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Bass pre amp


The Dude
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I come here as I know there is a wealth of knowledge and I feel this might be simple enough we can come up with a good solution. 

 

About 15 years ago I purchased a product from Ampeg called the SVTDI.  Which stood for SVT direct injection.  Basically a tube preamp designed around the pre amp section of their SVT bass amp.  This was great to give a little tube warmth when connecting to the house system.  It is was what also gave me the idea to run a tube pre amp into a SS power amp (bu thats a hole other story all on its own).

 

Now Behringer makes something similar for around $50.00, which may be worth it.  But from what I have read some of the gear isn't up to par like it used to be.  Plus I would like to build it into a rack mount case.   Now before we go further, what are my attentions with this gadget.  Well I have a camper down on a lake that we visit just about every weekend during the summer.  I was thinking of picking up an acoustic electric bass to jam some tunes (if funds allow if not I have two electric basses I can choose from) as my buddy often brings his acoustic guitar along with a banjo.  If all goes as well we will have our own bluegrass band by the end of summer.

 

Now for the rest of the gear, I have acquired a couple of different rack mount power amps so I have that covered.  I was looking at housing these in one of those road cases.  This would power a Table tuba located somewhere on the deck. 

 

What do you folks think, any ideas for a simple tube pre amp running something like a 12ax7 or some other readily available pre amp tube.

 

 

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Well, since it is to be used for a musical instrument input, then driving some unspecified amplifier, I would think you would be able to build it fairly inexpensively. I'd consider using the 1st half of the 12AX7 as the gain-stage and using the 2nd half of the tube as a cathode follower to lower output impedance and allow you to drive a decent length of cable to your amp.

 

TS Matt.

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http://www.ampeg.com/pdf/svtdi.pdf

 

Most guitar amps don't have a balanced input. Keyboard amps do I guess. The device is meant to drive a long cable from a stage to a soundboard. The block diagram does show what looks like a common cathode gain stage directly coupled to a cathode follower. The cathode follower will have a low output impedance and the output transformer lowers it even further and converts to balanced. Make note of the -14db gain.

 

I find it strange because with all passive pickups there will be a resonant spike in the midrange, the placement is dependent upon the parallel capacitance of the cable and the inductance of the pickup. I think this is why most guitar amplifier tone stacks will scoop the mids to to flatten things out. Of course if you were sending the signal to a soundboard with a competent engineer he would adjust the sound accordingly. But most people that wouldn't know this or don't have the ability to flatten the mids might end up with a lumpy sound with boosted mids.

 

 

 

Other than that I am sure someone here that builds amps could make you something very similar relatively easy.

 

-Cindy

Edited by CindyJarvis
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Just curious….if you're jamming with you're buddy and he's on an acoustic or banjo (un-amplified) why do you need an amp, get an acoustic bass like you mentioned.

A lot of times bass acoustic bass is low that with multiple guitars going it gets drowned out. So to make up for it you could add a amp. Or if i dont puck one up this year, i can all ways use my electric bass.

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Yes. You should still be able to drive a reasonably long cable like 10'-20', pick a higher transconductance tube and get a lower output impedance, plenty of current too, 5mA-10mA.

How much gain do you want? How about adjustable gain? Tone controls? The world is your oyster.

-Cindy

I dont know how much gain i want/need. I do know a volume pot would be nice. No tone controls.

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I think I figured it out.
 
 

P1010121

 

 

 

I biased the follower so that it is really close to 0v so the two stages interact with one another. It should be rich in second order harmonic distortion. Gain is around 18db. You could always stick a volume pot at the input too but I assume your instrument has volume controls.

 

 

What do you guys think?

 

 

-Cindy

Edited by CindyJarvis
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I can draw something up if the tube guys haven't already.

A couple more questions. Are your bass' pickups active or passive. Same question for the acoustic you plan on getting.

-Cindy

Good question, lets just say both.  As one bass is passive and the other is active.  Don't know what the acoustic will be, I am guessing passive but if they have a built in eq than active. 

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What do you guys think?

 

I don't know if the technical stuff is correct as I am no tech.  However it does make me want to go dig for parts.  Most of those look like basic common parts.

 

What are the voltages on the caps? Are any of them electrolytic?   What does the 1 meg represent?

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The caps can be what ever dielectric you want, Aluminum are the cheapest. The 47uF must be 10v or higher and the 10uF must be 250v or higher. As for the 10uF cap, if you want to go with a higher quality film cap you can lower the value depending on what you think it will be driving. Basically if it is being plugged into an amp I don't think it will see anything lower than 100k, this means you could use .33uF.

 

10uF was kind of a safe value so you wouldn't get any bass rolloff while driving heavy loads.

 

 

The 1Meg is a load resistance which will be in parallel with whatever you plug in.

 

-Cindy

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It seems like the original Ampeg device was intended to add tube sound when a guy would have to plug into a board instead of an MI amp.

Pretty much. The problem is that the classic Ampeg sound just can't be duplicated on a typical solid state amp or PA system. An Ampeg tube amp doesn't just add warmth, it has its own sound, just a very distinct growl. Ideally you'd have a full Ampeg tube amp but you can get most of that sound with just a tube preamp which is what I imagine this separate preamp is doing, which is the same idea as their other solid state amps where they have a tube preamp. I realize that's what they claim but I would be shocked if it sounds good on an acoustic upright bass. Adding an Ampeg tone to an upright bass seems odd but maybe it's awesome. I've played on Ampeg, Gallien Krueger, Fender, played in a band with some Mesa Boogie, it's interesting how the different bass amps sound so different.

Edited by MetropolisLakeOutfitters
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I'd be rather surprised if a two stage triode preamp will give the "Ampeg sound" of the Ampeg tube guitar amp. That sounds comes from saturating an output transformer with attending speaker.

"Warmth" is a vague concept. Unless it's designed into this little pre stage, it's not a guarantee. Right?

I've played a lot of Fender amps and I can't imagine trying to get the sound of a Deluxe from anything but the amp itself.

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  • 2 weeks later...

Ok, just thinking out loud and trying to utilize some gear I all ready own.  But would there be a way to install a bi pass on a tube preamp (my vta 6 with phono input) that would send the input signal (the bass in this case), to a dsp of some sort (minidsp in this case).  That would only send the low frequencies to a bass amp to the sub woofer. 

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