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More Ruminations on Screen Drive/Crazy Drive
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, 441 views)
File Type: jpg rsz_26lx6_g2_70_7vs.jpg (66.9 KB, 437 views)
File Type: jpg rsz_26lx6_crazydrive_55v_5,4vs.jpg (71.9 KB, 436 views)
File Type: jpg Crazy_TFH.JPG (172.3 KB, 433 views)
File Type: jpg rsz_40kg6_g1_1p5vst_78vg2_50_50.jpg (82.9 KB, 429 views)
File Type: jpg rsz_40kg6_g2_5p5vst_50_50.jpg (76.0 KB, 20 views)
File Type: jpg rsz_40kg6_twin_3p5vst_888rg2g1_2770rg1k_50_50.jpg (74.9 KB, 24 views)
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Old 17th October 2018, 08:15 PM   #202
DIYBras is offline DIYBras  Brazil
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18GV8Single2W+2W
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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Old 14th February 2020, 08:52 AM   #205
Sorento is offline Sorento  Europe
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MJ^9510_(03)40KG6A-single-power-amp_SoyaS.pdf - Google Drive
Looks a lot like "crazy drive" published 10/1995 ...
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Old 14th February 2020, 10:34 AM   #206
OldHector is offline OldHector
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Synola-509/Paravicini.pdf

Does this one count?

What part does the suppresaid play in this circuit? It is not tied to the cathode.
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Old 14th February 2020, 12:53 PM   #207
DIYBras is offline DIYBras  Brazil
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Quote:
Originally Posted by Sorento View Post
MJ^9510_(03)40KG6A-single-power-amp_SoyaS.pdf - Google Drive
Looks a lot like "crazy drive" published 10/1995 ...
Basically, but the devil are in details...
Dual Drive for sure, but:
The g1 DC bias is negative relative to cathode. This results in different transfer curve than both g1 and g2 being DC positive relative to cathode.
The AC signal will travel under these bias conditions.
For me (interpreting the gm curves from my test results) the gm curves for g1 negative and g2 positive resembled something like square law transfer (or higher!), and for g1 and g2 positive is closer to linear law gm.
Linear law is the key for Crazy Drive I suppose
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Old 14th February 2020, 12:56 PM   #208
DIYBras is offline DIYBras  Brazil
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Quote:
Originally Posted by OldHector View Post
Synola-509/Paravicini.pdf

Does this one count?

What part does the suppresaid play in this circuit? It is not tied to the cathode.
Basically screen drive with a curious cathode bias (g1 is not signal driven).

The anode suppressor/compensation capacitors are tied to the feedback line.
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Old 14th February 2020, 10:12 PM   #209
Sorento is offline Sorento  Europe
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Quote:
Originally Posted by OldHector View Post
Synola-509/Paravicini.pdf

Does this one count?.
Don't think so, both are just single drive albeit screen driven. No ac signal on g1.
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Old 15th February 2020, 01:57 AM   #210
jhstewart9 is offline jhstewart9  Canada
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Default Some Evidence of Crazy Drive Before WW2?

Referring to the Class B curves for the RCA 49, looks a lot like a pentode. Running in Class B as a zero bias triode was a way to get more audio out of a small tube. That was common in the 30s. The 49 data sheet is copied from the RCA/ Cunningham RC-11Tube Manual dated 1933. But the 49 & others like it had to be driven by a low mu triode thru an interstage transformer. The 6AC5GT is a very high mu audio triode that is normally driven by a low or medium mu triode by a direct connexion, not RC or IT. Several triodes were suitable as drivers.
The next step was put it all in one glass bottle. The 6B5 & 6N6G are the result. The symbol used for these tubes actually shews a driver triode direct connected to the included pentode. And the pinout plugs right into the socket of a corresponding pentode. So 6B5 subs for a 42 & 6N6G subs for a 6F6G. The resulting cct is self biasing, no cathode bias resister required. There are/were 25 volt heater versions as well. But tubes such as this never caught on.
These tubes looked interesting in 2003 but very expensive. As an alternative any common audio power pentode when screen driven has a high mu. Turns out a 6F6 does well for the experiment. I drove the 6F6 with a 6SN7, both triodes in parallel. The results are in the attachment. The results also shew the driver triode plate can be used as a UL connexion to the OPT. Output is limited by plate dissipation just as it is in the normal hookup.
I went no further, many others had built amplifiers both SE & PP based on this principle. The all triode UL PP cct is a possibility for those with time to kill.
But it did lead to a convenient way to create pentode sub-circuits in Electronic Workbench software which come with not pentode models.
Not much new in toobz…………….or under the sun.
Some here on DIY have already seen a version of this a couple of years ago.Still looks like crazy drive to me!!
Attached Images
File Type: jpg 6AC5 SE Amplifier.jpg (40.9 KB, 25 views)
File Type: jpg Rockolab PP 6B5 Amplifier 12W.jpg (217.2 KB, 35 views)
File Type: jpg Triode UL.jpg (39.9 KB, 37 views)
Attached Files
File Type: pdf 6SN7 6F6 Amp B.pdf (13.2 KB, 8 views)
File Type: pdf 49 RCA 2.pdf (67.3 KB, 5 views)
File Type: pdf 6B5 Tungsol.pdf (64.3 KB, 5 views)
File Type: pdf 6N6G RCA.pdf (39.5 KB, 3 views)
File Type: pdf 6AC5GT RCA.pdf (105.1 KB, 6 views)
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