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More Ruminations on Screen Drive/Crazy Drive
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Old 30th August 2018, 05:54 PM   #201
smoking-amp is offline smoking-amp  United States
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Crazy drive uses HV direct screen V drive, with a resistor from g2 to g1 (call that Rg2g1) and another resistor from grid1 to cathode (call that Rg1k).

The main drive is then via HV grid2 voltage drive, with grid1 acting as a current driven drive at low plate current, which compensates for gm2 being low at low plate current. The grid1 current drive saturates as plate current increases due to Rg2g1 and grid1 current dragging on it, while gm2 picks up with increased plate current and Vg2. The net effect is to get constant -total- gm drive of the tube. The resistor Rg2g1 is selected to get this compensation correct. Rg1k just acts as a pull down to help turn the tube off, but also affects the compensation some.

a) 26LX6 normal grid1 drive
b) 26LX6 grid2 only V drive
c) 26LX6 Crazy/Twin drive
d) 12HL7 Crazy/Twin drive
e) 40KG6 normal grid1 drive
f) 40KG6 grid2 only V drive
g) 40KG6 Crazy/Twin drive
Attached Images
File Type: jpg rsz_26lx6_p.jpg (51.4 KB, 187 views)
File Type: jpg rsz_26lx6_g2_70_7vs.jpg (66.9 KB, 181 views)
File Type: jpg rsz_26lx6_crazydrive_55v_5,4vs.jpg (71.9 KB, 179 views)
File Type: jpg Crazy_TFH.JPG (172.3 KB, 176 views)
File Type: jpg rsz_40kg6_g1_1p5vst_78vg2_50_50.jpg (82.9 KB, 178 views)
File Type: jpg rsz_40kg6_g2_5p5vst_50_50.jpg (76.0 KB, 10 views)
File Type: jpg rsz_40kg6_twin_3p5vst_888rg2g1_2770rg1k_50_50.jpg (74.9 KB, 9 views)
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Old 17th October 2018, 08:15 PM   #202
DIYBras is offline DIYBras  Brazil
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Not exactly Crazy Drive, due to CCS making the divider ratio 1:1 for G2/G1, relying only on G1 current (if any) to make the division. Interesting different project nonetheless, albeit with high global NFB.
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Old 17th October 2018, 08:21 PM   #203
Kay Pirinha is offline Kay Pirinha  Germany
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Unfortunately my Japanese crystal ball is being serviced at the moment. What does this CCS in the pentode's plate circuit do?


Best regards!
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Old 17th October 2018, 08:30 PM   #204
DIYBras is offline DIYBras  Brazil
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My crystal ball showed a message:
Since the diff input (2SJ109) forces the anode voltage to zero (input is ground referenced), I believe that the designer don't trusted to rely only on output trafo DCR for stabilizing the anode current. Is one of these complicated "exercise" projects...
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