Bass/guitar tube preamp with small power stage

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Hi!

I've made quite a few solid state amps and stompboxes, but this is my first tube project, and I'm a novice in this area. The idea is to make a preamp for bass (though it should work for guitar as well) that produces a fat line level tube sound that I can then patch to a big SS power amp.

My goal is to get an amp that can deliver a nice clean sound, but that will distort when pushed, much like an ordinary tube head.

The basic principle is a 12AX7 preamp with a 6L6GC (or similar) power stage. I would then use a false load and attenuate the speaker output to line level. I've thrown in a Hamond 125DSE 3W output transformer in this schematic, but I could use the 5W or 10W version if this one doesn't cut it.

My major concerns are:
1) How should I bias the 6L6 to match the output transformer, or should I have a larger trafo to begin with? The idea is to be able to overdrive the output tube if need be.
2) How does the transformer power rating impact the sound?
3) What plate voltage (or B+ from the power trafo) should I aim for? I don't need a huge amount of gain, but the output tube should overdrive when pushed. (This is really a smaller concern, because the resistors in the PSU are cheap and easy to change if proven wrong.)

Other comments or questions are welcome as well! (Normally I would just try and see how it works out, but changing transformers will get expensive in the long run...)

fw_tube_pre-poweramp_0.9.png


/Fredde
 

Beachcoma

Member
2012-07-24 10:10 am
Looking at the 6L6GC, in single ended ClassA it'll put out roughly 10W. In trying to get that "overdriven power stage" sound of a screaming Mesa 400+ or Ampeg, keep in mind that a large part of the sound is not the output tube but the undersized Xformer. Nonetheless a 5W will be better suited to getting a bit of clean headroom as well.

As for the B+, 300 to 350V will get you headroom and overdrive possibilities, as your preamp section seems to be able to put quite a hot signal into the power tube.

You might want to up the capacitance in the PSU a bit though, if you want it to be dead quiet. Exchanging the 22uF to 47uF should dampen ripple to acceptable levels.

Also, your 10W load might bite the bullet early unless you force some airflow over it. I would consider paralleling two 10W 16R power resistors for this project.
 
Looking at the 6L6GC, in single ended ClassA it'll put out roughly 10W. In trying to get that "overdriven power stage" sound of a screaming Mesa 400+ or Ampeg, keep in mind that a large part of the sound is not the output tube but the undersized Xformer. Nonetheless a 5W will be better suited to getting a bit of clean headroom as well.

Yes, I've been reading in on this a bit after my first post, and the output trafo indeed seems to be more important than some particular power tube. Since quietness is important for me, I'm thinking I might change the out the power section for a push-pull stage with a 12AU7 or 12AT7 (like in the FireFly amp). This would also allow for a slightly smaller power trafo, and a smaller output trafo. I'm thinking the 5W or 3W version of the Hammond 125 series. I'm not sure how low you can go without risking burning the trafo.
 
6V6 output ? or ecc99 ?

your priority is not clear to me .... a preamp which holds a small power amp, or a small power amp with preamp line out
Sorry if I was unclear, what I meant was to replace the single ended 6L6 stage in the schematic with a push pull 12AU7 stage.

Like Doug Hammonds FireFly:

An externally hosted image should be here but it was not working when we last tested it.
 

tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
ahh, ok, I see, thanks...how about ecc99

btw, single ended output is generally considered better sounding on overdrive guitar...but I play bass, so I might be ok with pushpull :D

but 'soundwise', I might think about a 6V6 pre/headphone/power combi
 
ahh, ok, I see, thanks...how about ecc99

btw, single ended output is generally considered better sounding on overdrive guitar...but I play bass, so I might be ok with pushpull :D

but 'soundwise', I might think about a 6V6 pre/headphone/power combi
Well, this is intended as a bass preamp. I don't know anything about the ecc99. 12AT7 and 12AU7 (ecc81 and ecc82) are the ones I've usually seen in schematics over this kind of power stage. What would be the benefits of using the ecc99? It seems to have the same pinout, but would it require any modifications to the design?
 
very tough preamp/driver tube

google ecc99 mini amp (its on DIYstompboxes)

might be possible to make it stronger and better

:p and patiently waiting to see what the ManicMidget is about

Fred Nachbaur's Vacuum Tube Projects

Ok thanks! Im only wondering if it's necessary since I'll only need line level output, and the only important thing is to match the power tube with the OT. Im not sure how the ecc99 is compared to a 12AU7 soundwise, but if they're roughly the same, a 12AU7 should work equally well (but maybe with a smaller OT).

Also, the ecc99 is physically bigger and I'm planning to fit everything into a 2U rack housing.
 

tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
Well, this is intended as a bass preamp.

just thinking out loud :clown:...but since you study guitar amps to get inspiration, may be important to consider that they could be designed for overdrive guitar sound....and you may not need that for bass

whatever, I kind of liked your first idea/schematic

don't know why, but I find this site... 'toxicating' :D

LONE WOLF BLUES COMPANY


Sorry if I was unclear, what I meant was to replace the single ended 6L6 stage in the schematic with a push pull 12AU7 stage.

Like Doug Hammonds FireFly:

you mean only the output stage ?

heck, sometimes there is simply just too much stuff and ideas to try....
 
just thinking out loud :clown:...but since you study guitar amps to get inspiration, may be important to consider that they could be designed for overdrive guitar sound....and you may not need that for bass

whatever, I kind of liked your first idea/schematic

don't know why, but I find this site... 'toxicating' :D

LONE WOLF BLUES COMPANY
Yes, I've thought a lot about that and it's much easier to find schematics for guitar. The idea is to get a clean sound but with the possibility for a *slight* overdrive when pushed. I guess I'll just have to experiment.

One important thing is to get the right B+ voltage for appropriate headroom, but that can be fixed pretty easily by changing the RC-resistors in the PSU. Cathode resistors and capacitors are also easy to change. The difficulty, as I see it, is to get the right output transformer on the first try, since these cost money. I see that the ecc99 mini amp you mentioned uses a Hammond 125B which is 5W, so maybe the 3W version would be enough with 12AU7 as power tubes.

But: I've read that an OT will saturate easier at low frequencies, so I might need a bigger one for bass. I'm not sure about this one though.
 
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