RE: 12bh7 substitution in pilot sa 260

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Do It Yourself (DIY) paradise for tube and SET project builders.

Posted by Eli Duttman (A) on February 27, 2012 at 18:06:38

In Reply to: 12bh7 substitution in pilot sa 260 posted by pennyford on February 26, 2012 at 07:59:20:

A look at the schematic shows the 'U7 (1 section/channel) being used in a common cathode gain block, outside the NFB loop. The 'U7 is NOT linear. Switching to the pin compatible and linear 12BH7 rated to (and did) offer a sonic improvement.

As the posts below indicate, you are probably safe, as long as you don't use the preamp power socket.

If you decide to install 5R4s, be certain to use the tough "potato masher" variety. The B+ rail voltage can be made "perfect" by tweaking the value of R48 downwards.

The bias rectifier (M1) rates to be selenium, which makes it a ticking toxic time bomb. I suggest you replace M1 with a pair of 1 A./100 PIV Schottky diodes. Doing so may require the value of R49 to increase.

FWIW, a nice, linear, medium μ twin triode that draws only 300 mA. of heater current is available, the 7AF7. To use the 7AF7, sheet metal "surgery" replacing the Noval socket used by the 'U7 with a Loctal socket is necessary. If you look at the data sheet linked below, you will see that the 7AF7 is similar to the 7N7. The 7N7 is electrically equivalent to the 6SN7 and the 6CG7/6FQ7. ;>)

Eli D.

7AF7 Data Sheet (Open in New Window)

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Follow Ups Full Thread

Topic - 12bh7 substitution in pilot sa 260 - pennyford 07:59:20 02/26/12 (14)

RE: 12bh7 substitution in pilot sa 260 - pennyford 18:23:23 03/03/12 (0)

RE: 12bh7 substitution in pilot sa 260 - Eli Duttman 18:06:38 02/27/12 (0)

RE: 12bh7 substitution in pilot sa 260 - Cleantimestream 11:06:11 02/26/12 (11)

RE: 12bh7 substitution in pilot sa 260 - pennyford 18:57:32 02/26/12 (10)

RE: 12bh7 substitution in pilot sa 260 - Cleantimestream 00:13:40 02/27/12 (5)

RE: 12bh7 substitution in pilot sa 260 - pennyford 04:48:51 02/27/12 (4)

RE: 12bh7 substitution in pilot sa 260 - Neff 20:29:12 02/27/12 (0)

RE: 12bh7 substitution in pilot sa 260 - Tom Bavis 08:14:14 02/27/12 (2)

Rectifier substitutions in Pilot SA-260... - Interstage Tranny 14:21:38 02/27/12 (1)

RE: Rectifier substitutions in Pilot SA-260... - Eli Duttman 17:26:20 02/27/12 (0)

RE: 12bh7 substitution in pilot sa 260 - Eli Duttman 19:10:59 02/26/12 (3)

I think the answer is yes - Tre' 10:06:08 02/27/12 (2)

I think the answer is ? - Interstage Tranny 14:04:19 02/27/12 (1)

I messed up, again - Tre' 16:07:42 02/27/12 (0)

Follow Ups

RE: 12bh7 substitution in pilot sa 260 - Eli Duttman 18:06:38 02/27/12 (0)

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