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Old 25th February 2010, 02:27 AM   #91
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The combined g2,g1/Mu drive scheme seems like a practical solution for positive g1 and g2.

This only requires half the positive voltages of positive g1 only drive (and g2 need only go half as positve compared to fixed pos. g2 and neg. g1 drive). Grid currents scale like square law of pos. voltages I think. The plate voltage can then drop lower too (before going below g2 voltage).
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Old 25th February 2010, 02:49 AM   #92
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Quote:
Originally Posted by smoking-amp View Post
The combined g2,g1/Mu drive scheme seems like a practical solution for positive g1 and g2.

This only requires half the positive voltages of positive g1 only drive (and g2 need only go half as positve compared to fixed pos. g2 and neg. g1 drive). Grid currents scale like square law of pos. voltages I think. The plate voltage can then drop lower too (before going below g2 voltage).
Totally. I think it makes a 2 stage screen drive amp a possibility. A couple of MOSFET followers and <=150Vpk drive stage (which is not that hard) traded off against a stable screen supply and a negative bias supply that you don't need... At the lower drive voltages vs. g2-only drive it would work with parallel (shunt) plate-grid feedback. Plate efficiency goes up a few % in class AB for some test cases of 50W PP sweep tube amps using 17W tubes. Quiescent current can also I think be reduced as the transfer function seems more linear.

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Old 25th February 2010, 03:32 AM   #93
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Default 6p1

6P1 is same construction as 6AQ5
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Old 25th February 2010, 04:33 AM   #94
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FYI, tubes are usually built with nickel, an excellent ferromagnetic element (and half the content of such legendary alloys as permalloy). Ni's curie temperature is 358C, so most of the components inside an operating power tube will be demagnetized (above the curie temp.), though not necessarily a tube on the curve tracer.

When you apply a toroidial magnet, you're bringing a dipole field near, which is basically all fringing flux. This means the field is diverging above and below the magnet. Since electron paths are bent perpendicular to the field lines, you will get an axial spiraling, which will tend to move the beam of a beam tetrode away from the plate seam and towards the suppressor plates and grid supports. Since the field is divergent, electrons will also be bent up or down, which will increase screen current. One might speculate on the effect of secondary emission (which seems to be worsened, although only at high current, as in a pentode, not to the point of negative resistance at low current density, as in a beam tetrode), but I don't have a convincing explanation for that.

And note this doesn't even take into account the channeling of flux inside the magnetic components, which should act to reduce divergence (except at the open end, on top) and make a somewhat more uniform distribution of magnetism along the length, butbe left with localized spots of higher intensity (such as supports, corners, cold spots, etc.).

Hmm, now I wonder what a tube does when hot. That could be tested by building a standard magnetization fixture (i.e., a glorified solenoid around the tube, with calibrated currents applied), and testing after running at rated power for a few minutes (so the structure is hot, past the curie temperature in places). Then watch how it changes as it cools down.

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Old 25th February 2010, 02:15 PM   #95
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As TubeSteve says, 6P1P appears to be identical in specs to a 6AQ5, which has a g2/g1 Mu of 9.5. So I rigged up the 6P1P 4400 (hysteretic) with g2,g1/Mu drive using a 1K Ohm resistor from g2 to g1, and a 100 Ohm resistor from g1 to cathode. Drive on g2 at 5.3 V/step.

Plot1: g2,g1/Mu, 50 V/div horiz., 5 ma/div vert., 5.3 V steps

Plot2: g2 only, ditto, g1 at 0 V

Plot3: g2 only, but changed to 2 mA/div vert.

Seems to still have hysteresis in g2,g1/Mu mode. Still a bit in g2 drive only too. Plot 1 versus Plot2 verifies that each grid is doing half the control.

I also tried banging this tube hard on a vinyl floor mat to see if I could affect the g2/g1 alignment, but couldn't see any difference, and the hysteresis stayed the same too.
-------------------------------
On the g2 current with positive g1 thing again, I've been thinking that maybe a tube with 180 degree g2/g1 alignment (g2 wires exactly in between the g1 wires) might be the ideal arrangement to avoid focusing current onto g2 wires. If I ever see some very badly aligned tube I'll test that.
--------------------------------
edit: added the non hysteretic 3950 6P1P tube in g2,g1/Mu mode for comparison, same scale as Plot1. Curves look great in g2,g1/Mu, no rounding at all.
Attached Images
File Type: jpg 6P1P_g2g1Mu.jpg (24.9 KB, 292 views)
File Type: jpg 6P1P_g2only.jpg (22.8 KB, 287 views)
File Type: jpg 6P1P_g2only2.jpg (24.3 KB, 285 views)
File Type: jpg 6P1P_3950_g2g1Mu.jpg (22.5 KB, 281 views)
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Last edited by smoking-amp; 25th February 2010 at 02:38 PM.
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Old 25th February 2010, 04:23 PM   #96
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"When you apply a toroidial magnet"

I tried a simple axially symmetric rectifier tube, 19Y3, in a magnetic field from a stack of strong toroid magnets here. They are stacked about 4 inches high and slipped over the tube for the axial case. Perpendicular case has the tube stuck to the end face. Scale for plots is 2 V/div horiz. and 10 mA/div vert.

Plot1: no field

Plot2: axial field case

Plot3: perpendicular case

I tried both directions of the magnetic field for the axial case, and they were the same as expected. Also tried both polarity fields for the perp. case, and the same result.

Looks like an axial field works well for increasing the plate resistance.
Should be interesting to try this on an axial sym. triode to see if it affects the Mu or not.

Edit: added earlier plots as plot 4 and 5 here of the 6P1P in normal g1 drive mode for comparison with the previous g2,g1/Mu and g2 plots shown above. Plot4 has 100 V on g2 and Plot5 has 200 V on g2.
Attached Images
File Type: jpg 19Y3_nomagnet.jpg (19.3 KB, 273 views)
File Type: jpg 19Y3_axialmagnet.jpg (18.5 KB, 6 views)
File Type: jpg 19Y3_perpmagnet.jpg (19.7 KB, 5 views)
File Type: jpg 6P1P_3950_100Vg2.jpg (39.8 KB, 8 views)
File Type: jpg 6P1P_3950_top0.jpg (28.8 KB, 9 views)
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Last edited by smoking-amp; 25th February 2010 at 04:43 PM.
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Old 25th February 2010, 05:23 PM   #97
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Here are some curves for a 6S4A triode with axial magnetic fields. This tube has a circular cathode, the grid is crimped around the cathode (nearly round) from further out posts and the plate is flat type.

Plot1: normal triode operation, 50 V/div horiz., 5 mA/div vert., 1.2 V steps g1

Plot2: axial field along tube, ditto, same grid bias as above: 0V grid for max curve

Plot3: axial field along tube, ditto, adjusted grid bias positive for max curve

Seems that the electrons can't get past the grid with positive grid in Plot3. Must be spiralling into the posts.

Appears that Mu and gm got decreased in Plot2
Attached Images
File Type: jpg 6S4A_normal.jpg (26.0 KB, 28 views)
File Type: jpg 6S4A_axialmagnet.jpg (22.4 KB, 28 views)
File Type: jpg 6S4A_axialmagnet_posg1.jpg (20.8 KB, 21 views)
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Last edited by smoking-amp; 25th February 2010 at 05:41 PM.
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Old 5th March 2010, 10:47 AM   #98
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this discussion was very interesting, are there any new points / ideas about G3 voltage supply?
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Old 5th March 2010, 03:24 PM   #99
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Quote:
Originally Posted by smoking-amp View Post
Looks like an axial field works well for increasing the plate resistance.
Hmmm... Is a current limiting rectifier already patented? Just put a rectifier tube inside of a choke ...
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Old 5th March 2010, 03:35 PM   #100
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Originally Posted by smoking-amp View Post
Still, mysterious why some tubes do the hysteresis thing and others don't.
Different tension of the 1'st grid, so temperature changes geometry differently?
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