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Old 23rd October 2007, 04:36 PM   #1
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Default Building bass tube preamp. (Stupid) questions?

Hi, everyone. It's my first post here.

So, Im planning to build an bass preamp. Wanting to use Alembic F-2b cirquit (this one http://www.geofex.com/FX_images/alembpre.gif). BUT, I would like to use 6SN7 type tube instead 12ax7 or similar. Info about 6sn7 http://www.mclink.it/com/audiomatica...tubes/6sn7.htm and 12ax7

I know that original version of the F-2b runs 300V into plate.

My question is how should i modify the schematics to use the 6sn7 tube. Especially - what to do with the powering section.

I've found this Hammond power transformer :

TRANSFORMER, POWER, HAMMOND, 300-0-300 V, 100 mA

P-T372BX

* Universal Primary - 100, 110, 120, 200, 220, 240 VAC 50/60 Hz.
* High voltage secondary designed for full wave center tapped use.
* 50 VAC bias tap from high voltage secondary C.T.
* Black epoxy painted to match our P-T1650 series of output transformers .
* Class A insulation (105 degrees C).
* Hi-Pot test of 2,000 V RMS.
* Windings are concentric wound for low stray field and low noise

Secondary: 300-0-300 V, 100 mA
Filament winding 1 : 5 VCT, 2 A
Filament winding 2 : 6.3 VCT, 3 A
Mounting centers: 2.5" x 2.19" vertical mount
Weight: 4.8 lbs.

Should i give more voltages to plate (as 6sn7-s max plate voltage is 450V) - maybe 400V?

And how should i connect the wires - can or can't i just connect wires from transformer to plate and filaments??

thanks,

ps.

sorry for my bad english
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Old 23rd October 2007, 05:11 PM   #2
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The 12AX7 and 6SN7 are so completely different that you should consider a different solution. Pick one or the other, either you must have the Alembic circuit or you must have 6SN7's.
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Old 23rd October 2007, 07:01 PM   #3
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Agreed.

6SN7GT isn't the best sounding tube in a guitar pre unless playing super clean, IMO.

Cheers!
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Old 23rd October 2007, 07:50 PM   #4
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ok, thanks for replies. And yes, i want that clean sound. My intention was to make something like the lowend truevoice preamp

http://www.head-fi.org/forums/showthread.php?p=3233782

How to make that? Does anyone have a simple schematics for preamp that uses 6sn7 tube?
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Old 23rd October 2007, 09:11 PM   #5
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operating a 6SN7 at 300V is not a problem. you may have to adjust grid bias a little, but otherwise it should be fine. just look at the characteristic chart and see what needs to be different. the max plate voltage is exactly that ...MAX....

things change a bit with a different plate voltage, but a good curve chart makes it relatively easy to figure out plate load resistors, grid bias, etc...
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Old 24th October 2007, 04:10 PM   #6
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ok, i think im going to use the original alembic diagram. My concern is the power section.

I found this schematics from internet

http://moosapotamus.net/IDEAS/F2B/AlembicF2B_PSU.gif

what do you think about it? Decided not to buy the Hammond transformer. How should i modify this schematic to use 220v to 12 vDC power adaptors? What to add, to get around 300v from the high voltage section?

thanks,

ps. I'm a real newbie, this is my first DIY project.
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Old 24th October 2007, 06:44 PM   #7
gerryc is offline gerryc  United States
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Quote:
Originally posted by madisqbu
ok, thanks for replies. And yes, i want that clean sound. My intention was to make something like the lowend truevoice preamp

http://www.head-fi.org/forums/showthread.php?p=3233782

How to make that? Does anyone have a simple schematics for preamp that uses 6sn7 tube?
You might want to consider using a 6SL7 instead for the Alembic preamp for voltage gain reasons. It has a mu of 70 compared to 100 of the 12AX7, while the 6SN7 is only around 20. You would probably have to adjust the cathode resistors slightly to set the bias correctly (to get the plate voltage at idle to be around B+/2). You'll still have lower distortion than a 12AX7, not quite as low as with the 6SN7, but at that point it's probably moot anyway for a bass guitar preamp.
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Old 24th October 2007, 09:35 PM   #8
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The Alembic pre is based around a Fender Bassman type cct and unless you're bass is passive with Barts, it has too much gain. A 6SL7, 12AT7 or even a 6DJ8 or 6N1P are probably better options. On another forum there is a similar DIY bass pre discussion started and I'm going to breadboard some of these to see what works best.

I hope that you're aware that the Fender stack doesn't really have a flat setting. It gets close at 2-10-2.
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Old 25th October 2007, 11:31 AM   #9
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Brett, thanks for information. I'm planning to add a active/passive swiching opion to the preamp (i'm playing active bass, nordies with audere pre). Actually i would like to omit the controls, using just gain and volume pot.
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Old 25th October 2007, 07:43 PM   #10
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Quote:
Originally posted by madisqbu
Brett, thanks for information. I'm planning to add a active/passive swiching opion to the preamp (i'm playing active bass, nordies with audere pre). Actually i would like to omit the controls, using just gain and volume pot.
Omit the tone controls on the bass or the preamp?

Nordstrand pups and an Audere; you must be a Talkbasser.
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