Cathode follower driver stage

[Joel]

I've noticed a few designs lately, including a 1949 article on a "Peerless" amp, using cathode followers coupled to the grids of the output triodes. A more recent example was the "2001 2A3" amp project on the angela instruments site.
My question is: what are the benefits? and are they worth the additional tube & signal path?

Also, isn't any benefit in the JE Labs version offset by the fact that it's RC coupled to the ouput grid?

[dhaen]

Joel,

In my experience you can get alot more undistorted power, as there is a clean slide from class A1 to A2. Even though a valve doesn't say it'll do A2, it can still dissapate a little grid power.

Not sure about the micro-details though

Cheers,

[Joel]

Thanks Dhaen,

Yes, I thought that might be one of the justifications for it. BUT...
a) Is drawing grid current even a good idea with tubes not specifically designed for it??? ie. My fragile old globe 71A's don't look like they have too hefty of a grid.

b) In a design where the CF is rc-coupled to the output grid, how does the follower supply current through the coupling capacitor? Doesn't that block the dc current in the plate-cathode circuit?

[dhaen]

Hi Joel,

a) Of course you sould be careful, but what can happen? For any grid deformation to take place, the grid must get very hot. Calculate how much grid power is being produced, it'll be milliwatts for music. You'll probably have to set up a test circuit to establish the dynamic resistance.

b) As for as AC coupling; of course it doesn't work for A2, as the bias point moves through rectification. Even though some fairly respected circuit designers suggest it slides a little into A2, I believe it's rubbish.
There can other advantages though. If the valve being driven has high input capacitance, or to avoid parasitic oscillations.

Cheers,

[Joel]

Originally posted by dhaen
There can other advantages though. If the valve being driven has high input capacitance, or to avoid parasitic oscillations.

Thanks for the input again Dhaen!
That's really what I was looking for, as both of those conditions describe the situation in the PSE thread - paralleled tubes have more of a tendency toward parasitics, and DHT's have a high input capacitance too, if I'm not mistaken.

Joel

[dhaen]

and DHT's have a high input capacitance too, if I'm not mistaken.

I don't know if they have any higher input capacitance than IHT's.
Generally the higher the mu, the higher the capacitance, for a given size of valve.

Anyway, what's a down to earth DIY'er like you want with DHT's?
Are you looking for some micro-details?

Cheers,

[fdegrove]

Hi,

Just can't leave you two alone,can I?

a) Is drawing grid current even a good idea with tubes not specifically designed for it???

What this has to do with cathode followers is beyond me.
Let me guess:you saw a circuit where the CF was driving an output tube that was biased positive,hence drawing grid current?

Why would one use cathode followers that have no gain,even detract from the overall gain:

First of all not all CF topologies are the same but generally these are used to:

Linearize the signal from the gainstage preceding them.
(they rely on 100% feedback)

Provide low output Z to drive the next stage effectively without loss.(other than the mu -1 gain)

They can swing a very respectable amount of voltage and provide healthy current.

Drive a bunch of paralelled tubes in an output stage with very wide bandwidth to combat the combined Miller effect of the tubes.

Effectively buffer the previous stage from the rest of the world.

There must be a few other applications but I presume this will keep you all busy for a while.

b) In a design where the CF is rc-coupled to the output grid, how does the follower supply current through the coupling capacitor? Doesn't that block the dc current in the plate-cathode circuit?

Joel,do you understand how a coupling cap works?

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

Also you can still direct couple even when the next stage is drawing grid current,it just becomes that much harder to drive.

Ciao,

[Joel]

Originally posted by dhaen
I don't know if they have any higher input capacitance than IHT's. Generally the higher the mu, the higher the capacitance, for a given size of valve.

Guess I could just look it up - but I was feeling lazy.

Anyway, what's a down to earth DIY'er like you want with DHT's?
Are you looking for some micro-details?

Haha. It's a fair question - since I'm so anti-voodoo.
Well, I guess I just thought it'd be fun to build a circuit or two with some really old tubes. They were all outside my budget except used 71a's. End result is in the "my current 71a amp" post. And luckily it sounds good too.

I think the real motivation was just to make a "DHT SET" amp, that everyone's been raving about, for a quarter of the money they spent.

[dhaen]

Frank,

I believe Joel's question was rhetorical.....

[fdegrove]

Hello,

My question is: what are the benefits? and are they worth the additional tube & signal path?

For a rhetorical question it did get quite a few answers.

Ciao, (-5 minutes)