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Old 24th July 2006, 08:39 PM   #1
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Default Bass guitar preamp

Hi!!!!

I'm looking for a tube bass-preamp schematic. Does anyone have a circuit ? I searched for half hour and didn't find anything.

Thanks!!!
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Old 24th July 2006, 11:07 PM   #2
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I am searching for info on the exact same subject. So far I haven't found much. Im looking to build a VERY simple preamp, preferably with tubes.
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Old 24th July 2006, 11:30 PM   #3
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Why don't we try and figure something out together? The only thing I've come up with so far is a 6N1P and BUF634. I can't find much info on signal levels and such. Do you have any idea what the normal signal level from a passive bass pickup is? The gain of the 6N1P is 33, and from what I can figure out that seem to be roughly 15dB.
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Old 25th July 2006, 02:52 AM   #4
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Hi!!!!

I dont know how much I can help to design a tube amp. What I can offer is some tube manuals/datasheets, my father used to build tubes stuff. But he dont know about hi-fi, only radio amps......

I Think that I have lot of 12ax7 tubes for the project.

I will use an active bass guitar so I need both inputs. I dont know how to design a preamp for bass, I dont know nothing about impedances of bass pickups.

If there is anything i can do, tell me. I also have some tube manuals, schematics, but they're like 30 years old.

thanks!!!!
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Old 25th July 2006, 07:32 AM   #5
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Just tell me what kind of features you want. Im going to buy my first bass guitar in the next 2 weeks. I have never played any kind of string instrument before, so you probably have a lot of things to teach me about what kind of effects and tone controllers would be nice to have. My first idea was to just build a small gain stage with a low mu (gain) tube and just a buffer but it looks like the signal might me a bit high to start with. I was aiming for just a bit over normal home stereo equipment line level but assuming closer to 2-300mV from the guitar Im afraid i will blow up the opamp or buffer on the output. I cant do any measurements before I get my bass.
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Old 25th July 2006, 07:44 AM   #6
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You could try to build the one in the picture. It's a very simple schematic and sounds very good indeed, I recently built one myself. The original Alembic unit has two identical channels (which are modelled after Blackfaced Fender amps). I built a preamp with only one channel, and added "bass-shift" and "treble-shift" functions like present in the Mesa Boogie Bass400. I have built that preamp as well (a long time ago), and didn't like it much.

I did redesign the powersupply with DC heater supply, and B+ with a overkill amount of capacitance (had a bunch of caps lying around).

The preamp is easy to build and sounds super, I would say a fine candidate for a first project (if you treat the HT with the appropriate amount of caution).
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Old 25th July 2006, 07:47 AM   #7
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And this is the Mesa Boogie Bass 400 schematic:
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Old 25th July 2006, 08:13 AM   #8
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Looks nice and simple but since you use both halfes of the 12AX7, wont that produce way too much gain? Have you measured you "maximum" output signal voltage?
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Old 25th July 2006, 08:36 AM   #9
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Hello Mikael,

I am not much of an EE guy (rather ME) so I didn't measure anything, but this schematic is built by Alembic so I just considered it a finished design. I did hear someone say there is a mistake in the schematic somewhere.
But you are right, when using my (active) bass, the output is quite "hot", I am thinking of adding a 6db pad.
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Old 25th July 2006, 01:07 PM   #10
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thank you all for the replies !!! interesting schematics.

I think I will build the preamp stage of this one:

Click the image to open in full size.

As a power amplifier, I have a LM3886 gainclone,... do you think that I can use the output of the master pot as output to feed the gainclone?

thanks!!!
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