12ax7 gain stage incorporated into Tiny Giant, possible?

[perpetual-amateur]

So with the recent posting of the Ruby Tubey vero layout and my impending plans toward a Tiny Giant build, I got to thinking, could the 12ax7 gain stage from the RT design be incorporated into the Tiny Giant design to arrive at a low voltage 20w hybrid amp?

The Ruby already has a 072 pre, so it looks to my unskilled eyes like the valve stage might be able to be crammed in there.

Thoughts?

[rocket88]

i've been thinking the same thing actually. i've been thinking about making a little low wattage amp for awhile, especially after seeing this, from zvex.

[perpetual-amateur]

That's pretty sick, but what if we could build something about the same footprint that pushes out 20 watts?

[rocket88]

Agreed. Even if it was a in a 1590b, I'd be down with that.

[IvIark]

It's totally do-able, you just need to insert the 12AX7 stages between the Tiny Giant preamp and power stages in exactly the same way as it's done in the Tuby.
In fact you could just use the existing Tiny Giant layout and just add a small daughterboard for the tube stage connections.  It would be an interesting experiment.

[perpetual-amateur]

Oh Nelly, this is my new white whale!

[perpetual-amateur]

Ok, I've been looking at the schems for both for a while and it damn near looks like you could insert that daughter-board right in the path before the volume control, but the Ruby's pre is configured differently, with a gain control and a bunch of different values.  Since the TDA on the Giant is just a clean power amp, wouldn't most of the mojo come from the Ruby's voicing of the 072?  And wouldn't that gain control be a pretty critical element?

[IvIark]

Yep, I'd be more likely to use the Ruby and omit the 386 for the Tiny Giant power stage

[motterpaul]

""Yep, I'd be more likely to use the Ruby and omit the 386 for the Tiny Giant power stage""

You know I am a novice, but I built one circuit using the LM386 as a gain stage and it was way too wooly. Now I know that in the tube circuit the 386 is there solely as an amp - but I can't help wondering if the 386 is also getting pushed pretty hard, into clipping,

Because in my mind a 12ax7 pre should have very sweet and consistent overdrive; some "hair" yes, but not like a fuzz tone. The demo of the 12ax7 Ruby posted here sounds to me almost exactly like the LM386 circuit I built from here (and I do know the difference in design, I am just saying).

My question is this - why not just build the 12ax7 pre-amp with NO amp stage - to really see how it sounds, and then you can start getting ideas on how you want to boost it. This is the normal procedure here anyway.

Personally, I would LOVE the knowledge of how to build a good VERO 12ax pre-amp, but I do not care for the sound of the Ruby one and I am opting not to try it now. I have faith in you guys that you can really dig into that tube and get all kinds of sounds out of it - by just skipping the amp stage all together for now.

[perpetual-amateur]

The point is well taken that some of the dirt is coming from the LM386, but I also agree that that's dirt I don't need.  I'd be happy with a moderately overdriven tube sound even if it lacked real dirt, that could always be added via a pedal.

So here's what I've worked up so far:

I started with the edge formatted vero layout from Insulator over at diystompboxes

(Read the post for an explanation about the elements of his layout, specifically his treatment of the TDA pins, this is a verified layout fwiw.)

I reworked the TL072 section from the schematic and lvlark's layout for the Ruby and came up with a small daughterboard for the 12ax7 section.

Here is the current state of affairs:

EDIT: (Image temporarily removed due to major errors)
Edit to the edit... here is the updated image.  (Not saying there aren't major errors, just that I fixed the ones I found.  I could use some feedback.)

3rd Edit... taken back down for update in later post.


The parts on the upper board that are outlined in white are altered from the original layout and should correspond with the Ruby design.

I'm a complete newbie hack at this so I'd appreciate any corrections, comments, etc.

[motterpaul]

Perpetual amateur...

Thank you for reading my comments and taking the time contemplate what I was saying. Of course, I was just guessing (and still don't know) if the hair I heard was coming from the 386, but in any case I am very happy and impressed that you manged to do what I am still too amateur to do - design a discrete 12ax7 pre-amp. I will taking apart some odd piece I have here to retrieve the proper tube socket and start working on this right away.

I can't wait to hear this...

[perpetual-amateur]

Just to be clear, this is a design for a full hybrid amp, not a tube pre.

On Oct 13, 2014 12:21 AM, "motterpaul [via Guitar FX Layouts]" <[hidden email]> wrote:

Perpetual amateur...

Thank you for reading my comments and taking the time contemplate what I was saying. Of course, I was just guessing (and still don't know) if the hair I heard was coming from the 386, but in any case I am very happy and impressed that you manged to do what I am still too amateur to do - design a discrete 12ax7 pre-amp. I will taking apart some odd piece I have here to retrieve the proper tube socket and start working on this right away.

I can't wait to hear this...

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[perpetual-amateur]

Bump...

I could use some other eyes on this before my parts show up and I end up breadboarding some dysfunctional monstrosity...

See the re-revisited post from Oct. 11th above.

Thanks in advance.

[motterpaul]

I apologize for not getting started, but I had to accompany my wife to San Diego through this weekend, last minute notice, so I am away from my workbench. I was just looking into it when I had to leave.

[perpetual-amateur]

Well, I built this on a breadboard, and it was unstable but worked well enough to urge me forward. So, I built it on vero and put it in a makeshift chassis to eliminate flying leads, loose connections and lack of shielding as variables.  

It's still pretty unstable.

It starts out fine, once the tube warms up it sounds pretty good for a minute or two but eventually it start to fart out and then cut out.  I can speed up the process by cranking the gain or volume, or swinging the tone knob into the bass region too much.  My current theory is that there is just too much gain in the earlier stages for the TDA7240.   In Taylor's original schem for the Tiny Giant, the gain in the pre stage is about 3 and it's a pretty stable little circuit.  (Someone correct me if I'm wrong here) but in the Tuby Ruby we have a potential gain of about 50 in the TL072, followed by almost 40 for each of the 12AX7 stages.

I have tamed the problem a fraction by switching to a 12AU7 and suspect that upping the grid stopper at the first tube stage and adding on before the second could calm things down a bit, but frankly a barely know what I'm doing here and these are my first experiments with tubes.

I'd love some input.  Thanks in advance.

Here is a revised layout.  As you can see I have a modified BMP tone stack in there and have socketed the shit out of things to make tweaking things a lot easier...
RD3.png


Here's a couple of shots of the proto just for fun...

IMG_20141114_224943166.jpg

IMG_20141114_224851464.jpg