GTFO - Full tube high-gain pedal (2x 12AX7)

[J0K3RX]

Has anybody built this yet? I have the board etched and I am still in the process of collecting the parts, I have most of them already except for the caps... Any gut shots would be greatly appreciated!!!

[Mtmattan]

I'm also in the process of building this GREAT project. Finally a tube
pedal with a HV SMPS power supply rather than a McTube or EH and their bulky transformers set up on reverse etc. Can't wait to finish this build. Thanks again!!

[Gaetano Capuano]

reall nice   

[gtudoran]

I will try to post some pictures tomorow, with the prototype. The PCB also changed a little (more optimized for the space - i will also post it.

Best regards,
Gabriel Tudoran
Analog Sound

Has anybody built this yet? I have the board etched and I am still in the process of collecting the parts, I have most of them already except for the caps... Any gut shots would be greatly appreciated!!!

[Mtmattan]

In regard to the caps, which ones are the High Voltage caps? As far as I can see it is the PS caps (C8-11) and the tube Coupling caps (C3-6). Are there any other HV caps? C12-13 maybe?

Thanks so much!!

[J0K3RX]

In regard to the caps, which ones are the High Voltage caps? As far as I can see it is the PS caps (C8-11) and the tube Coupling caps (C3-6). Are there any other HV caps? C12-13 maybe?

Thanks so much!!

+1... I think I pretty much have it figure out but a clear listing of the recommended voltages would be nice! I also have 1/2 watt resistors for all, is that overkill? I also noticed from the diagram that some of the resistors look like jumpers but on the schematic they are actually resistors? There are a few other mistakes as well but don't get me wrong, I'm not bitchin! Thanks for this kick ass pedal and I hope I can get it working!!

[gtudoran]

Hello guys,

Thank you a lot for your positive feedback. I've worked a little more on the layout. For this layout i've used this model of tube sokets: http://www.banzaimusic.com/Tube-Socket-Noval-PC-PMV.html

The schematic is the samme regarding the signal path, but in addition to this, i've made some mods on the supply path: added 2 resistors and 2 HV caps (10uF each) to have seppartaed HV supply for each triode.

The PDF with the details: http://dl.dropbox.com/u/15275178/blast%20from%20the%20past/GTFO.pdf
BOM with voltage ratings: http://dl.dropbox.com/u/15275178/blast%20from%20the%20past/bom.txt
The PCB ready for print: http://dl.dropbox.com/u/15275178/blast%20from%20the%20past/GTFO-PCB.pdf

All used resistors are 1/4w no exceptions, regarding jumpers - can you show me where did you saw them? In this version there are no jumpers on the board

Best regards,
Gabriel Tudoran
Analog Sound

[seedlings]

Perhaps I'm dense today, but is the high voltage output at the junction of R22 and C11?



CHAD

[gtudoran]

  Right on the spot - the HV output is on on junction of R22 and C11

Best regards,
Gabriel Tudoran
Analog Sound

Perhaps I'm dense today, but is the high voltage output at the junction of R22 and C11?



CHAD

[amptramp]

You show L1 as a capacitor - better change that to an inductor of 100 microhenries rather than a capacitor of 100 microfarads.  Do you have a value for pot 5?  And the arrow on the pot indicates clockwise movement.  Clockwise reduces the output voltage in this case as shown - you might want to change that.

[J0K3RX]

You show L1 as a capacitor - better change that to an inductor of 100 microhenries rather than a capacitor of 100 microfarads.  Do you have a value for pot 5?  And the arrow on the pot indicates clockwise movement.  Clockwise reduces the output voltage in this case as shown - you might want to change that.

I think somebody pointed out the "cap\inductor" mixup a few pages back...  I also think he said that anything from 2k to 5k would work for the pot..?

[gtudoran]

Yap, it was, if i remember well, a post of mine back few pages. The cap was put it there only for the PCB foot print (i'm a little too lazy and the trim-pot could be anything between 1k and 5k - the trimmer controls the CV of the 555 timer - the HV variation is acheeved by FRQ. variation - IIRW bigger FREQ. smaller HV output, smaller FREQ. bigger HV output (hope i'm not mistaken).

Best regards,
Gabriel Tudoran
Analog Sound

[PRR]

Ideal values for the lower side of POT5:

300V - 0.44K
250V - 0.53K
200V - 0.66K
150V - 0.88K
100V - 1.32K

It may be close-enough to just pick a voltage goal and use an appropriate fixed resistor. The tube doesn't care if it comes out 230V or 260V.

I do not know if real parts will kick-up as high as 300V.

Unless you need under 133V, a 1K pot is ample and won't be as fussy to adjust.

Keep T1 away from strong heat. The output voltage will drop as T1 gets hot. With T1 at ambient, "250V" will wander from 254V on a hot summer stage to 245V at a winter concert. That hardly matters, but lean T1 against a tube or power resistor and it could drop from 250 to 210. Even that is no big deal except if you take voltage readings and switch off/on a lot it will drive you crazy.

> the trimmer controls the CV of the 555

Actually the trimmer directly controls the base voltage of Q1, which yanks the CV.

T1 will shift from full-off to full-on in a narrow range around 0.6V.

So if all is well, T1 base will be very near 0.6V, and you can predict divider (220K+pot) ratio from 0.6V against desired output.

[gtudoran]

Ok... so it's true ... PRR is not human - how the heck did you predict the voltages so accurate?! Jesus...  Thank you again for taking time to post on this topic.


Best regards,
Gabriel Tudoran
Analog Sound

[tubegeek]

Here is a modified version of the 555 SMPS schematic which addresses some questions people had.

[guitarrob]

I am considering building this on the protoboard and trying it out.

[Minion]

Hi , I am wanting to build this curcuit but I have a few questions/clarifications on the project .....

Firstly I can"t seem to find the dimentions of the PCB layout , in inches or centimeters is fine .......

The mosfet , will any 400v+ mosfet work ??

I have some toshiba TO-247 2sk2698 that are 500v , will they work ??

Thanx a lot !!!

[waltk]

The mosfet , will any 400v+ mosfet work ??

No.  Other MOSFETs will work with reduced efficiency, or not at all.  The critical parameter is the "on" resistance of the MOSFET - it should be <= 0.5 ohm. Source: Mick Bailey on page 2 of this thread - http://www.diystompboxes.com/smfforum/index.php?topic=74088.20

I found this to be true in practice when I tried a different MOSFET that otherwise should have worked.

[Minion]

The mosfet , will any 400v+ mosfet work ??

No.  Other MOSFETs will work with reduced efficiency, or not at all.  The critical parameter is the "on" resistance of the MOSFET - it should be <= 0.5 ohm. Source: Mick Bailey on page 2 of this thread - http://www.diystompboxes.com/smfforum/index.php?topic=74088.20

I found this to be true in practice when I tried a different MOSFET that otherwise should have worked.

Thanx , I just read that post and it says that the on resistance has to be 0.5 ohms or lower or the voltage with sag and the PSU will loose effeciency ......


In that case my Toshiba 2sk2698 Will work well in this case as it has a on resistance of 0.35 ohms .....

Thanx

[guitarrob]

For the L1, The 100uH inductor...this spec means it needs to be a 2-3A inductor?
I am pretty sure I have the right ones in mind but just making sure.