Help with Shaka Tube

[stenns]

Project: Shaka tube in hammond 1590BB Enclosure from Diyguitarist.com

http://www.diyguitarist.com/DIYStompboxes/ST1590BB.htm

Followed plan (PCB) exactly as depicted using TL072 dual opamp.

Makes a harsh, distorted sound only when you strum hard on the strings at full volume.  Gain controll on device is non-functional.  I originally had centred the problem to the preamp stage using the probe device attached to the tip of the input jack [when tip jumpered to grids of triodes, relatively un-distorted signal].  TL072 has been replaced for the third time now to no avail.

Uses 12v AC from a radioshack answering machine adaptor to power the 12ax7.

Voltages:

-V...........-11.25v
+V...........14.51v
Bias..........-11.25 (This is the highest bias voltage that can be acheived.  project material suggests ~13.0v)
Tube pins
2..............-11.30v
1...............7.10v
3.............-11.12v
8.............-11.12v
7.............-11.35v
6................6.79v
TL072 pins
1...............14.13v
2...............-0.02v
3................0.0v
4................-11.53v
5................0.0v
6................-10.13v
7................-10.13v
8.................14.63v

Please suggest something! Thankyou!

Ps if it will help, I can record the nasty noise it makes and attach the mp3.

[QSQCaito]

I think you tl072.. or soemthing near is not ok..


You should be getting you positive voltage en the pin number 8 and 0volts on pin 4. are you sure, the voltages for the pin you posted coincede with the voltages of the pins in the drawing, see its
1          8
2          7
3          6
4          5


bye bye.. is all i can do for you sry , you could also put a link to shcem and/or pcb.. recheck everything, read the debugg post etc(and ssearch)

bye bye

dac}



EDIT: IM RECHECKING, IT MIGHT EB WRONG.. I'LL JUST LEAVE IT FOR NOW...

[stenns]

ahhah excellent thanks for the reply!

i searched the posts and found nothing

Here's the schematic:   http://www.diystompboxes.com/pedals/schems/shaka_tube_redrawn.gif

Could you please give me a break-down of the preamp?  ie. describe the signal path from input past the chip to the first grid of the 12ax7?  I think that would help very much.  Here's what I tried though:


notice the 250pf capacitor in parallel with the gain pot?  I figured if it were faulty it would short out and bypass the gain selection.  It was originally a 250 pf silver mica labelled 250 +/- 5%, 500v SM.  I replaced it with two itty bitty ceramic capacitors labelled 471 (47 times 10^1 = 470)  in series [ 1 / (2/470) = 250.]  still nothing.  If you think that by having a faulty cap there will cause the tl072 to become faulty, I will try replacing that now also, but if not i do not want to waste one (I only have 5 of the chip).


also, I have an mp3 of the sound......which I cannot attach.  If you think it would help i can email it to you it is only 400kb.  Thanks!

[QSQCaito]

Would you tell upto where do you get signal flowing with your signal tracer?

Beware with caps, Caps in parallel are like resistors in series, they add up. Caps in series(i think) are like resistors in paralell..R_total=             1           
                                                             1/r1+1/r2+1/r3
i think it was like these, cant remember exactly..

so if you put them in parallel you would get 940, instead of 250, that could make a difference, don't know.. im an electronics learner yet.

I see it's a TL072 used as single opamo right? are you getting any output from lug1?

Here it's a better tl072 diagram..




bye bye..

hope it helps..

DAC

[stenns]

K so I get clear undistorted signal path at pin 2 of the tube, (working backwards along the schematic) and between the 10k resistor and the .01uf cap, however after that capacitor things get iffy.  sometimes, at pin 1 of the tl072, there is no sound - press a bit - adjust pin position - loud crakling noises - broken chip noises - and then finnally some sound.  applying signal at the input allways generates dirty nasty sound.  How about that .01uf cap?

[Paul Marossy]

stenns-

So sorry about that, but there's a mistake on the TL072 layout which I haven't got fixed yet. 

To fix it, add a jumper between the end of the 250pF cap going to the gain control and Pin 1 of the opamp. It should work correctly once you do that. 

[stenns]

Ah wonderful!  I am just glad it is finally fixed!  AND it sounds great.

Do you know of a simple mod to get more bottom end?

[bancika]

try increasing second coupling cap to 0.022

[stenns]

try increasing second coupling cap to 0.022

is this the .01uf cap between tone pot and ground?

[Paul Marossy]

I am just glad it is finally fixed!  AND it sounds great.

Awesome. Sorry about the headache, I'll fix that PCB layout as soon as I can get to it.

is this the .01uf cap between tone pot and ground?

That's not what I would call the second coupling cap, but increasing that last cap from 0.01uF to 0.022uF would give you more bass. 

[stenns]

I replaced the coupling capacitor(I figured out that this is the .01uf between the plate of the second triode and the gain pot) but am still thinking I could use some more bass.  Aside from increasing this capacitor's value further, is there another capacitor that I could change?  I was thinking the .01uf capacitor proceeding the tl072 and preceeding the 10k resistor, changing it to a .022uf.  Whadduya think?  oh yah and thanks!

[bancika]

yeah, you could try increasing one or both 0.01uf caps, but the thing with bass is that if you add too much bass in first gan stages (before clipping occures) you cen end up with ugly farting bass instead of nice tight bass. Maybe you could try modifying tone stack to increase bass response

[Paul Marossy]

I would also recommend trying the tone cap first.

[fikri]

yeah, you could try increasing one or both 0.01uf caps, but the thing with bass is that if you add too much bass in first gan stages (before clipping occures) you cen end up with ugly farting bass instead of nice tight bass. Maybe you could try modifying tone stack to increase bass response

Yes, thats right, try to play with the tone control instead. And you could try to change the opamp section with another circuit to do some more experimentation. I have done it with a red fuzz circuit. and it sounds great. check it out here : http://fikri_ilyas.blogs.friendster.com/photos/new_project/

Have fun

[Paul Marossy]

Just to let y'all know, I just uploaded the corrected TL072 PCB layout at my website.