Bass Pre amp Build

[Lynxtrap]

Two channels, the first based on an Ampeg B15 and the second one is more like a Fender.

They share a third gain stage, added for some extra harmonic build up and sustain (or to simulate a PI if you see it that way). There's no need for the gain really, but I feel the third stage really added something soundwise, and I'm pretty happy about it. It sings and blooms in a way that I think would be hard to obtain with only two stages.

After that there's a cathode follower for low output impedance.

The "Ampeg" channel was pretty good from the get go, but I would probably not include that mid shift pot if I made another one.

The second channel was more problematic with the tone stack being the main problem, as I saw it. After starting a thread about it in the Dumble section here, I ended up with a Skyliner with a Jazz/Rock switch.

There was still a bit too much bottom end coming through, and after some testing I ended up with a 4.7 cap on the first cathode. Smaller than that had too much effect, especially when tuning down the E-string to D and B (like it would be with a 5 string bass).
To add a little more top end I found that a cap across the volume pot worked best, and tamed a little with a series resistor.

Channel 2 was still a bit overpowering with a 12AX7, compared to Channel 1, and I had come as far as I could with the voltage dividers at the mixer stage. Not that it wouldn't work, but I decided to put a 5751 in there and that will stay for the moment.

I'm sure there will be some more minor tweaking when I get my bass player buddy to run a second test through a real bass power amp and speaker.

The build isn't particularly beautiful to look at, not that I find it to be of great importance as long as it sounds good without noise. I planned the layout myself from scratch, and if I ever make another one I would probably be able to optimize a few things and minimize the amount of wire... I guess this can be viewed as a prototype.

Bass preamp rev5.jpg

Pre front.JPG

Pre side.jpg

[Reeltarded]

Nice looking. Skyliner is weird, huh?

[roberto]

Just add a class D power amp with SMPS on top of it, and you'll get a rackable lightweight bass amp.

[Lynxtrap]

Nice looking. Skyliner is weird, huh?

Thanks! Well, tone stacks are weird if you ask me
The Skyliner works much better than the standard Fender IMO.

[Lynxtrap]

By the way, a question: I don't understand why I get such high plate voltage on V2B and V3A, 227V and 239V compared to 203V on V2A.

I can't find the paper with the numbers I measured a couple of weeks ago, but I don't recall this kind of difference then.
Things that have changed since then: new filter caps with higher voltage rating, different tone stack in Ch2, the 5751 in V2, plus some small resistor and cap changes here and there.

[Stevem]

The better a tube is ( stronger) the more current it will consume and in turn the lower the plate voltage will be along with a higher voltage across the cathode.

[Lynxtrap]

The better a tube is ( stronger) the more current it will consume and in turn the lower the plate voltage will be along with a higher voltage across the cathode.

Yes, but still the difference between the halves of V2 seems big to me. V2B is a biased a little colder, but would 1.5K vs 2.2K on the cathodes pull the plate down from 227V to 203V, both fed from the same node?
I was starting to suspect a funky resistor or tube.

[Reeltarded]

It seems like a lot, but it's ten percent. I don't generally like the response difference of a softer substitution, myself. Put an AU in that hole and think about that for a second or two..

Meh.

I would use AX7 and figure out the issues with a circuit change.

Oh sorry looked at the schmatic and reread what you said. It should be closer both sides of that tube.

[stelligan]

Thanks for sharing. Have been looking for a B-15 type setup for bass tracking and decided to build either and amp or preamp. Good info.

[Lynxtrap]

Thanks for sharing. Have been looking for a B-15 type setup for bass tracking and decided to build either and amp or preamp. Good info.

You're welcome, good luck with the build!
If I had to choose one of the channels in my build, for a pretty neutral bass preamp with some options, it would be the Ampeg.
Just by looking at it, I thought it would be a bit tame and dull, but that's not the case. But as mentioned, I think the extra stage adds its bit to the equation as well.

I also think this version of the Ampeg pre has an advantage in the volume pot being placed before the second stage with the EQ after that stage.
In conjunction with the third stage and master volume, you can drive it to a "sweet spot" where you get some sustain, overtones and maybe a bit of compression, still without audible overdrive.

[Lynxtrap]

I've tested it through sereral amps and lining it into my computer soundcard without problems.
Tonight a brought it to a band rehearsal where the bass player plugged it into the effect return of his rig, using only the power amp.
Then, maybe becaus the amp cabinet had a tweeter, there was a pretty disturbing amount of hiss coming from the preamp.
The hiss follows the channel volume pots and master, also with nothing plugged in.

Where would you start looking for this problem?

Here is the schematic again:

[Lynxtrap]

I don't generally like the response difference of a softer substitution, myself. Put an AU in that hole and think about that for a second or two..

Meh.

I would use AX7 and figure out the issues with a circuit change.

After the latest test with the right player, instrument and power amp/speaker, I am totally with you on that!
I had been playing my cheap Ibanez through it, into a just as cheap fullrange 40W keyboard amp, and thought the 5751 worked just fine.

During the real test, the tone stack was tweaked (as related in the other thread), but something was still not right. Frequency response was decent, but both me and the bass player missed the sustain, overtones, and most of all the touch response that the channel had last time we tested it.

At first I couldn't understand what the reason for such a big change could be. I saw the 5751 is a well tried and trusted tube, only with a little less gain than a 12AX7.
But I finally pulled it and put the Tung-Sol 12AX7 back in. Whoa! There it was. The magic was back.
We're not talking small nuances, more like night and day. I have never experienced anything similar with tube swaps in guitar amps. Eye opening for sure.

[Reeltarded]

Lower output tubes always seem like a bandaid on an unresolved other issue. I'd rather struggle with the unknown any day of the week.

[Lynxtrap]

Who knows, someone might find this in 15 and try to build something like it
So I thought I'd upload the latest, and hopefully the last, version of this one.

Just wish I could get going with the worst part (for me): the wood work. I will make it a small head. Already built one version without taking the thickness of the tolex into accout, so there goes that.

TF Double-B preamp rev6.jpg