What is this Mosfet doing?

[Otsismi]

I've been building a lot of the valvecaster circuits out there and recently came across this circuit for the Persuader by ModKits. What is the Mosfet doing in this circuit? What purpose does it serve?

http://www.modkitsdiy.com/sites/default/files/product_files/the_persuader_schematic.pdf

[induction]

That's a ZVex SHO circuit. It's being used as a gain stage.

[Otsismi]

Will using the gain stage give you more distortion or simply more volume?

[induction]

I've never built a valvecaster, so someone can correct me if necessary, but it will probably give you more distortion. The valvecaster is a booster/overdrive circuit (it's two gain stages in series). This circuit has a SHO boosting the front end, so now you have 3 gain stages. At high gain settings, the valvecaster will run out of headroom (it won't have enough voltage available to cleanly amplifiy the input signal) and it will start to distort on its own (instead of getting louder, it will clip the peaks of the signal at it's maximum available voltage - this is what 'overdriving a gain stage' means). With an extra gain stage up front, it will run out of headroom with a lower amplitude input signal, so it will distort faster.

The rule of thumb is that boosting the front end of a dirt circuit gives more dirt, boosting the back end gives more volume.*


*There are situations when this isn't true, but I doubt that this is one of them.

[psychedelicfish]

I take it you mean input when you say "front end" and the "back end" means output?

[mistahead]

I take it you mean input when you say "front end" and the "back end" means output?

I'd have assumed so... but I am guesing you're trying to clarify more than query?

Try the magic three - clean loud boost, Distortion+ / Electra type dirt, Phaser, (or four - throw a wah in there too) and mix up the order of the pedals - this is why I need to put a wah at the back of my chain as well one day, another four or so phasers strategically located, and vibe/trems littered through the whole chain.

Just have to consider most dirt circuits we use just pick up the signal, boost the hell out of it and crush it back through a volume knob. In the case for this thread the boost and crush portion is the mosfet's job and the tube is acting as a dual gain stage/clipping stage further boosting and crushing the signal through its volume control.

[psychedelicfish]

If you look at the 12AX7 section of the circuit, the grid bias resistor is 4k7, which is extremely low for a normal guitar pickup to drive. The MOSFET is not only being used to boost the input signal, but also to present the guitar with a nice high impedance input.

[Otsismi]

Right on. Thanks guys. Anyone know if this circuit would accept any other 12a-7 tubes without modification?

[alambike]

Hello,

I am trying to build the Persuarder on my own and I found in this topic some good information. How I only construct pedal circuits from the Tagboard blog and never developed a circuit from the electronic scheme, I would like to know if anyone do it with the Persuader.

Other thing, I was talking with a DIYstomboxes member about this question and He told me that is possible I conect the Output of the VZEX SHO to the Input of the Valvecaster. So, is it a simple way to sum the effects ??

Thank you very much.

[antonis]

is possible I conect the Output of the VZEX SHO to the Input of the Valvecaster. So, is it a simple way to sum the effects ??

I'm not sure about what you mean with "summing"...

There are totaly different results when you connect in series (add one to the other) two effects than you connect them in parallel (mix them)...

[GibsonGM]

Hello,

I am trying to build the Persuarder on my own and I found in this topic some good information. How I only construct pedal circuits from the Tagboard blog and never developed a circuit from the electronic scheme, I would like to know if anyone do it with the Persuader.

Other thing, I was talking with a DIYstomboxes member about this question and He told me that is possible I conect the Output of the VZEX SHO to the Input of the Valvecaster. So, is it a simple way to sum the effects ??

Thank you very much.

If you take the OUT of the SHO and input it into the Valvecaster, then yes, you are "summing" the effects, sort of.   You are running them in series, from one to the next.   

So, you boost the signal with the SHO, and hit the Valvecaster with it...this will result in a hotter signal being input, of course.  Which is likely to drive the Valvecaster harder, resulting in more clipping.  As noted above, less output volume since the tube will compress and distort rather than just give you a bigger output. 

You will also notice some audible differences....maybe a brighter signal, more punch, pick sensitivity, and so on.     The SHO is a GAIN stage, so it is amplifying the voltage going to the next stage, with appropriate consequences....it is a booster.

Probably sounds pretty d*mn wild   Esp. if you build it so it can be switched in and out.   

[alambike]

is possible I conect the Output of the VZEX SHO to the Input of the Valvecaster. So, is it a simple way to sum the effects ??

I'm not sure about what you mean with "summing"...

There are totaly different results when you connect in series (add one to the other) two effects than you connect them in parallel (mix them)...

How could I know if the circuit is in series or parallel.

I connect the out of SHO to the in of the Valvecaster and it worked very well, but after 20min the pedal stopped suddenly.

I noticed that the tone control (of the valvecaster) was not working. I changed the electrolytic capacitor (of the valvecaster board - 1uF) and the pedal came back to live again. Well, only for more 20 min and then stopped again.

Why this capacitor is breaking ? and it  is working only for a few minutes ?

[duck_arse]

alam, can you provide us with a circuit diagram showing how you've connected the two? you may be missing a pull-down resistor somewhere, allowing a cap to float to a point the valve don't like.

if you look on the circuit in the first post, measure the voltage at the points shown as "0V" when your pedal is working, and then the same points when it stops.

[alambike]

Hello,

I build on this way :

[antonis]

I don't know if the cable 2 is (propably) connected to the tube's grid but it isn't a good idea to leave the 10μF electro connected to the 47nF without a pull down resistor between the two of them...

Also, if the 1μF cap was bad it shouldn't work even the Output (no signal at all or TOO much amplified DC..)

[alambike]

I disconnect the SHO from the system and the pedal works again with the same electronic components.

Is there a correct way to plug both circuits (sho and valvecaster) in series or parallel ??

[GibsonGM]

I don't know if the cable 2 is (propably) connected to the tube's grid but it isn't a good idea to leave the 10μF electro connected to the 47nF without a pull down resistor between the two of them...

Also, if the 1μF cap was bad it shouldn't work even the Output (no signal at all or TOO much amplified DC..)

There is a 100k pulldown at the output of the SHO....he's having a weird problem here.  I suggested maybe his cap is rated 6.3V or something (??)   

Either that, or maybe there is a part missing in either of his builds?  IS that 100k there, and connected to ground?  That sort of thing...

[antonis]

There is a 100k pulldown at the output of the SHO....

You're right - I'm blind..

@alambike: I'm still confused about what you want to do with these pedals..

[duck_arse]

now we get to ask for photos of what you have built. please can we have photos of wot you bilt?

[alambike]

The photos from my the two connections:







The ideia is:

I would like to add the distortion effect on the Valvecaster, so I decided to plug the Sho With the Valvecaster circuit. The problem appears after some minutes playing with the two boards plugged.

Is there any solution to decrease the SHO gain or change the SHO circuit for other that could perform the same function (that is, add a distortion controlled by an pot)


thank you very much