Re: Substitute for 1/2 a 12AX7?

Posted by audiodude (A) on March 10, 2002 at 11:42:03

In Reply to: Substitute for 1/2 a 12AX7? posted by David Anderson on March 8, 2002 at 14:11:38:

While it is true that my driver circuit is similar to the mullard 5-20, there are substantial differences in the design including the use of limited local feedback in the driver and input stages - this slightly improves linearity and reduces dependance on tube characteristics. I actually designed this circuit more than 13 yrs ago & long before I saw the article on the 5-20. Goes to show that there really isn't that much new under the sun. Note that for best performance the supply voltage to the driver stages should be in the range of +360V - 380V so you might have to adjust the value of R22 from about 8.2K to 10K. I generally use 10K at this location.

Someone suggested strapping the two triodes in parallel - this isn't necessarily a bad idea.. However his suggestion as to how to do it with my design is incorrect. Reduce the plate load resistor R21-100K to 1/2 of its current value, say 51.1K, note that you must also reduce R20 - 100K supply decoupling/dropping resistor to 51.1K, connect the grids in parallel with just one grid stopper resistor (R1), and use independent cathode resistors (R3) connected to the feedback node of R4 - 47.5 ohms, R5-1K. Do not add bypass capacitors across the cathode resistors!!

David - I am an asylum member and you can email directly if you wish for further assistance.. You might want the latest schematic..

Kevin Kennedy
Former & thoroughily disillusioned audio guru?

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Follow Ups Full Thread

Topic - Substitute for 1/2 a 12AX7? - David Anderson 14:11:38 03/8/02 (13)

The free 1/2 ECC83 is... - PÃ¤r 23:39:22 03/10/02 (0)

Re: Substitute for 1/2 a 12AX7? - audiodude 03/10/0211:42:03 03/10/02 (3)

Re: Substitute for 1/2 a 12AX7? - bigfella 19:51:07 03/12/02 (0)

Re: Substitute for 1/2 a 12AX7? - David Anderson 06:47:04 03/11/02 (0)

Re: Substitute for 1/2 a 12AX7? - sajti 08:26:01 03/11/02 (0)

Re: Substitute for 1/2 a 12AX7? - audiodude 11:17:32 03/10/02 (0)

Re: Substitute for 1/2 a 12AX7? - kjt 06:49:53 03/9/02 (0)

Re: Substitute for 1/2 a 12AX7? - Eli Duttman 14:57:39 03/8/02 (5)

Re: Substitute for 1/2 a 12AX7? - David Anderson 19:48:24 03/8/02 (4)

Re: Substitute for 1/2 a 12AX7? - Eli Duttman 21:16:42 03/8/02 (2)

Re: Substitute for 1/2 a 12AX7? - David Anderson 00:22:13 03/9/02 (1)

Re: Substitute for 1/2 a 12AX7? - Eli Duttman 09:38:04 03/9/02 (0)

Re: Substitute for 1/2 a 12AX7? - Gary Kaufman 20:41:31 03/8/02 (0)

Follow Ups

Re: Substitute for 1/2 a 12AX7? - audiodude 03/10/0211:42:03 03/10/02 (3)

Re: Substitute for 1/2 a 12AX7? - bigfella 19:51:07 03/12/02 (0)

Re: Substitute for 1/2 a 12AX7? - David Anderson 06:47:04 03/11/02 (0)

Re: Substitute for 1/2 a 12AX7? - sajti 08:26:01 03/11/02 (0)

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