Bass/guitar tube preamp with small power stage - diyAudio
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Old 15th February 2013, 10:09 PM   #1
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Default Bass/guitar tube preamp with small power stage

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...)

Click the image to open in full size.

/Fredde
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Old 15th February 2013, 10:32 PM   #2
tinitus is offline tinitus  Europe
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looks interesting
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Old 17th February 2013, 10:13 AM   #3
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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.
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Old 17th February 2013, 10:49 AM   #4
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Watching this with interest, as I'm planning on doing something similar over the Easter holidays.

How much difference will the dummy load make?
A speaker is not only resistive, so I was wondering about adding a series inductor ~1mH to simulate voice coil inductance.

Thoughts?

Chris
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Old 18th February 2013, 12:22 AM   #5
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Hello Fredde79,

do a google search for the Garnet Herzog. This was a distortion box that Garnet amps came up with for Randy Bachmann of The Guess Who and BTO. You may get some ideas from there also.

Peace,

Dave Gerecke
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Old 18th February 2013, 07:25 AM   #6
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Quote:
Originally Posted by Beachcoma View Post
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.
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Old 18th February 2013, 07:36 AM   #7
tinitus is offline tinitus  Europe
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SE 6V6 output ? or ecc99 PP ?

your priority is not clear to me .... a preamp which holds a small power amp, or a small power amp with preamp line out
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Old 18th February 2013, 07:51 AM   #8
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Quote:
Originally Posted by tinitus View Post
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:

Click the image to open in full size.
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Old 18th February 2013, 08:04 AM   #9
tinitus is offline tinitus  Europe
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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

but 'soundwise', I might think about a 6V6 pre/headphone/power combi
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Old 18th February 2013, 08:54 AM   #10
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Quote:
Originally Posted by tinitus View Post
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

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?
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