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Old 5th March 2011, 12:59 PM   #1
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Default Converting 12FQ8 for 12AX7 , HELP !!!

How do i convert this 12FQ8 tube pre amp for 12AX7 use ?
http://www.instructables.com/files/o...28FVW22RXO.pdf
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Old 5th March 2011, 02:21 PM   #2
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You cant. Sorry but a 12ax7 wont work at 12-16v b+, end of story.
Also the 12fq8 is a very odd ball tube. Its got 4 plates instead of 2 like the 12ax7, and it has a shared cathode instead of 2 like the 12ax7.

If you can give us some info we can pick you out a better design. AKA you are cheap and just have 12ax7's running around, do you need it to work at 12volts, stuff like that.

Have you tried the other 2 variations that use normal tubes?

http://www.diystompboxes.com/smfforu...?topic=63479.0

http://www.dogstar.dantimax.dk/tubestuf/mctube.htm even uses 12ax7. Its power supply is a little funky but can easily be made normal.

Last edited by nightanole; 5th March 2011 at 02:24 PM.
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Old 5th March 2011, 03:01 PM   #3
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Quote:
How do i convert this 12FQ8 tube pre amp for 12AX7 use ?
Why don't just try to make the same circuit with 12AX7 and try what the result is ?
It is not a big risk at all. I would do so.
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Old 5th March 2011, 04:15 PM   #4
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Quote:
Originally Posted by nightanole View Post
You cant. Sorry but a 12ax7 wont work at 12-16v b+, end of story.
Also the 12fq8 is a very odd ball tube. Its got 4 plates instead of 2 like the 12ax7, and it has a shared cathode instead of 2 like the 12ax7.

If you can give us some info we can pick you out a better design. AKA you are cheap and just have 12ax7's running around, do you need it to work at 12volts, stuff like that.

Have you tried the other 2 variations that use normal tubes?

Tube boost + overdrive running off a 9 volt battery

The Real McTube II even uses 12ax7. Its power supply is a little funky but can easily be made normal.
that oddball tube 12FQ8 are only stocked as NOS tubes , 50$ ( oh yeah , i live in singapore )

And , the only cheap option is 12AX7 , no , i'm beginner and have no tubes at all .

I sort of saw the diystompboxes , and didn't give a , nevermind .

What sort of info are you looking for ?

Last edited by DJDestiny; 5th March 2011 at 04:22 PM.
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Old 5th March 2011, 04:19 PM   #5
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Quote:
Originally Posted by artosalo View Post
Why don't just try to make the same circuit with 12AX7 and try what the result is ?
It is not a big risk at all. I would do so.
Big risk ?
I don't know , if anything goes wrong , 6.50 down the drain .
I'm a student , so no funding at all . ( Unless somebody asks me to produce one more )
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Old 5th March 2011, 07:18 PM   #6
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Since i have no clue what you are after. a 12ax7 has a gain of 100 and will never work at low voltages. I have no clue what gain the 12fq8 is.

try a 12au7, they are $5 or so with a gain of 17 and is the best shot at 12 volts just like your circuit.

http://www.nj7p.org/Tube4.php?tube=12AU7

http://www.netads.com/~meo/Guitar/Tubes/dual-triodes.pl


now lets rearrange the pins so it works with the circuit.

12FQ8.
1 becomes pin 1 on 12au7
2 becomes pin 2 on 12au7
2 becomes pin 3 on 12au7
3 becomes pin 1 on 12au7
4 becomes pin 4 on 12au7
5 becomes pin 5 on 12au7
6 becomes pin 6 on 12au7
7 becomes pin 7 on 12au7
8 becomes pin 6 on 12au7
9 becomes pin 8 on 12au7
9 becomes pin 3 on 12au7

pin 9 is not used on the 12au7 or 12ax7 when you are using 12v heaters. If you are still hell bent on a 12ax7 then this pinout will work too.

As far as playing 20 questions. We now know you are a beginner,not in the usa, and a student with a low budget. Next question, what do you want to do with this? Is this for a battery powered guitar pickup box or something?

Last edited by nightanole; 5th March 2011 at 07:39 PM.
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Old 5th March 2011, 08:32 PM   #7
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Quote:
Originally Posted by nightanole View Post
Since i have no clue what you are after. a 12ax7 has a gain of 100 and will never work at low voltages. I have no clue what gain the 12fq8 is.
The 12FQ8 has a gain of 95, and by all rights probably shouldn't work at such low plate voltages either. The RCA datasheets show operating conditions with "normal" plate voltages around 250.

If you go find and read the original "ValveLitZer" article, it says the "designer" fully expected it not to work, but he had a bunch of 12FQ8 and wanted to mess with them. It's not intended to pass clean audio, but rather to add a lot of "character".

The ValveLiTzer: Low-voltage Tube Booster

Someone even asked (back in 2009) whether it works with a 12AX7. Apparently the author answered the question, but didn't post the answer publicly.
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Old 5th March 2011, 08:34 PM   #8
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Quote:
I have no clue what gain the 12fq8 is.
12FQ8 is very similar to 12AX7 when both anodes are connected together.
See here: http://www.mif.pg.gda.pl/homepages/f...93/1/12FQ8.pdf
So, if 12FQ8 has been suitable for the circuit shown, then 12AX7 will definitely work better with same component values than 12AU7.
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Old 6th March 2011, 05:33 AM   #9
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@nightanole
Yeah , i wanted to make a ValveLitzer .
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Old 6th March 2011, 05:39 AM   #10
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@nightanole
Yeah , i wanted to make a ValveLitzer .
But , i need it to run on 12v
So i'm not sure what tube to use .
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