12AX7 tube preamplifier for bass guitar (Need help)

[fingerboy21]

I am building a tube bass preamplifier and I would like some suggestions on what I can do to improve my current design. I am interested in getting a very flat response as well as the ability to overdrive slightly when the gain pot is in the higher region. I can use either one or two 12AX7 tubes.



Thank you!

[bob91343]

You are missing grid return resistors on both tubes. What is a 50% pot? No resistance value shown. I assume you have figured out the operating point and the plate load resistors are the proper value. I didn't check.

That's a lot of gain, and you will easily overdrive it I am sure. If you have voltage gain of, say, 70 or so per stage, that's a total gain of about 5000.

[fingerboy21]

The pots are 1MOhm, the 50% is just showing the position of the pot at the moment. I don't know much about grid return resistors, could you please explain? Thanks!

[mashaffer]

The grid of each tube needs a reliable DC path to ground. This resistor from grid to ground is often referred to as the grid leak resistor. With out it your tube will not bias up properly. If you remove the 0.1u cap from the input of the first tube for example you would have a 2K grid leak resistor (the 1K would be a grid stopper if soldered directly to the tube socket).

The coupling capacitor between the two stages should be moved to before the pot. This will remove all DC from the pot. The pot can act as a grid leak then but I would recommend using a smaller pot and adding a fixed grid leak of about 1 meg to preserve bias when the pot's wiper goes open.

Are you sure that your guitars pickup likes a 2K ohm load? seems awfully low. You might also want to consider whether a 12AX7 in common cathode has enough stones to drive the EQ. You might need a follower in between.

[Miniwatt]

I don't like the EQ at the end, I'd put it in between the stages.

[fingerboy21]

I have an active bass guitar. It's impedance is lower because it has an onboard preamp. So there should be a resistor from pin 2 and pin 7 directly to ground?

Thanks, I appreciate the fast responses.

[mastodon]

So there should be a resistor from pin 2 and pin 7 directly to ground?

Yes, and a fairly big one at that. Something in the range 100k - 1Meg.

The 2k resistor at the input still looks rediculously small. Assuming that your bass outputs something comparable to line level, I'd still use 10k or bigger. But in a guitar/bass amp for regular (passive) pickups, 1M is a usual value.

Also, the grid stopper resistor at the input (1k), I'd swap it with the decoupling cap, and the resistor should be soldered directly to the tube socket (i.e. as close to the tube as possible).

I agree with Minitwatt's remark on the EQ position, but for a specific advice, it would be nice to know the entire chain of devices and their properties (i.e. input/output levels and impedances) and the result you're after.

Apart from the above, the circuit you devised will work as intended (if you ground the grids, that is): it will amplify like crazy and drive the second triode into distortion with the gain pot opened up. Depending on the output level of your active bass, the input triode might distort mildly as well (although it certainly won't if you keep that 2k resistor at the input); this could be a drawback, as your only way of controlling that would be the volume adjustment on the bass itself.

Note that you will have to provide a really clean B+. The circuit you designed provides very high gain, and has poor power supply ripple rejection. So it will likely hum like a Chevy V8. Edit: since you tied both anodes to the same B+ supply, I suppose the second stage will cancel out any ripple that the 1st stage sees. Also, I spotted your other thread in which you are designing a power supply for this unit. Looks to me like you'll tackle that one, as long as you build neatly.

Oh, before I forget: although the second stage will distort, it will be an asymmetric distortion that may just what you're looking for. However, it's not going to be a metal-shred-total-fuzz-abstract-shizzle type of distortion.

[Dragondreams]

Originally Posted by fingerboy21

I have an active bass guitar. It's impedance is lower because it has an onboard preamp.

I'd still be tempted to use a higher impedance input, even though the bass is active. 100K or more is the norm.

Originally Posted by fingerboy21

So there should be a resistor from pin 2 and pin 7 directly to ground?

You got it. Values from 470K to 1M are a good starting point.

As already mentioned, put the pots AFTER the coupling caps so you don't get DC on the pots, which can make them sound "scratchy" when you turn them.

Also mentioned is the idea of sticking the eq at the end of the two stages. You might find that you lose quite a bit of gain, depending on the flavour of eq you plan to use. Consider adding another 12AX7 as a cathode follower to drive the EQ section.

Paul.
Edited... by the time I'd finished typing, mastodon had already answered the questions.

[fingerboy21]

Haha, a slight overdrive is what I'm going for, but NOT like shredding distortion haha. I would like to have to set the gain control at about 70-80% to get some overdrive.

My EQ is a 2 band parametric equalizer that uses Op-amps with pots to control gain, Q, and frequency center. I also have a balanced line driver to be used at the output so I can used balanced cables into a recording console or long cable lengths.

Ill work up a more complete circuit for you guys to check out.

Thank you so much, you guys kick ***.

[fingerboy21]

I don't know how to implement a cathode follower since this is my first exposure to tube engineering. We didn't learn any tube stuff at my college in my Electrical Engineering Technology major, so my education on tubes is all self-learned. This is actually my senior project.