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Newbie question: What characteristics are best for a cathode follower?

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Windcrest77

Windcrest77

Member
Joined 2019
#1
I need to understand what dual triode tube is best to use as a cathode follower. So if I had 5 tubes to pick from, which is best, a tiebreaker.

What characteristics of a tube might make tube "A" a better cathode follower than tube "B" or tube "C", etc.? In the context of a CF stage that follows a grounded cathode stage that then either drives a power tube stage or a line out.
 
#3
Also determine which is needed, a low source impedance or a low load impedance capability.
Even if the CF has a low source impedance, it may not be able to drive a low load impedance
with low distortion.
 
#6
PRR said:
Why do you think you need a cathode follower?

That may hint what parameter you think is important.
Click to expand...

I want to do this experiment on the breadboard to see what happens by modifying the Broski EL84 schematic below, I have this running as is on the bench but want to try things out for fun...

Replace the simple cathodyne Splitter with a high quality Jensen input Transformer to get the split phase. Then follow up each side of the phase with two CF's. The Cathode followers will then drive the output tubes.

So basically use CF's to buffer between a transformer Phase Splitter and the output tubes 6BQ5's. Imagine circuit below with a transformer Splitter instead of a tube and feeding two CF's into each side. The 470k divider would become the secondary load of the transformer. The transformer is 1 : 1.414. I tested this transformer into a virtual ground voltage divider load on the scope and it works nice, so I want to see if it can perform as a splitter basically. No NFB is planned. The 470k will be reduced greatly following nearer the test Circuits loading on the PDF last page.

https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-11p4-11.pdf


Capture.PNG
 
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#9
Eli Duttman said:
Transconductance (gm) is (IMO) the most important parameter. High gm equates to a low O/P impedance.
Click to expand...


If both triodes have a gm of 10000 µmhos (10mA/V) but one has a mu of 60, the other 20, which one would you choose? I'm assuming the lower the mu and higher the gm, the better the performance?

How about a CF using a pentode? I assume there's reason I've not seen it in any popular designs.
 
#10
The first tube in that circuit has a u of 33. Even if it only has half that much gain, you'll need a transformer with a pretty steep ratio (per side) to replace the tube stage. I don't see how you're going to replace the gain with a 1:1.4 transformer. Are you talking about keeping the first stage and replacing only the splitter half of the tube with a single secondary transformer? That seems like adding one tube and one transformer to accomplish the same thing the circuit already accomplishes. What's the point of that?
 
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#11
dgta said:
The first tube in that circuit has a u of 33. Even if it only has half that much gain, you'll need a transformer with a pretty steep ratio (per side) to replace the tube stage. I don't see how you're going to replace the gain with a 1:1.4 transformer. Are you talking about keeping the first stage and replacing only the splitter half of the tube with a single secondary transformer? That seems like adding one tube and one transformer to accomplish the same thing the circuit already accomplishes. What's the point of that?
Click to expand...

My purpose here is to experiment and learn about using a transformer as a Splitter, loading it, maybe using a shunt volume control across the secondary, a no capacitor Amp, etc. Yes and to learn how much gain I really need for my DAC, and if I need to keep the first stage too, etc. I'm not necessarily heading towards an end like an experienced designer would just having fun learning without a simulator. I could just put what I have on the breadboard in a chassis now, but I want to start making my own mistakes!
 
#13
piano3:
Think of it this way: what's the gain of a CF? How much feedback is there? Can you apply more feedback than there is already? What's the distortion? Can you theoretically get less distortion?

Koda:
For a given gm, higher mu means higher rp. Is that beneficial to your goals?
 
#15
Generalizations are just that.

Pentode or Beam Power/Beam Tetrode tubes . . .
They have relatively High rp
If it also has High Transconductance, then . . .
If it is used as a Cathode follower, it will have very low output impedance.
 
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#16
dgta: The fact that the feedback fraction is unity is only part of the distortion reduction due to the feedback factor which includes mu. It is mu which you are trading off for the benefits of distortion and output resistance.
 
#17
A cathode follower performance can be improved by using a high transconductance tube, whether it is a triode, pentode, beam power, or beam tetrode (beam power) tube.
Transconductance is paramount.

For a common cathode stage (the output is from the plate) generally can benefit from a low rp.
low rp is paramount.
 
#19
I would assume if it's a triode and it's good as a CF, it'll be good as a cathodyne.

Personally, I've used 6SN7 and the triode half of 6F12P and I prefer the latter. I wouldn't use a 12AX7 for either position, but at least as a splitter you can use a driver after it. There's no point in driving a CF with a CF is there?
 
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