TUBE STAR - 12V-50V tube overdrive

[Renegadrian]

I started playing around with my breadboard on a mesa boogie schem - the 12ax7 doesn't sound good with the changes I made, but it's good with a 12au7.
It uses 12V to power the heaters and 50V on the plates - I used the voltage multiplier I built, you can find it here -->http://w1.859.telia.com/~u85920178/power/vconv_01.htm

I made some changes, took out some components, and gave it a cool name...it's just a 5 minutes build now, so I guess there are some minor changes to be made, but it sounds good even at this stage...
In my breadboarded version I didn't put the final 25n cap...
So here it is, version 0.1

[km-r]

whats wrong with the 12ax7? i like the look of this circuit... Lone star?

[Ice-9]

Hi, Nice looking circuit there, i will have to breadboard this one and try it out. I have one question though, in the schematic you have the 1n and 560k resistor that are parrallel between valve stages shorted out with a wire . Have i misread the schematic ?

[punkin]

Nice! What sort of tonal characteristics does it have? is it clean or crunchy? I like the looks of this.

[Ripthorn]

Hi, Nice looking circuit there, i will have to breadboard this one and try it out. I have one question though, in the schematic you have the 1n and 560k resistor that are parrallel between valve stages shorted out with a wire . Have i misread the schematic ?

My guess is that where there is the wire there is supposed to be a gain pot, but I definitely don't think there should be a short.  But other than that, it is a nice looking circuit.

[km-r]

yep, that shouldnt be... but i might be the 2m2 resistor that acts like a gain pot that was maxed...

the cathode bypass capacitor is in series with a 47k resistor... i think the 47k resistor will null any effects of the bypass cap in the audible spectrum. correct me if im wrong...

[Ripthorn]

The 47k resistor will simply limit the difference between the treble and bass response.  So instead of being maybe 6 or 7 dB, it will be more like 3 or 4 dB (where the treble is higher than the bass).  I actually have that resistor that can be bypassed with an spst switch on my current tube project, it is great for a little flavor change.

[Caferacernoc]

What a coincidence. I was playing around on LTSpice yesterday and came up with something very similar. I was simulating a valvecaster type unit run on 36 volts(four 9 volt batteries is the plan) and I came up with similar values as you except no 47k in series with the bypass cap.

[Renegadrian]

thx for the replies - Let me point you where it all began...this is a stripped out and modified version of the mesa boogie preamp you can find here--> http://www.freeinfosociety.com/electronics/schematics/audio/pictures/mesaboogiedualrectifier1.gif

It has a good crunch, not the metal distortion I wanted but hey it's so nice - you can add a pot for the gain, I just went over it...
I tried all the combinations on the first cathode, like 2k2+1µF, 2k2+47k, 2k2+47k and 1µF, 47k alone, 1µF alone, 47k+1µF
No big differences, but hey you can try them....

[Ripthorn]

On my project I recently swapped out the 1k8 cathode resistor for an 820R and it added a little more saturation to it.  I think that a general rule is the lower the cathode resistor value, the more gain you will get, but then again, I just started in on these things.

[kurtlives]

That partially bypass cathode cap on the first stage has a pretty big limiting resistor.

From my experience 1K - 15K work best with 12AU7 designs like this.

[Renegadrian]

I'll try that tonight
Also, the original res on the second plate was 100k, but 47k seemed a little better to my ears...

[frequencycentral]

Neat see you experimenting Adriano. A few observations:

Is that a short between the two stages, or a mistake in the schematic?

It's more usual to have the grid leak resistor (2M2) the other side of the grid stopper resistor (470K).

The grid stopper seems a high value, tubedummy: Values can be 1.5K->100K. Larger values roll off more highs and reduce gain between stages.

Why no input cap? Did you try it with one?

So maybe add an input cap, get rid of the short, move the 2M2 to right next to the grid, and in it's place have a 1M interstage gain control, and have a play about with the value of the grid stopper.

[German]

Input caps is usual for non-tube projects.
There is possible an input resistor - 33-75K.

[Renegadrian]

Checked back the breadboard, yeah I should remove that line from the schematic...

Tried to follow some of your hints before I finally go to bed...
german, tried a 47k res. between the input and pin 2 - no big deal...
tried other res. value with the cathode cap - no big deal too
finally I got rid of the cathode cap/res. couple...
lowered the remaining res. to 470r - thx Ripthorn
tried to move the 2m2 res. after the 470k - no big difference...

Updated and added a quick sound sample

[Ripthorn]

That sounds really great, Adriano.  Was that clip through an amp, cab sim, or direct?  I think I may have to pick up a 12au7 and give this thing a shot.  It may have to wait, as my submini amp is taking a lot of time and I want to make sure I do it right.

[Eb7+9]

I like the fact that you have no capacitor bypassing in either stage, and that you're using a quieter hi-fi tube ...
good job !

[Renegadrian]

Brian, the clip was (quickly) recorded with this setup...
Guitar-->Tube Star-->PC audio card. No amps or amp simulators.
Maybe I'll see if I can plug it into an amp when I come home from work...

thx to everybody for the nice words and suggestions...

[Ripthorn]

You know, you actually inspired me to give this a shot, so I just bought a lot of 6 12au7's on ebay.  They were cheap and I have some 6aq5's hanging around, so I have pedal and amp possibilities galore.  This is definitely going on the board as soon as the tubes come.

[liddokun]

That sounds really great! I'll have to breadboard this guy as well. Good thing I have one spare 12AU7 in my closet.