What is a cathode follower?

[bluefoxicy]

I am having trouble trying to figure out just what in the fuck a cathode follower is. I found this information:

The cathode follower is typically used where a low impedance output is desired, since the output impedance of most valve circuits is rather high (equal to the value of the plate load resistor). Simply attaching a low impedance load to a voltage amplifier stage will cause the output level to be dramatically reduced, so the current amplifier (cathode follower) is a useful stage. The output impedance of the circuit of Figure 1.3 can be expected to be about 1/10th the value of the cathode resistance Rk2 - but this is highly dependent on the valve itself, and its operating current. The available current is very low, so the circuit will not be able to drive a load much less than Rk2, or 47k. Remember that output impedance and drive capability are not related.

And this

since—as compared to a grounded-cathode amplifier—the cathode follower offers a much lower output impedance (Zo), the output transformer’s winding ratio can be equally reduced, allowing the use of 500-ohm primary instead of the 5,000-ohm primary needed for a grounded-cathode amplifier. Right? Absolutely dead wrong! It is as wrong as believing that holding a magnifying glass over a gold coin will not only make the coin appear larger, but weigh more as well. Just as the coin’s actual size and weight remain unaltered, the triode’s transconductance (gm), amplification factor (mu), and plate resistance (rp) remain unaltered, as nothing inside the tube’s envelope has changed.

The topology changes, not the tube’s internal functioning. The tube has no idea what circuit topology it finds itself in; it only knows plate, grid, and cathode voltages and currents. If we want to change the tube’s rp or gm, we have to break the glass envelope’s seal and alter the tube’s internal structure.

So what value primary impedance is best for a push-pull or single-ended cathode-follower-based power amplifier? The answer: use the same (I repeat, the exact same) impedance that you would use had the transformer connected to the plates in a grounded-cathode amplifier. Cathode-follower operation does not change the IV (the current/voltage) dynamics that the tube must undergo nor does it artificially lower the tube’s intrinsic rp.

... wtf? The "load" is the OT.

[jrfrond]

Very simply put, a cathode follower is a tube triode stage whose input is the grid, and the output, rather than being taken off the plate, is taken off the cathode. The plate is generally connected directly to the power supply, and the cathode has a resistor going to ground as the tube's load. The output is directly out of the cathode at the top of the load resistor.

A cathode follower never has a voltage gain of more than 1, so generally Vin = Vout. What it does offer is CURRENT gain, and is useful for driving current-hogging circuits like tone stacks. It is also used for impedance matching. The cathode follower has a high input impedance and a low output impedance. The output is also in-phase with the input.

Here is an article I wrote for the 300Guitars website concerning Phase Inverters. Cathode Followers are explained in a bit more detail here:

http://300guitars.com/index.php/arti...hase-inverter/

[Enzo]

Blue, I don't know, but I think the second quote box in your post was someone discussing using a cathode follower as a power amp. You won't see that in a guitar amp, so it would have been a hypothetical discussion. We use cathode followers in the preamp though.

And I would also point out that the transformer is not the load. The speaker is the load. The impedance of the speaker is reflected back to the tubes by the transformer. The impedance is "transformed." If the transformer were the load, then we would not have to worry about running the amps without speakers.

[bluefoxicy]

A cathode follower never has a voltage gain of more than 1, so generally Vin = Vout. What it doe offer is CURRENT gain, and is useful for driving current-hogging circuits like tone stacks.

Hmm. I need to research this more and figure out how to do a 12AX7 or 12AU7 circuit to drive a tone stack leading into a 12AX7 preamp....

[tubeswell]

Hmm. I need to research this more and figure out how to do a 12AX7 or 12AU7 circuit to drive a tone stack leading into a 12AX7 preamp....

http://www.ampwares.com/ffg/schem/ba...f6-a_schem.gif

[Enzo]

COmpare an old Fender design with an old Marshall design. There is nothing particularly high tech about cathode followers.

[mooreamps]

A cathode follower is indeed a current amplifier gain stage.

-g

[bluefoxicy]

ok I really don't get it from that diagram, I'll have to look around more and try to figure it out. Thanks though guys!

[Paul P]

Imagine a really long garden hose hooked up to your house's water system.
If you don't open your spray gun the pressure at the gun will reach the same
pressure as your household water supply. If you turn your spray gun on the
pressure will start out high but immediately drop to a much lower pressure.
Even with a lot of pressure at the house end of the hose (high B+ voltage)
you won't get much water (current) out of the hose (plate resistor). This is
how a voltage amplifying preamp tube works with the output being the variation
in voltage (pressure). In this hose analogy the tube's grid is the spray gun
trigger which requires little power to control a big variation in pressure at the
output of the hose.

With a cathode follower you don't have a plate resistor. It's like your
hose is huge. Now if you turn the spray gun on you can get as much water
(current) as you want out the end. Unless your water supply is huge
the pressure won't be as high though.

The usefulness of a cathode follower is to supply current to a lot of
circuits which would cause too much of a voltage drop if you were to
use the tube as a voltage amplifier. Just like you couldn't supply three
houses with your garden hose because they wouldn't have any pressure,
but you could supply them with a 6" pipe.

Paul P

[jrfrond]

Good analogy Paul.

[tubeswell]

ok I really don't get it from that diagram, I'll have to look around more and try to figure it out. Thanks though guys!

In the 5F6-A circuit, the CF is the gain stage in front of the tone stack. The previous gain stage plate goes straight to the CF stage grid with no attentuation in between in this case (i.e. what's called a 'direct coupled pair'). The CF stage plate voltage and grid voltage are quite high. The signal is picked up off the cathode of the CF stage (where the AC swing is in phase with the grid - like the cathode part of a cathodyne phase inverter) and fed straight into the tone stack.