Lets build a All-Tube Bass Preamp.

[BassTweaker]

Hello folks!

This idea of mine has been growing and growing in my head and recently I have finally got some time to put some effort into it too..

The idea is to build/design a All-Tube Bass Preamp (no transistors, no fet's, no op-amps, just TUBES).

Time to lay down some specs?

- All Tube!
- No overdrive function whatsoever, all clean signal in and out (Gain from unity to +10dB?)
- 4-band active EQ (Bass, Lo-Mid, High-Mid and Treble)
- Possibly a DI output (if possible?)
- Other goodies?

I've got a basic degree in electronics, BUT I KNOW VERY LITTLE ABOUT TUBES
So this is where I'm gonna need your help people, your knowledge on how to solder together a reliable and good sounding preamp.

First off, feel free to comment and suggest improvements. If you see something that's wierd or just plain wrong to you, please tell me and I'll see what we can do to improve the circuit.
Here's a quick mock-up of the initial idea I did in LiveSPICE...



So what do you people think? What's the first thing that's wrong with this circuit???

Cheers,

"Bobby"

[PeanutNore]

The first thing wrong is the 12 volt supply which is about 15-20 times too low.

The second thing is that the first triode isn't going to amplify anything without an anode resistor.

The third thing is that your "gain" control isn't going to control the gain but rather the bias point. And as a cathode resistor the value is 20 to 50x too large.

The fourth thing is that the anode resistor in the second triode is way too large for the cathode resistor (and for a 12AU7 to work well) if you want to operate in the linear range.

I would run the whole thing on 220 volts, replace the gain pot with a 1.8k resistor, and use a 22k anode resistor on both triodes. If you want a gain control before the tone stack, use a pot as a voltage divider. Varying the gain of the triode would mean using a pot with a huge DC voltage across it - if you replaced the 22k anode resistor with a 10k resistor and a 10k variable resistor in series that would control the gain, but that seems like a recipe for a fire or a bad shock.

[PeanutNore]

It looks like a 15k anode resistor and a 820 ohm cathode resistor give you a nice operating point if you want a 12AU7 to stay clean and linear.

[Chuck H]

There are plenty of good examples of tube preamps for bass. There are a couple of things in your post that imply a failing lack of experience for a project like this. There are a lot more in your proposed design. You might do well to start with a known circuit and then come here to ask about desired modifications. This might give you a better understanding of the circuits and relieve us of the task of designing your entire project.

[loudthud]

A variable gain stage, the kind where voltage gain is controlled by global feedback, is nice to have as a first stage. It can accommodate most any bass guitar and has a low output impedance to drive all those tone controls. This takes at least two triodes. Still, the impedances in the tone networks need to go up about 10 times. The active tone stage also needs a low output impedance to operate properly with all the feedback to the control networks. A 12AU7 doesn't have enough open loop gain to get the job done in that position, although as a cathode follower it can provide plenty of drive.

[PeanutNore]

Listen to Chuck, he knows what he's talking about. I studied the circuits of other tube amps for about 5 years before I felt comfortable building one and I'm learning new stuff all the time still. Read everything you can, read Merlin's book "Designing Tube Preamps for Guitar and Bass", study any amp schematic you can get your hands on. Especially read all the warnings about the danger from high voltage power supplies and charged filter capacitors before you touch anything.

[Chuck H]

Especially read all the warnings about the danger from high voltage power supplies and charged filter capacitors before you touch anything.

Because being dead sucks.

[mozwell]

I built this about 10 years ago, the amp is still going strong. I love how this sounds.
The DI is solid state, as is the graphic, but you could just leave the whole graphic eq out. I don't normally switch it in these days.
Included in the zip file are details of the inductor i made for this preamp.
The only thing i would change is to replace the output volume pot with a fixed resistor. As there is DC on the pot, its a bit scratchy when you turn it. If you want the master volume pot, it would not be too hard to rearrange this slightly to AC couple the pot. The bright input switch i would replace by a -15dB pad for active basses, but both my basses work fine with the preamp the way it is now so im not going to dig into it & change it around.

This powers a 400W solid state power amp into my 15", the whole amp is really like an SVT on a budget.

[Tom Phillips]

...
- All Tube!
- No overdrive function whatsoever, all clean signal in and out (Gain from unity to +10dB?)
- 4-band active EQ (Bass, Lo-Mid, High-Mid and Treble)
- Possibly a DI output (if possible?)
- Other goodies?...

My comment is that none of your goals seem to be related to a vacuum tube based design other than the "All Tube" requirement.
I like building tube circuits but I don't really see the need for tubes based on your stated goals.

[Chuck H]

Indeed! The only criteria stated that would require tubes for improvement would be "All Tube".

As to other goodies (since it's part of the original subject), how about a small measure of compression? Always good for a bass amp. And certainly more easily done with silicooties (if you've ever built a tube compressor). Oh... And make it a five band EQ. Not having control of the high mids/low treble can be a real problem when establishing a mix.

[überfuzz]

Because being dead sucks.

I think he's on the safe side as long as he uses the 9V stated in the schematics as B+...

[J M Fahey]

Bass Tube preamp design peaked around 1972, over 40 years ago, so save your time, build an SVT preamp and enjoy.

FWIW I do design and sell Bass preamps .... SS of course; but for the customer who **insists** on "all tube" , I don't preach or try hard to convince him, I just plain build my version of Ampeg SVT 1972 (I already made the front panel and PCB silkscreen so why reinvent the wheel?) , mount it in a Rack, and call it a day.

Everybody happy.

[loudthud]

Well, it's been 24 hours and BassTweaker hasn't been back.

I'm wondering if the four band Baxandall is that usefull. The bands overlap quite a bit. Something like the Mesa 5 band graphic or the Fender Studio Bass with gyrators might be more usefull because of the higher Q of the bands.