A quick tube (valve) fuzz box

[DrNomis_44]

Finally got to solder this all together today. So far... it’s not working. However, I’m reasonably confidant it’s just a bad solder join somewhere, so will attack it with fresh eyes tomorrow.

It’s been a lot of fun working on this. I’m still very new to reading schematics, and I’ve been enjoying trying to understand what’s actually happening.

So what exactly is it not doing?

[DrNomis_44]

After a bit of mmmm-ing and ahhhh-ing, I've decided to replace the existing circuitry in my Doctor Overdrive pedal with circuitry based on Lawry's tube fuzz box, that way I can use the existing pots and etc since there's a 250k log, 25k lin, and a 1M log pot already installed in the case, I can re-build the Doctor Overdrive circuit board in another case at a later stage, it doesn't get used much so it's no big deal anyway.


Just been working on drawing-up a version of the Purgatory pedal schematic with some mods that are worth trying, going to scan it into my computer using the scan function of my new HP printer I bought recently, once it's posted I'll explain the mods I did to the circuit.

Here we go:




Mods To The Stock Purgatory Pedal Schematic:

* Added an extra 68k resistor to the 1M resistor on pin 2 of V1, this is something you see in a lot of Marshall amp circuits, it's basically a grid-stopper to prevent self-oscillation.

* Change V1 A's cathode resistor from 470 Ohms to 820 Ohms and put a 680nF cap in parallel with it, this increases the gain of the first stage a bit (maybe change the 220k anode resistor to 100k too), this should give a Marshall-like tone.

* Change V1 B's cathode resistor from 470 Ohms to 1k and put a 1uF/25V cap in parallel with it, this will increase the gain a bit more.

* Added Q1, an N-Channel Mosfet, which adds a buffer to the Purgatory pedal circuit to enable V1 to better drive the tone stack, this is similar to what's done in most Marshall amp circuits except that one half of a 12AX7 tube is used to form the buffer.

* Added the Marshall Tonestack to the circuit, this enables greater tonal flexibility, you could also use the Fender Tonestack if you wanted to get more of a Mesa/Boogie tone.


Note that I haven't had a chance to try these mods yet so I don't know how they will sound, but they should work in theory.


Note 2: The N-Channel Mosfet used for Q1 needs to be rated higher than the +HT supply voltage generated by the inverter, most N-Channel Mosfets intended for VHF/UHF applications will also work at audio frequencies too.


Note 3: As Marcel has just pointed out, one other possible mod is to change V1 B's cathode resistor to 39k and omit the 1uF/25V bypass cap to get even closer to the Marshall tone.

[JohnH]

So what exactly is it not doing?

Well currently it’s not doing anything (at least in the way of making noise lol)

I haven’t had a chance to look at it again as we’ve had people over every night since. I had a brief fiddle with it after I initially posted, and can confirm voltage up to a point. Annoyingly I only have a very very cheap multimeter, which doesn’t have a continuity function.

Hopefully can get back to it tomorrow night

[Marcel]

After a bit of mmmm-ing and ahhhh-ing, I've decided to replace the existing circuitry in my Doctor Overdrive pedal with circuitry based on Lawry's tube fuzz box, that way I can use the existing pots and etc since there's a 250k log, 25k lin, and a 1M log pot already installed in the case, I can re-build the Doctor Overdrive circuit board in another case at a later stage, it doesn't get used much so it's no big deal anyway.


Just been working on drawing-up a version of the Purgatory pedal schematic with some mods that are worth trying, going to scan it into my computer using the scan function of my new HP printer I bought recently, once it's posted I'll explain the mods I did to the circuit.

Here we go:




Mods To The Stock Purgatory Pedal Schematic:

* Added an extra 68k resistor to the 1M resistor on pin 2 of V1, this is something you see in a lot of Marshall amp circuits, it's basically a grid-stopper to prevent self-oscillation.

* Change V1 A's cathode resistor from 470 Ohms to 820 Ohms and put a 680nF cap in parallel with it, this increases the gain of the first stage a bit (maybe change the 220k anode resistor to 100k too), this should give a Marshall-like tone.

* Change V1 B's cathode resistor from 470 Ohms to 1k and put a 1uF/25V cap in parallel with it, this will increase the gain a bit more.

* Added Q1, an N-Channel Mosfet, which adds a buffer to the Purgatory pedal circuit to enable V1 to better drive the tone stack, this is similar to what's done in most Marshall amp circuits except that one half of a 12AX7 tube is used to form the buffer.

* Added the Marshall Tonestack to the circuit, this enables greater tonal flexibility, you could also use the Fender Tonestack if you wanted to get more of a Mesa/Boogie tone.


Note that I haven't had a chance to try these mods yet so I don't know how they will sound, but they should work in theory.


Note 2: The N-Channel Mosfet used for Q1 needs to be rated higher than the +HT supply voltage generated by the inverter, most N-Channel Mosfets intended for VHF/UHF applications will also work at audio frequencies too.

Looks to me that you have become an alchemist magician Doc, doing the whole transforming Lead into Gold thing.... or more rather from 'Purgatory pedal' into 'plugin Marshall front end + tone stack'...

Personally I'm surprised you didn't up V1b cathode to 39k .... lol ... ;-)

[DrNomis_44]

Looks to me that you have become an alchemist magician Doc, doing the whole transforming Lead into Gold thing.... or more rather from 'Purgatory pedal' into 'plugin Marshall front end + tone stack'...

Personally I'm surprised you didn't up V1b cathode to 39k .... lol ... ;-)

Now that you mention that, I'm surprised at that little oversight on my part, I'll make an addendum to my post.....cheers mate!


I'm going to try building it with that 39k cathode resistor mod cause I'm intrigued about what it might sound like.


So, the next thing I need to work out is how I'm going to build the circuit, I just happen to have some blank pad-per-hole circuit prototyping board that I recently bought from my local Jaycar Electronics shop, what could be interesting for my build is to do something similar to a Turret board, and use some PCB pins as Turrets, there should be no issues with the + 70V HT exceeding the dielectric strength of the phenolic insulation of the pad-per-hole board, I'm guessing that it can withstand at least 300V DC or so, the inverter circuit can be built on a separate piece of pad-per-hole board.

[JohnH]

I found my attempt at this in a drawer today. I think i tidied my studio for some reason and totally forgot to pull it back out.

Anyway, looking at it again tonight I couldn't see any issues, I went over the schematic about a dozen times. Then I realised, I'm pretty sure I've wired the tube socket backwards.

Just to double check, if I'm looking at the bottom of the tube, with the gap between the pins at the top, pin #1 is the first pin to the right of the gap? Because I've wired it as if the first pin is to the left.

[DrNomis_44]

Been ages since I thought-up that Purgatory Pedal mod I posted, got side-tracked with some other things that needed my attention.

[DrNomis_44]

I found my attempt at this in a drawer today. I think i tidied my studio for some reason and totally forgot to pull it back out.

Anyway, looking at it again tonight I couldn't see any issues, I went over the schematic about a dozen times. Then I realised, I'm pretty sure I've wired the tube socket backwards.

Just to double check, if I'm looking at the bottom of the tube, with the gap between the pins at the top, pin #1 is the first pin to the right of the gap? Because I've wired it as if the first pin is to the left.

If you're looking at the pins with them facing you, pin 1 will be the one on the left-hand side of the large gap in the pins, from pin 1 you just keep counting the pins clockwise from it.

[JohnH]

I think this is where I'm getting myself confused. If I have the pins facing me and start with pin 1 being to the left of the gap, and then go clockwise around, pin 2 would be the first pin on the right side of the gap (see below), but that doesn't seem right.



I thought it should be like this?

http://www.dogstar.dantimax.dk/tubestuf/pinout.htm

[JohnH]

Been ages since I thought-up that Purgatory Pedal mod I posted, got side-tracked with some other things that needed my attention.

Ha ha ha, same. Life gets busy