), and the rate of flow of water (gallons per minute, i.e. current).

Current is the rate that a quantity of charge moves past a point in 1 second. Since electrons have a discrete amount of charge per electrons, you could look at it as how many electrons move past a point in 1 second, but the number will be huge. So an ampere is 1 coulomb per second.

Anyway, you next need to suss out ohm's law and understand voltage drops.

For most typical guitar situations, a voltage is applied to an input, and that changes the current flowing through the tube. The changing current may make itself known by creating voltage drops in external circuit components, which is then a changing voltage that we could use as an output.

The tube doesn't care much that you take a useful output from the plate or cathode. It doesn't care if your "input" is on the grid, cathode or plate. But the "useful arrangements" that have been discovered take advantage of features in some circuits, and ignore the circuits that don't appear that provide any useful advantage.

[aro]

The action is simple. Understanding could be complicated.

What opened the door for me was "Inside the Vacuum Tube" by John F, Rider, which I found here http://www.tubebooks.org/technical_books_online.htm. That book, very much kind of a 'Electrons movement for very Dummies' is what did it for me. I was becoming very discouraged, not being able to understand basic stuff, but after reading it, Merlin's book is now so much more accessible to me.

[HotBluePlates]

Well, there's a lot that I now understand about tube electronics. But nobody is born knowing this stuff.

I got a reprint of the RCA tube manual of 1959 a long time ago when I first started out, and also read parts of the Radiotron Designer's Handbook. However, I did not know enough at the time to see that what was written was the plain facts, and that it "should have" been very simple for me to see and understand what was going on.

What it really takes is someone or some book to finally explain things in a way that turns the lightbulb on for you, and then you start to see how simple and straightforward it all really is. The hard part is that "simple" seems anything but simple until you already understand what's going on.

I just wish I had access to stuff like the Rider book when I was first starting out. It might have saved a lot of years of fumbling around half-knowing stuff.  

[PRR]

> RCA tube manual of 1959 ..and... Radiotron Designer's Handbook. However, I did not know enough at the time to see that what was written was the plain facts, and that it "should have" been very simple

A lot of people try to understand electronics without understanding ELECTRICITY.

Ya gotta know batteries and lamps and electron-pipes FIRST. Resistors, all that boring stuff.