Overdrive circuit

[Geo]

Hello everyone... long time no post.... lately (the last two or three years?!) I've been less focused on electronics and more focused on actually using my gear. But, the bug has bit me again, and I need a foot-controlled distortion device anyway.

So, I am thinking about this circuit. It's basically a Plexi topology. The "normal" channel is engaged by the main footswitch, taking the pedal from bypass to overdrive. A boost footswitch kicks in the bright channel, which has its own drive control and bright boost control. These two channels slam V2a for distortion, and the cathode follower buffers the output. I left off the tone stack because it's too many knobs for a reasonably sized pedal, and to my ears, the Marshall stack doesn't have much range to it anyway.

I didn't draw the power supply in this schematic. I'm not sure whether to go with an isolation xfmr and a voltage doubler, or something with a higher voltage secondary. Anyway, assume the voltages to be typical of tube guitar preamps. Also not sure how to add, say, diode indicators for the overdrive and boost channels--but I don't necessarily need these, as the loudness of my P-90 buzz should tell me what's switched on. 

Aside from the missing power supply, does anyone see a problem with this circuit, or have any suggestions?

[VMS]

Hello,

I think there should be a capacitor between the cathode follower and that 500k pot.


If it pops when you switch the pedal to bypass, then this thread might be helpful:

http://www.el34world.com/Forum/index.php?topic=9461.0


Also, you probably already know this, but Doug has a tube preamp pedal at his mods and projects page:

http://www.el34world.com/schematics.htm#Mods

...might give you some ideas for the power supply.

[rzenc]

I would do it differently, but that's just my $$0.02
If it is supposed to drive a somewhat long cable, then I believe you would benefit whether 12AU7 is used as C.F. since C.F.'s offers very low impedance in order to drive following high impedances, like a guitar amp for instance. However, I'm not sure about it, your 500K pot will raise the impedance a lot - I believe it is in series with Zout from C.F. - meaning it would waste a lot of signal - which may not be 'bad'. 
Also, what VMS said about the missing coupling cap. the way it is drawn you will have D.C. on your pot and jack and cable.....possibly enough to get you zapped...

Hope this helps.
Best Regards

Rzenc

[Geo]

Thanks guys. Didn't even think about DC on the pot!

Would a lower value pot be better do you think? 1k or 4.7k?

[rzenc]

Thanks guys. Didn't even think about DC on the pot!

Would a lower value pot be better do you think? 1k or 4.7k?

It`s actually about the Zout of your circuit. If you want a stomp box??? it would be better to have a low Zout in order to drive a High Z device - other stomp boxes or amp - and to achieve this it`s necessary to redesign the voltage divider you have after the 100K resistor on cathode of final triode. As you scale down resistance, it will be necessary to scale up capacitances in order to keep freq. response about the same.

Hope this helps
Best Regards

Rzenc

[rzenc]

I found an example on Merlin's book, hope he doesn't mind sharing it. I gave him full credit on the drawing. If I made something silly, pm me.

With Respect
Best Regards

Rzenc

EDIT:
I will suggest using a film cap instead of e-cap, such as Solen Fast Metalized Polypropylene - 22uF @ 630V - but they are very big... 42mm diameter X 58mm lenght.

[jojokeo]

I found an example on Merlin's book, hope he doesn't mind sharing it. I gave him full credit on the drawing. If I made something silly, pm me.

His name is Blencowe 

[rzenc]

  it was too late 

[Geo]

Interesting... I know so little... 

Thanks for the help rzenc! If/when I build this, I'll report back with how it sounds.

[rzenc]

You are welcome mate!

Best Regards

Rzenc

[Geo]

Okay, here's a schematic with the modified cathode follower and the power supply from Hoffman's tube preamp.

Any problems?

Thanks again!

[VMS]

Disclaimer: I don't know if this is the right way to do this, but here is how I would wire the input switch (see the picture below).

Also, don't take voltage from point A and that cathode follower coupling capacitor might be a tad small (not 100% sure about this one).

[rzenc]

Disclaimer: I don't know if this is the right way to do this, but here is how I would wire the input switch (see the picture below).

not sure either...

Also, don't take voltage from point A and that cathode follower coupling capacitor might be a tad small (not 100% sure about this one).

Cap size is selected upon desired -3dB Freq. Using Zout from C.F.: ((1)/(2*3,14*2300*0,00000002)) yields @ 3K5 Hz it will shave all bass content... on the above example from Merlin's book there is a 22uF cap, not 0.2uF, which is 1100 times smaller...

I'm curious to know how it will turn out...

Hope this helps
Best Regards

Rzenc

[Geo]

Cap size is selected upon desired -3dB Freq. Using Zout from C.F.: ((1)/(2*3,14*2300*0,00000002)) yields @ 3K5 Hz it will shave all bass content... on the above example from Merlin's book there is a 22uF cap, not 0.2uF, which is 1100 times smaller...

I'm curious to know how it will turn out...

Hope this helps
Best Regards

Rzenc

Oops, I thought I saw a decimal! Thanks--I really don't know this stuff like you guys do, so it's great to have someone look it over.

[jojokeo]

For extra gain/boost tweaking you could add a cathode bypass cap on V2a and reduce V2b's cathode down to around 56k.