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Local feedback between grid-cathode
Local feedback between grid-cathode
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Old 20th August 2019, 08:24 PM   #91
SpreadSpectrum is offline SpreadSpectrum  United States
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Well, the tetrode cathode follower, Unity-Coupled, and Circlotron are all pentodes with voltage feedback to the grid. They perform much better than UL but it really isn't a fair comparison to UL since there is so much feedback (50% - 100%). A fair comparison would be to construct a circuit with just enough plate-grid feedback to give the same amount of gain as the UL circuit.

I have no doubt that this would give less distortion than UL because you are applying the error correction to the electrode that gave rise to the error in the first place, not some other electrode with other characteristics.
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Old 20th August 2019, 09:31 PM   #92
Wavebourn is offline Wavebourn  United States
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Local feedback between grid-cathode
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Originally Posted by SpreadSpectrum View Post
I have no doubt that this would give less distortion than UL because you are applying the error correction to the electrode that gave rise to the error in the first place, not some other electrode with other characteristics.
Exactly. The name "Ultra Linear" is misleading and is responsible for it's undeserved popularity.
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Old 21st August 2019, 12:10 PM   #93
Ketje is offline Ketje  Belgium
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Here are the dynamic Ia-Vg1 curves for the 3 choises with the KT88.
One can see that the UL is the more linear.To call it ultra is to much honor
Mona
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File Type: png ul-pt-tr.png (21.0 KB, 105 views)
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Old 21st August 2019, 02:50 PM   #94
jhstewart9 is offline jhstewart9  Canada
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Default Amplifiers & Superlatives

Langford Smiths Paper UL vs CFB, a good read.
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Old 21st August 2019, 02:51 PM   #95
jazbo8 is offline jazbo8
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I think you meant Williamson & Walker...
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Old 21st August 2019, 03:56 PM   #96
smoking-amp is offline smoking-amp  United States
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Here are some Vg1 to Ip curve traces for 26LX6 (50 mA per div Vert.)
a) grid 1 drive
b) Crazy drive (grid1 and grid2 special combo) (5.4 V steps)

c) 26LX6 low current end in Crazy Drive (10 mA per div Vert.) (showing low end for class aB overlap)

d) 36LW6 normal plate curves in Crazy drive (50 mA/div) (5 V steps)
e) 36LW6 plate curves for grid 1 drive
f) 26LX6 plate curves for grid 1 drive
g) 26LX6 plate curves for Crazy drive

Ultra linear, hmmm
Attached Images
File Type: jpg 26LX6_grid1_drive_VtoI.jpg (82.6 KB, 81 views)
File Type: jpg 26LX6_Crazy_drive_VtoI.jpg (79.1 KB, 81 views)
File Type: jpg 26LX6_Crazy_drive_VtoI_10mA_lowI.JPG (293.7 KB, 76 views)
File Type: jpg 36LW6_Crazy_drive.jpg (62.3 KB, 80 views)
File Type: jpg 36LW6_grid1_drive.jpg (56.2 KB, 3 views)
File Type: jpg 26LX6_grid1_drive.jpg (51.4 KB, 1 views)
File Type: jpg 26LX6_Crazy_drive.jpg (71.9 KB, 1 views)

Last edited by smoking-amp; 21st August 2019 at 04:18 PM.
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Old 21st August 2019, 04:14 PM   #97
SpreadSpectrum is offline SpreadSpectrum  United States
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I just want to point out that in QUAD output stage in the Williamson and Walker paper, the cathode feedback stage also has negative feedback to the screen. So that paper is really comparing UL with UL+CFB. Neither of the output stages examined are Pentode/Tetrode+CFB. If you want to combine Pentode and CFB you have to keep the voltage between screen and cathode constant which requires extra effort in floating supplies or extra OT windings or special cases (like Unity-Coupled config).
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Old 21st August 2019, 11:01 PM   #98
gabdx is offline gabdx  Canada
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To me UL is way superior to UL+CFB,

A great amount of UL is enough and GNF works a lot better to reduce any amplifier parts coloration, reducing GNF and using then CFB corrects only the output stage.

GNF reduces the output Z enough, with UL, so that the CFB is useless and there is no benefits from reducing the output Z more.

To me, with thd levels inaudible, UL was the clear winner over UL + CFB.

However I am curious about pentode + CFB, which will enable same gain levels with a decent amount of GNF.

In other words, the pentode higher gain, plus the CFB plus around same level of GNF is the highest gain, highest feedback solution.

Logically the higher gain, higher feedback levels should sound better, we will see.

Table 1 in the article refers to stability. this is where pentode is weak.

I am thinking about running the KT120 with a G2 at higher voltage than the anode... regulated.
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