My First Working FX Design

[Paul Marossy]

I have been fooling around with a Shaka Tube variant the last few weeks, and I think I finally have something that is worth sharing....

It uses the same power supply scheme as the Shaka Tube, and the tube section is similar, but I tweaked all of the resistor values and added bypass caps to the cathodes. Instead of the opamp, I used a J201 FET. At the output, I placed an LM386 to boost the output, an idea I got from the Ruby Tuby, I think. The tone control is the same as the Shaka Tube, but it would probably sound cool with a Big Muff style tone control or the AMZ version of it.

It probably needs a little more tweaking, but it actually sounds pretty good, to me. It's kind of a high gain sound, fairly smooth sounding, too. I think that the output level may still be a little bit low, though. I'll have to do a little more testing tonight. If anyone is interested, I'll post a link to the schematic when I get it scanned in tonight. (and maybe I'll attempt a sound clip in the next few days)

[aron]

Yeah post it! COOL!

As soon as I saw cathode bypass caps, I knew it; higher gain.

I'm looking forward to seeing the schematic!

[slajeune]

Hi Paul,

yes, DO post a schematic of your design.  I have just started working on FX (and tubes for that matter) and I always like to see new tube designs using low voltage.  Especially if it sounds cool!

I really like the idea of an LM386 at the end of the chain that's why I am using it in the ruby tuby!

Cheers,
Stephane.

[donald stringer]

I would be interested also, tubes are a strange animal to me though. Could you post a no brainer picture showing which wire go where on the tube.

[Paul Marossy]

Donald:

Look at this sheet, it will show you each pin. This would be looking at the tube from the bottom.

http://www.diyguitarist.com/GuitarAmps/MV_Chassis.htm

(It's in the lower right corner)

[slajeune]

Hi Donald,

On a schematic, the tube pins are always numbered.  Here is how to identify the pins on a tube.  Look at the pins of the tube.  The gap should be facing down.  Reading clock wise, you have pin 1 on the lower left (first one to the left of the gap).  Pin 9 is the pin to the right of the gap.

Hope this helps.

Stephane.

[Paul Marossy]

OK, here's a schematic:

http://www.diyguitarist.com/PDF_Files/PJM-TubeOverdriveV1.pdf

Sorry for the large file size. I don't know how to get my scanner to get a clear image without making the file size large.  It's as small as I can get it. :oops:

The voltages shown are just what I was measuring with the xfmr I was using for testing. I think it sounds pretty good as it is, but I still am kinda testing it out...


Comments, suggestions welcome.  

[Ansil]

looks cool i love the 386 there man

[Paul Marossy]

That 386 section is basically right off of the data sheet. It is set up for a gain of 50.

[Joep]

Hi Paul,

I noticed 13,9V from the power transformer. That might be a bit to high for the poor tube.... Use a 8.2Ohm resistor is series with the heathers to get it down a bit.

Joep

[slajeune]

Hi Paul,

thanks, very interesting design!  I am intrigued how it sounds.  The 3 diodes to ground before the tone control would make it a hard clipping circuit.

If you make them switchable, you could pick between tube only distortion (soft) and tube + diodes clipping (hard).

The low output level is mostly due to the tone control (low pass filter) I think.  It will definately cut the output a lot.  You can try to increase the 386 gain.  If I remember correctly, simply increase the resistance between pins 1 and 8 of the LM386.  When I added the same type of tone control to the ruby tuby, the output droped significantly.

Cheers,
Stephane.

[Mark Hammer]

Just urious.  Why the choice of a dash-four 386, as opposed to any of the other 386's, given that one assumes it to be driving a medium-to-high impedance load?

[Paul Marossy]

I know that input voltage is too high, I was just using a quick and dirty xfmr for testing.

I suggest using the LM386N-4 since the it can handle a maximum supply voltage of 15 volts instead of the 12 volts that the N1 thru N3 can handle, according to the data sheet I have. Although my LM386N-1 seems to handle nearly 13 volts without any trouble...

A switch for the diodes would probably be pretty cool. I meant for this to be really high gain. It seems to be pretty versatile as far as tone goes. It goes from fairly mild to over the top - kind of like a Smash Drive Tube, mostly because of how the tone control works. (I was using my guitar with a preamp built in it, so that probably factors in) Some people would probably call the tone "complex". I did notice that the tone stack dumps a lot of the signal, but the LM386 at the end of the circuit solves that problem. I have a pretty loud output now! I did put a trimpot at the input of the LM386 section in addition to the volume control so you can set the max. volume level going to it.

It sounds pretty cool with the bridge pickup (humbucker) and the neck/middle combination on a Strat. I'll try to get a sound clip or two recorded tonight. I am also going to look at the waveforms thru-out the circuit and see what is happening at each stage. This is fun!  

[Jay Doyle]

Paul,

This is really small but you may want to add a 1M-4M7 resistor on the other side of the output cap to prevent clicks when switching.

Great job with the design!

[Paul Marossy]

Thanks Jay.  

Yeah, I think that would probably be a good idea. I am going to test it out with an amp today or tomorrow and see if I get any pops when switching. So far, I have been doing all of my testing with headphones on.

I'll also post a pic of the thing later today.

[Ansil]

can't wait to hear it paul  ps the n4 is a max of 18v on the national datasheet,  but i have another sheet that says 22v so it is fine

[smoguzbenjamin]

I'm looking forward to some soundclips

[Paul Marossy]

OK, here is a few pictures of my build. This whole thing started out as me wanting to use up a tube socket I had left over from my Firefly build...

Exterior: (it didn't take that long to drill all those holes)
http://www.diyguitarist.com/Images/PJM-TOD-Ext.jpg

Interior: (another perf job...)
http://www.diyguitarist.com/Images/PJM-TOD-Int1.jpg

Interior: (view from another angle)
http://www.diyguitarist.com/Images/PJM-TOD-Int2.jpg

Tube Socket: (detail of mounting)
http://www.diyguitarist.com/Images/PJM-TOD-Socket.jpg

Things are kinda tight in there, but not as bad as the pictures make it look. In case anyone is wondering, the fourth pot is a "countour control" od sorts. It's basically a 100K variable resistor in series with the input. It kind messes with the harmonics a little bit. Just an idea I was trying out...

Sound clips coming next.

[Paul Marossy]

OK, here is the soundclip. This is one of the few times that I have recorded anything on my computer, so be forewarned, I think it may be a little bit loud...

It starts out with the neck pickup (humbucker), gain about 3/4 from max, tone control about 1/3 up from bassy end. Then I switch to the bridge pickup (humbucker), turn the gain up to max, and the tone control to about 3/4 from the treble end. Then I switch back to the neck pickup and once again to the bridge pickup. I added a little bit of reverb so I wouldn't sound too dry and I also used my Bulldog cabinet simulator.

This is just a clip to show the heavier side of it. You can also get a lot milder overdrive sound with it, too. And, it seems to clean up pretty good when you roll back the guitar volume.

http://www.diyguitarist.com/Sounds/PJM-TubeOD1.wav

I ended up keeping it a .WAV file, since it sounded better than when I converted it to MP3, and the file size is quite a bit smaller...
And BTW, using a scope with a 1kHz sine wave test signal, right out of the FET stage, I have asymmetrical soft clipping and at the output, asymmetrical hard clipping. The waveforms in the stages in between are interesting... I'll have to look at them with different frequencies and see how it changes.

[Paul Marossy]

A little update: When I turn the tone control all the way to the treble end, it can get an ugly oscillation, kind of gated sounding, too. I think it may have to do with the LM386 section having too much gain. Any thoughts?