Building bass tube preamp. (Stupid) questions?

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madisqbu

Member
Joined 2007
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/audiomatic.../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
 
unclejed613

unclejed613

Member
Joined 2006
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...
 
M

madisqbu

Member
Joined 2007
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.
 
G

gerryc

Member
Joined 2005
madisqbu said:
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?
Click to expand...

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.
 
Brett

Brett

Member
Joined 2002
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.
 
Brett

Brett

Member
Joined 2002
madisqbu said:
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.
Click to expand...
Omit the tone controls on the bass or the preamp?

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

madisqbu

Member
Joined 2007
Yes Brett, I am a talkbasser.;)

I modified the schematic a bit - erased some parts from it (when being correct). What do you think - can I do such modifications - will it work?
 

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Brett

Brett

Member
Joined 2002
madisqbu said:
Yes Brett, I am a talkbasser.;)

I modified the schematic a bit - erased some parts from it (when being correct). What do you think - can I do such modifications - will it work?
Click to expand...
Too much gain. Each stage has 36dB or so and removing the Fender stack whic has an insertion loss of 20dB flat or 14dB in the centre detents is too much. Way too much.

Redesign around a higher transconductance tube like the 6N1P.
 
M

madisqbu

Member
Joined 2007
:xeye: oh, this is getting too complicated for me, i think i'll stay to the original design. But, yes, my original purpose was to make something like a tube di (with gain and vol controls) what could also power a power amp. So its something like A-designs REDDI or (as mentioned above) Lowend truevoice. Does anyone know how to make something like this? Schematics? should i instead look for mic preamp schematics, when i want to make something like it? My goal is to make something as simple as possible and sounding as neutral/flat as possible?

greetings,


Madis
 
G

gerryc

Member
Joined 2005
madisqbu said:
:xeye: oh, this is getting too complicated for me, i think i'll stay to the original design. But, yes, my original purpose was to make something like a tube di (with gain and vol controls) what could also power a power amp. So its something like A-designs REDDI or (as mentioned above) Lowend truevoice. Does anyone know how to make something like this? Schematics? should i instead look for mic preamp schematics, when i want to make something like it? My goal is to make something as simple as possible and sounding as neutral/flat as possible?

greetings,


Madis
Click to expand...

If you want to stay to the original design, the only thing I would suggest is to replace the Fender-style tone stack with a Baxandall arrangement (look at the preamp section of almost any Ampeg bass amp, e.g. http://www.drtube.com/guitamp.htm#Ampeg ) and you can pretty much just lift the bass/treble arrangement and substitute it into the F-2b circuit. The reason that I say this is that you stated that you wanted something that is as neutral / flat as possible. You can get that with the Baxandall arrangement, but not with the Fender style tone controls. Check out Duncan Amps tone stack calculator and play around yourself.
 
M

madisqbu

Member
Joined 2007
to gerryc. Good idea. I did a little search and found this - http://amps.zugster.net/articles/tone-stacks#Baxandall

As I understand, this version is made for using with guitar amps. What should i modify? (for example: replace the 500k pots with 250k pots?, or just loo ampeg preamp schematics and take values (resistors/caps) from there or from original (alembics) design.

So as you already noticed, i'm a bit (not to say very) confused.

regards,


Madis
 
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