Cathode Follower

[C Moore]

I am trying to figure out what causes the high voltage on the cathode of a Cat Follow.?
Guess I am talking about your "typical" AX7 in a guitar preamp. No plate load resistor and a high value cat resistor. How does that add up to high Cat Voltage.?
Thank You

[markr14850]

Not trying to be a smart ass, but... It's a certain amount of current across a certain resistance.

[C Moore]

Not trying to be a smart ass, but... It's a certain amount of current across a certain resistance.

I don't know if you are being a smart ass. Maybe I look for complex answers. Is it really a simple E=IxR equation.
If quiescent voltage on the cat of an AX7 was 2 volts across a 1.5k resistor, that same current would cause a nominal voltage of 135 volts across a 100k. But is that all that is happening. I guess I just assumed it was something more....

[diagrammatiks]

in a dc coupled cathode follower the anode is at b+

the grid is at the plate voltage of the previous tube.

the cathode gets what's left and the bias is the difference set with the cathode resistor.

[Firestorm]

It helps (maybe) to remember that what we have been taught to think about electricity is backwards of what actually happens (thanks, Ben Franklin). A positive voltage represents a depletion of electrons relative to some other point which has a comparative surplus. Connect the plate to an electron-depleted B+ and connect the cathode to an electron-rich ground, current will flow in the tube, but the cathode will remain at ground potential as electrons attracted away by the plate are continually replenished. With a resistor in series from the cathode to ground, this replenishing flow of electrons is impeded and the cathode voltage will rise in accordance with Ohm's Law (becoming closer to the plate voltage). The larger the resistor, the closer to the plate the cathode becomes. Disconnect the cathode from anything and it will rise to the plate voltage (both equally electron-depleted).

[tubeswell]

In a triode, there is a voltage potential between the ground return and the HT supply.

If you put a resistor in series with the HT and the plate, you will get a voltage drop across the resistor whereby the voltage at the plate will be lower than the voltage at the HT supply. The larger the resistance value of the resistor, the bigger the voltage drop at the plate will be.

Likewise if you put a resistor in series with the cathode and the ground return, you will get a voltage drop between the cathode and the ground return, and the larger the resistance value of the resistor, the bigger the voltage drop between the cathode and the ground return will be.

Now if you take the resistor between the HT and the plate away and just connect the HT to the plate, then the plate will be at the same voltage potential as the HT supply. But if you leave the resistor in series with the cathode and the ground return, you will still get a voltage drop between the cathode and the ground return. The more current that flows through this resistor the smaller the voltage drop will be, and the less current that flows through this resistor, the greater the voltage drop will be. And this is how we get a signal swing.

[9pins]

the tube is a diode, picture a diode on top of a resistor, thats your follower
with a regular gain stage the resistance makes a voltage drop between the diode and the B+,
the larger the resistance the larger the current alteration and gain.
Put the diode back on the other end ,as a follower you only get the resistance
of the diode and not a lot gain, you get a high voltage.

[greekie]

Hey, leave Ben Franklin alone! He merely supplied us with a simple "rite of passage" that we must understand to do this stuff. Those who don't are better off not working with high voltage

[C Moore]

In a triode, there is a voltage potential between the ground return and the HT supply.

If you put a resistor in series with the HT and the plate, you will get a voltage drop across the resistor whereby the voltage at the plate will be lower than the voltage at the HT supply. The larger the resistance value of the resistor, the bigger the voltage drop at the plate will be.

Likewise if you put a resistor in series with the cathode and the ground return, you will get a voltage drop between the cathode and the ground return, and the larger the resistance value of the resistor, the bigger the voltage drop between the cathode and the ground return will be.

Now if you take the resistor between the HT and the plate away and just connect the HT to the plate, then the plate will be at the same voltage potential as the HT supply. But if you leave the resistor in series with the cathode and the ground return, you will still get a voltage drop between the cathode and the ground return. The more current that flows through this resistor the smaller the voltage drop will be, and the less current that flows through this resistor, the greater the voltage drop will be. And this is how we get a signal swing.

10-4
That explains it clearly at my level.
Thank You All