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Post subject: 5E3 circuit w/6SL7's
Posted: Thu Nov 04, 2010 3:39 pm
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Hi, this is my first post here so I thought I would start with a circuit question. Can I use 6SL7's in place of 12AX7's in the 5E3 circuit. The reason is a while back I bought a 5C3 chassis from Weber because he was out of the 5E3 style. I was going to use adapter plates that I purchased from him at the same time but I ended up using them on another project. I have already made the board so I just wanted to know if I need to change some component values for the 6SL7's.

Thanks,

Bigampdaddy[/img]


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Posted: Thu Nov 04, 2010 7:55 pm
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I think no problem. Several amps (like 1950-60's Ampeg combos) used 6SL7 or 6SN7 in the gain stages. Mu is 70 for the 6SL7 versus 100 for he 12AX7, so you MAY lose a bit of gain (depending on other factors like plate voltages, idle bias, and feedback specifics). I'd scope out some Ampeg schemos to see what parameters they used on their 6SL7's.

http://unofficialampeg.com/schematics/j-12-a.gif


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Posted: Thu Nov 04, 2010 8:01 pm
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BMW2002Ti wrote:
I think no problem. Several amps (like 1950-60's Ampeg combos) used 6SL7 or 6SN7 in the gain stages. Mu is 70 for the 6SL7 versus 100 for he 12AX7, so you MAY lose a bit of gain (depending on other factors like plate voltages, idle bias, and feedback specifics). I'd scope out some Ampeg schemos to see what parameters they used on their 6SL7's.

http://unofficialampeg.com/schematics/j-12-a.gif


+1

Most of the original K&F's and some of the early TV-front tweeds used noctals in the pre-amp. I think they'll work well and sound great.

Arjay

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Posted: Thu Nov 04, 2010 8:32 pm
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Yeah, both my Champion 600 (1949) & 800 (1952) run 6SJ7 octal pentodes in the gain section. Much browner, slower onset of overdrive... compared to similar circuit wih 12AX7. I kinda like the better, for these amps.


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Posted: Thu Nov 04, 2010 10:09 pm
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Indeed, the sonic character of the tweeds changed dramatically when Leo switched to the sub-miniature dual triodes in the pre-amps.

It's been a long time since I last heard one of the old woodies or a TV-front tweed......

Arjay

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Posted: Fri Nov 05, 2010 9:18 pm
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Hi Bigamp, guess you've noticed the seperate cathode design of the 6SL7(8BD basing). A 2700ohm resistor is the norm. The bypass caps would be the same as the 12AX7. The input grid leak resistor is usually 5meg. Of course, these are just starting points and open to tweaking. Art

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Post subject: Reply and some pics
Posted: Sat Nov 06, 2010 12:33 pm
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Wow, thank you for all the replies. I will take a look at the Ampeg schematic and the earlier Tweed ones also. I took a few pics but I don't see a way to download them. Do they have to be posted as a link to a web page or photobucket?

Thanks
Bigampdaddy


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