Cathode follower driver stage

[Joel]

Originally posted by fdegrove
Joel,do you understand how a coupling cap works?

Do you? (see below)

It blocks off DC voltage,NOT current nor AC voltage.
If it would nothing would work,would it??

Frank, I'm no engineer, but I'm sure dc cannot flow through a capacitor. Were you really saying that???

[fdegrove]

Hi,

I know,I know I can be a major PITA,but:

Generally the higher the mu, the higher the capacitance, for a given size of valve.

Not necessarily:

What the problem is is that this high mu amplifies it's input capacitance therefore limiting and rolling off the high frequency range.

Ciao,

[fdegrove]

Joel,

A capacitor will block DC voltage and DC current.
It will pass AC voltage and AC current.

Better this way?

Ciao,

[dhaen]

That's why I used the word "generally".
It allows for exceptions.................

Grenache-Syrah,

[Joel]

Yes, much better.

The fact that it blocks DC is exactly why I brought the point up to begin with! How can the CF "deliver" current if there is a cap in the way? I use the term in quotes because this is how it's often described in voodoo audio literature.

[fdegrove]

Joel,

If it passes the AC signal voltage,why should it then be unable to pass AC current?

What they mean by it's capability to deliver current is that when high current is called for the CF will be able to put it out even when the impedance gets on the low side.

This is due to it's low output impedance as opposed to say a run off the mill anode follower.

Of course you will choose a CF with low mu and low Rp (natural state of affairs) and the more this tube can dissipate the more it will be able to supply this current.

It is not that complicated really.

Ciao,

[Joel]

Thanks frank.
I wasn't arguing the point, I just wanted an explanation.

[fdegrove]

Joel,

As I said,no problem better to ask then to make mistakes like this one:

(other than the mu -1 gain)

Actually this is NOT correct.
I had it correctly in my mind but somehow managed to miswrite it
completely.

It should say: unity gain -1.
This means that this stage will have some insertion loss.

Sorry,my mistake,

[dhaen]

Of course you will choose a CF with low mu and low Rp (natural state of affairs)

I don't think you will notice the difference. If you're driving a low mu output valve, you'll probably want lots of peak-to-peak voltage, so the Va (Vp) and Ia (Ip) will be important.
Power pentodes are good for this.
None of the triode vs pentode arguments apply to cathode followers...

[fdegrove]

Hi,

I don't think you will notice the difference. If you're driving a low mu output valve, you'll probably want lots of peak-to-peak voltage, so the Va (Vp) and Ia (Ip) will be important.

True.
The lower the output impedance the easier it gets when the though gets going...

Power pentodes are good for this.

I never tried it,but penthodes are high Ri compared to triodes and are noisier as well.
Usually I go for White cathode followers using something like a 12BH7A.
I never had a situation where they couldn't handle it.
A penthode CF may be useful for driving a 845 for instance.
(admit it John,that was what you had in mind.)

Cheers,