How do I design a pentode constant current source plate load? - Page 2 - diyAudio
 How do I design a pentode constant current source plate load?
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diyAudio Moderator Emeritus

Join Date: Jan 2004
Location: Jakarta
Quote:
 the cathode of the pentode would have a lower output impedance, but it's sitting at such a high voltage that it would prevent direct coupling of the two stages.
Why would the voltage at the pentode cathode be significantly higher than at the triode plate?

Quote:
 Take the mu-follower output at the cathode! use a bigger coupling cap.
I'd agree that it's better to take the output from the pentode cathode (otherwise it's a waste of a perfectlty good cf), but what does that have to do with the size of the coupling cap? That should be determined with reference to the IP impedance of the following stage, surely?

diyAudio Member

Join Date: Jan 2009
Quote:
 Originally posted by ray_moth I'd agree that it's better to take the output from the pentode cathode (otherwise it's a waste of a perfectlty good cf), but what does that have to do with the size of the coupling cap? That should be determined with reference to the IP impedance of the following stage, surely?
I think I remember reading that there are really two sections that affect interstage low-frequency response; that there's a high pass filter created by the output impedance and the coupling capacitor in a voltage divider with the grid resistor, and then there's the grid resistor and coupling capacitor acting alone, and the cutoff goes to -12db/octave when the two response curves intersect. Maybe that has something to do with increasing the size of the coupling capacitor on the pentode cathode output? I don't know, I'd better stop now as I've begun confusing myself.

diyAudio Member

Join Date: Nov 2007
Location: Dallas
Quote:
 Originally posted by Bandersnatch Take the mu-follower output at the cathode! use a bigger coupling cap.
In all honesty, if you like the Triode driving Triode sound, I'd tap
the middle point of the current set resistor. This configuration is
SEPP, Triode / Anti-Triode.

It will seem like the lower triode has a virtual clone working in
parallel. Though it is actually working in Anti-Pullup. And the
drive capability seen here is exactly double what you'd have
if you tap the plate of the unassisted Triode by itself.

Or the same drive seen at the plate, but now the driven load
appears to have twice the input impedance. Same thing, only
different... You can look at this topology either way.

---------------------------------------------------

If you go all the way to the top of the current set resistor, to the
MuFollower point, low Triode's plate doesn't see the load at all.
All it sees is a constant current from the sink above. This gives
good pull up current from the top cathode, but weak constant
current pull down through the current set resistor at this tapping.

But be aware of circuits with unequal push and pull capabilities.
They often can not slew as difficult load as "output impedance"
might suggest. Or maybe they can ,but not at all frequencies of
interest.

But triode grids in A2 require higher pull up slew anyway, so if
thats the load you are driving, this imbalance in favor of pull up
might be beneficial.

diyAudio Member

Join Date: Mar 2008
Quote:
 Originally posted by kenpeter [B] ... If you go all the way to the top of the current set resistor, to the MuFollower point, low Triode's plate doesn't see the load at all. ...
This is not true because this stage isn't a buffer. E.g. See #3 the cathode load is in parallel to Rg2 in this calculation. This reduces r.

 21st January 2009, 07:34 AM #15 diyAudio Member   Join Date: Dec 2000 Location: Germany http://www7.taosnet.com/f10/mustage.html Think it is better to look here.
diyAudio Member

Join Date: Nov 2007
Location: Dallas
Quote:
 Originally posted by oldeurope This is not true because this stage isn't a buffer. E.g. See #3 the cathode load is in parallel to Rg2 in this calculation. This reduces r.
Im not sure we are talking about the same circuit.
Rg2 in Post#3 appears to be a screen resistor???
I think I may still get what you are driving at.

I concede that my statement was an exaggeration.
Lower plate of any Mu follower stack must see some
interaction with the load. Because the current sinking
cathode above is always imperfect (less than infinite
Gm) and therefore interacts with the load.

I merely intended to emphasise that Mu-Follower is
the least interactive tapping of the three choices
(SRPP, Anti-Triode SEPP, MuFollower) as seen from
the lower triode's plate. Which is good if you want to
track the linearity of Mu. But the most asymmetrical
in slew capability. Which could work for or against
you, depending what you are driving next.

diyAudio Member

Join Date: Mar 2008
Quote:
 Originally posted by kenpeter Im not sure we are talking about the same circuit. Rg2 in Post#3 appears to be a screen resistor??? ...
Yes. #3

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