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Old 5th March 2011, 01:49 PM   #11
DF96 is offline DF96  England
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Those embarrassed by the idea of having hidden feedback in their amplifiers could always look at it another way:
- an ideal triode is a linear voltage amplifier with a non-linear output impedance (so for best results have a very high load impedance)
- an ideal pentode is a non-linear transconductance amplifier with a very high output impedance (so for best results either use very small signals, or arrange in push-pull with external NFB to reduce the output impedance)
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Old 5th March 2011, 04:44 PM   #12
jcx is offline jcx  United States
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Quote:
Originally Posted by ruffrecords View Post
I remember this being debated ad nausem on rec.audio.tubes some years ago.

I suggested the following experiment. Wire up a pentode as a triode and measure its stage gain and distortion harmonics at a given frequency and output level.

Next wire up the same pentode as a pentode with NFB so that it has the same stage gain. Again measure the distortion harmonics at the same frequency and output level.

If a triode truly is a pentode with NFB then the results should be at least very similar i.e. the relative levels of the harmonics should be very similar.

In practice they are not.

Cheers

Ian
I earlier mentioned that grid-cathode, grid-plate voltages influence the current through similar nonlinear functions, Langmuir-Childs law - the internal feedback isn’t linear, can't be externally reproduced by a linear feedback


A perhaps amusing to modern eyes example of “category error”:

1522
In an effort to reduce the continuing fear surrounding the black magic of guns and black powder, a Bavarian necromancer states that rifles are more accurate than smoothbores because the spinning bullet doesn't allow a demon to gain purchase upon it.

1547
The Archbishop of Mainz has two members of a shooting club compete at 200 paces, one using lead balls and the other balls of silver blessed by the Church and etched with cross. The silver balls all miss the target while the lead balls scored hits on 19 out of 20 shots. This disproves the theory put forth by the Bavarian of 1522 and causes the Church to declare that round [sic, actually a "logical" test of rifled/spinning] bullets can indeed be controlled by demons.
Taconic Valley Rod and Gun Club ~ History

better:
http://www.trivia-library.com/b/mili...-the-rifle.htm

Last edited by jcx; 5th March 2011 at 05:08 PM.
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Old 5th March 2011, 05:12 PM   #13
Yvesm is offline Yvesm  France
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Well, well . . . So, just insure nobody attempted to bless your zero feed back amplifier and be happy !
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Old 5th March 2011, 05:21 PM   #14
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Quote:
Originally Posted by jcx View Post
I earlier mentioned that grid-cathode, grid-plate voltages influence the current through similar nonlinear functions, Langmuir-Childs law - the internal feedback isnít linear, can't be externally reproduced by a linear feedback
Surely you jest?

Cheers

Ian
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Old 5th March 2011, 05:56 PM   #15
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If the triode plate and grid both had exactly the same 3/2 power gm rules, then the results would be the same as linear feedback (for CCS loading, for real resistor loading the variable plate Rp has big effects on the gain), just an isomorphic mapping to 3/2 power space internally (like looking at a linear device thru curved mirrors). But the g1 suffers from grid wire proximity effects (island effect) to the cathode, which the plate does not. This makes g1 have closer to a square law power at low current, but not exact, and closer to 3/2 power law at high current. Causing the Mu to vary some with current level if you use an adjustable CCS load.

Keeping to low gm grid 1 tubes will get the g1 closer to 3/2 power law, matching the plate better, and so the Mu is more constant (assuming the load Z is well above the Rp).

Pentodes suffer the same grid 1 proximity effects as triodes. Grid 2 is usually far enough away to not. So triode wiring a pentode should give comparable results to a triode, except pentodes usually are designed with higher g1 gm, so somewhat less constant Mu than early triodes typically.

There are some triodes that are designed specifically to not have constant Mu, like 12aT7 or 6LQ8, ..... These typically have a round cathode, with the grid circularly crimped around the cathode, then a flat plate. These are designed for mixer use with high 2nd harmonic content. Or for P-P operation where the 2nd harmonic cancels and 3rd harmonic comes out low due to the near linear ramp of gm versus current, summing to nearly a constant gm. (these give lower 3rd harmonic than constant Mu tubes in P-P class A)

Then there is Schade like feedback for a pentode which mixes g1 proximity effect forward conductance with linear resistive feedback. The curves look similar to triode curves, but the Mu varies (increases) with current due to the mismatch of power laws. This should give something like 12AT7 curves (extra 2nd harmonic) for loaded power stages, but if the stage gain is high enough (low loading or CCS loading, driver stage duty for instance) can look like a very constant Mu device.

If the Schaded pentode is used in P-P class A, then the extra 2nd harmonic will cancel, and it will have lower than usual 3rd harmonic. Not a bad choice. It gives extra 2nd harmonic for SE use, and lower 3rd harmonic for P-P class A use. (but not for class AB for the 3rd H reduction)
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Old 5th March 2011, 06:19 PM   #16
DF96 is offline DF96  England
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A good summary!
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Old 5th March 2011, 06:39 PM   #17
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Then there's the perfect triode, an old low gm pentode wired in triode mode:

Could this be a perfectly linear triode?

That 6SN7 looks like it gets beat by any old pentode.

If you want to see great triodes, they are high current rated (big flat cathode) sweep tubes wired in triode mode.
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Last edited by smoking-amp; 5th March 2011 at 06:42 PM.
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Old 5th March 2011, 06:47 PM   #18
DF96 is offline DF96  England
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Yes, you basically want a valve with cathode, grid and anode (or g2) all the same shape (e.g. cylindrical or flat), and lowish mu so that g1 is not too near the cathode. This is why some old triodes are good: low mu, all elements cylindrical. Its nothing to do with their size (as some people think) but everything to do with their self-similar geometry.
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Old 5th March 2011, 07:06 PM   #19
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Agree. The reason I think the big flat cathode comes out favorably is that more of the structure is coplanar (cathode, grid, plate or g2) with less end effect from the cathode edge and grid supports. And some tubes just have more careful design to get all the geometry effects co-tracking (D3a). I doubt the sweep tubes were intentionally designed to be linear, I think it's just that the physics for getting high cathode current inadvertantly favors linearity.

That crazy 1E7G breaks some of the rules though. Filamentary cathodes must act like planar sources somehow I guess. It does have planar grids and plate geometry otherwise.
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Old 5th March 2011, 07:57 PM   #20
DF96 is offline DF96  England
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Just as a set of grid wires form an approximation to an equipotential plane, I guess a set of filament wires can do the same (although spacing will be much bigger). Maybe the space charge around each filament strand effectively increases its radius so the gaps become smaller.

Some valves don't do what we expect. The 6J6 is basically two planar triodes back to back, so it ought to be linear but it isn't.
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