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View Full Version : A quick tube (valve) fuzz box



lawry
02-02-2014, 05:55 AM
Well, as you are probably aware, I can't get stuck into my fretless build because of cyclone Dylan cleanup. So I decided to design/build a quick valve fuzz pedal on breadboard this afternoon using some bits I have lying around. I love designing valve stuff for guitars, they always sound so cool 8-) For those who are interested, it is a starved plate design using a 12AU7 and powered from 12 volts. I used a 12 volt transformer but a 12VAC plug pack would do. The 12 VAC power goes through a voltage multiplier to lift the plate voltage to around 50 volts and also feeds the valve heaters. This circuit has a tone control but I probably shouldn't have bothered. Anyway, it sounds pretty damn cool. I may just stick it in a box and use it on my pedalboard.
The 9v battery was removed in the final setup. It was just there in case the 12VAC was too noisy - but it wasn't :D

http://www.pitbullguitars.com/wp-content/uploads/2014/02/5v323-image.jpg

AJ
02-02-2014, 06:54 AM
Wow.. Lawry.. cool!
Can you do a reverb unit like that?

lawry
02-02-2014, 07:51 AM
Hey AJ. Reverb units need a bit more circuitry, much higher voltage (~300VDC or more) and higher current for the spring's driver transformer. It's doable, but it's not a quick or cheap project. Sorry man, if you were after an inexpensive knock-up reverb, it ain't that easy. However, this project only has about $50 in parts and you end up with a real valve distortion/overdrive unit.

stan
02-02-2014, 09:45 AM
that is awesome Lawry - you need to do a sound clip with it set up just like that

djdubya
02-02-2014, 11:17 AM
Lawry this looks real interesting to me. Could you post a schematic and parts list? I've built all sorts of gadgets and test boxes for work over the years but never anything with a valve in it. I'm guessing the starved plate design is creating distortion by running the valve at a lower than normal voltage? Stan's right - we need audio!

I passed through Mackay on the way up to Cairns last April - beautiful part of the world for sure. Hope old Dylan hasn't left too much of a mess and you can get back to guitar building soon.

pablopepper
02-02-2014, 11:58 AM
Nice one Larry, building pedals is something that I've wanted to do for a while. I found this website a while ago and their kits look great but I've yet to take the plunge.
http://buildyourownclone.com/

WeirdBits
03-02-2014, 12:09 AM
C'mon Lawry, you know the rules... schematic or it didn't happen ;)

lawry
03-02-2014, 01:07 AM
OK Guys,
I made a couple of small changes to the circuit since I posted it yesterday in order to make it easier for anyone who wants to build one. Nothing major, just swapped out the active multiplier for a passive one and removed the tone control. As requested, here are some sounds, and seeing how I always call my 'brand' Dirty Angel, it seemed appropriate to give the pedal a demonic name. Hence it is called the Purgatory Pedal. The guitar used is my Dirty Angel 'Viking' RC-1 build and the recording was done straight from the pedal into a Presonus 1818VSL USB Audio interface. The recordings are completely dry except for the pedal. Now, on to the sounds...
First 30 seconds is a clean reference signal using the bridge pickup.
The next 30 seconds is the bridge pickup with the gain on 3.
The next 15 seconds is the neck pickup with the gain on 3.
The last 30 seconds is the neck pickup with the gain on 7.

Above 7 the whole thing just gets muddy.
I recorded some sounds this morning and placed the file here...

https://soundcloud.com/dirty-angel-2/purgatory-tube-pedal-samples

Yep. There was a schematic here but it had a an error (forgot to delete a bit when I changed to the passive voltage multiplier) so I removed it so that no one would use it and get it wrong. I've redrawn the schematic and will post it soon along with a vero board layout.

AJ
03-02-2014, 01:24 AM
sound terrific Lawry !

WeirdBits
03-02-2014, 01:57 AM
Very nice.

kanelangmaid
03-02-2014, 10:19 AM
Sounds sweet Lawry, just tweaked my interest. Have been contemplating a valve amp build down the track this may be a nice fill in.

May have some questions for you down the track when I can spend some time checking your circuit diagram.

djdubya
03-02-2014, 10:31 AM
Thanks for posting the circuit Lawry. It sounds great! Reckon I might have a crack at building one of these in a couple of months.
@Kane building this would be a great warm up to building an amp.

lawry
03-02-2014, 11:42 AM
Thanks to all for your comments. I really built this out of frustration because I couldn't do any work on the guitar and needed to do something.
This circuit is a nice intro to valves and the voltage is safe, whereas a full-on valve amp can kill you before you even know you're dead if you're not careful. That doesn't mean you shouldn't build one, just be very careful. Especially if you're unfamiliar with high voltage electronics.
@Kane. Ask anything you like.

wokkaboy
04-02-2014, 04:57 AM
love the sounds Lozza, you really are an electronics guru !
nice axe playing too !
setting on 7 is heavy enough and sustain is good !

lawry
04-02-2014, 07:26 AM
Thanks Wokka. Agreed, the sound on 7 is as heavy as you need. Seriously thinking about buying a box and making it permanent pedal and If I do I'll probably put it on vero board. And if there's enough interest I'll post the layout for those who want it. We'll see.

VanHaydn
04-02-2014, 08:01 PM
Cool job Mate...
if you can do a track layout I can machine a couple out for ya, using this stuff
http://www.ebay.com.au/itm/Single-Sided-Circuit-Board-150x200mm-PCB-Copper-Clad-FR4-FR-4-1-5mm/190969262859?rt=nc&_trksid=p2047675.m1851&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%2 6asc%3D261%26meid%3D4582796485538710862%26pid%3D10 0005%26prg%3D1088%26rk%3D1%26rkt%3D5%26sd%3D400423 049476%26
I rekon you put a little kit together a few blokes will have one in a heartbeat! not that you have enough to do already hey! tehe!

lawry
05-02-2014, 09:21 AM
A CNCed PCB eh? I've etched a sh!t load over the decades but never had one ground away before. I'll think about it though.
The circuit is so simple that vero makes an accessible option at any time. But a 'limited edition' version does sound kinda cool.
Thanks

lawry
10-02-2014, 02:19 AM
Right then. It is absolutely @issing down here today with thunder and lightning aplenty. So it seemed like a good idea to finalise this project.

I have fully drawn the final schematic (and hopefully correctly), drawn up a vero board layout, then built and tested the end design. Needless to say, it sounds exactly like the previous clips. For those of you who want to build it there are a some important things to know.
1) the original design (and indeed mine) shows the 47uF capacitors as 63 volts. I recommend 100 volts because when I checked my voltage rail with a meter it was running around 70 volts. So the caps are a bit stressed. I'll change mine soon.
2) the layout drawing shows the components on the top of the board, not the copper side.
3) the copper side pic shows where you need to cut the tracks and where the two mounting holes are on mine.
4) firmly twist the power wires (and those that go to the valve heaters pins 4 & 5) to help reduce hum.

I have used a 12 volt transformer for testing but when I put it in a box I will use a 12 VAC plug pack. Also, I had not connected the foot switch or LED in the pics below. That will be done when I mount things in a box.

So, go for it...

http://www.pitbullguitars.com/wp-content/uploads/2014/02/7ud69-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/02/85y4s-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/02/862w8-image.jpg

lawry
10-02-2014, 02:25 AM
And here are some reality pics.
Copper side with tracks cut.
Copper side with components soldered in place.
Component side.

http://www.pitbullguitars.com/wp-content/uploads/2014/02/5725m-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/02/t5q71-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/02/160o6-image.jpg

lawry
10-02-2014, 02:28 AM
And finally, some assembled pics.
Just gotta buy a box and do the finishing off.
http://www.pitbullguitars.com/wp-content/uploads/2014/02/jk3n0-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/02/2no0d-image.jpg

WeirdBits
10-02-2014, 03:59 AM
That is a work or art Lawry, thank you for posting it for all our benefit.

What is the heat like from the 12AU7? Caged fully external mount or maybe socket inside box with half tube out through hole etc?

As an aside, have you checked out the AX84 (http://www.ax84.com) projects? And, if so, what do you think of the October (http://www.ax84.com/october.html) associate project? The Studio version (http://www.ax84.com/static/october/AX84_October_Studio_100325.pdf) (with FX loop and dual 6SN7's options) looks very interesting, particularly the preamp, but the Club is also tempting (albeit a lot more costly than something like the Lamington). I re-tubed my 5watter yesterday and your fuzz box project has now reignited my urge to build one... maybe

Brendan
10-02-2014, 04:33 AM
Looks great Lawry! Certainly a lot more advanced than mine! Reckon it'll be a cracker!

lawry
10-02-2014, 05:16 AM
Hey Scott. Thanks man.
There's a bit of heat from the valve heaters (about 2 watts) but I'm going to enclose it inside a box for robustness and drill air holes to let the heat out.
I've visited AX84 heaps of times. They have tons of valve related info and a great forum. I also had a look at the link for the pre and 2W amp and they look cool.
If you're interested in the 2W amp I built into the Vox box, see below. This is for info only. I accept no responsibility for any builds. But it works perfectly for me. The tag board is available from Jaycar.
Cheers
http://www.pitbullguitars.com/wp-content/uploads/2014/02/af577-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/02/yx713-image.jpg

WeirdBits
10-02-2014, 06:02 AM
You really are determined to get me playing with dangerous voltages, aren't you? ;)

Thanks for the schematic, it looks good, I'll file it away for future unsafe experiments.

gavinturner
13-02-2014, 03:16 AM
Hi Lawry, Thanks so much for posting the circuit, layout and all The other details. Your pedal sounds awesome! I'm definitely going to make one.
Sourcing the parts on the interweb now..

Cheers,
Gav.

lawry
14-02-2014, 10:53 PM
Hi Gav.
Just had a look at the schematic I posted and noticed an error. The gain pot is shown as 250kA (log) and should be 250kB (linear). If you put in a log pot there it will still work but you will get 90% of your drive in the first 10% of rotation (ish). The connection diagram shows the correct value though. I'll update the schematic soon. Sorry man.

lawry
01-06-2014, 07:15 AM
Right.
After making a deal with Scott (Weirdbits) that I will finish the Purgatory Pedal and he will start on his new hollowbody kit, I made a new start today. Originally I designed this pedal with a voltage multiplier that used a 12 volt AC plug pack but I have now redesigned it to use 12 volts DC with a multiplier based around a hex inverter chip.This lets me use my pedal board power supply that only has DC outputs. The new schematic and layout is shown below for those who want it. I've hooked it up to my amp and am very happy with the sound.
So... tomorrow it's off to Jaycar to buy an enclosure.
Your turn Weirdy.

http://www.pitbullguitars.com/wp-content/uploads/2014/05/x51gs-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/05/0r5ru-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/05/q79i8-image.jpg

WeirdBits
01-06-2014, 08:14 AM
Noooooooooooooooooooooo! That's what I get for making deals... but you're still only half way there ;) Good idea with the switch to DC.

lawry
02-06-2014, 03:34 AM
And, young Scott, you have mad a deal with the devil. Or at least his Dirty Angel. So be prepared for Purgatory.

Today's phase:
Got me a box and mounted everything inside. (Weird Note: For some reason it picks up more noise inside the box so I'll have to work on that).
However, it's getting closer.

Next step is to come up with some artwork to make the box look perty.
http://www.pitbullguitars.com/wp-content/uploads/2014/06/w498k-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/06/k1y7m-image.jpg

WeirdBits
22-06-2014, 01:54 AM
Did you get the increased noise issue sorted out, pot/tube proximity maybe?

I was about to order some bits for a couple of little side projects and I thought 'hmm... maybe a purgatory too', but I just use a 9V DC supply and don't really want to run a 12 as well. So, wizard of electrickery... any suggesions for a 9V version?

lawry
22-06-2014, 04:24 AM
Hi Scott.
Firstly:
Yep, got the noise sorted but can't honestly say which bit was the culprit. Once I tidied up the wiring and mounted everything properly it quietened down a lot. Theres still a bit of noise but no worse than any OD pedal I reckon.

Secondly:
9V will be fine but you'll end up with less HV (about 50V instead of 80V). It'll just fuzz-out a bit sooner for you. The only thing you'll have to alter is the heater wiring. Instead of going through the heaters in series (pins 4 & 5) you'll have to run them in parallel (pins 4 & 5 shorted together and out on pin 9). You'll also have to drop the 9 volts down to 6v by using a 6 volt regulator. You can buy an LM7806 and a couple of caps at Jaycar for about $3.00 I reckon. The heaters will draw about 300mA so you may need a small heat sink on the reg. I imagine you're OK with LM78XX series regulators so won't need a schematic. But if anyone else wants to do this please see the schematic below. It's a bit fuzzy because I took it without putting my glasses on. It's a bugger getting old!
Have fun.
http://www.pitbullguitars.com/wp-content/uploads/2014/06/87v0b-image.jpg

Gavin1393
22-06-2014, 06:15 AM
Still waiting for my valves and transformers to arrive.......

dingobass
22-06-2014, 08:33 AM
Still bemused by the amazing hieroglyphics.. :)

WeirdBits
22-06-2014, 08:37 AM
Thanks Lawry, that's sort of the path I was heading, much appreciated.

lawry
22-06-2014, 09:50 AM
@Gav & Weirdy. Really looking forward to your builds, men. Should be very cool!
@DB. LOL. Hieroglyphics huh? Sorta like electronics as a second language . Hehehe.

WeirdBits
26-07-2014, 06:45 AM
Lawry, regarding your Purgatory V2...

On the schematic C8 is listed as a 47uF electro, but appears to be marked as 10uF on the layout pic. I'm guessing 100v 47uF is the correct value? Also, do you have a preference for type of caps in the power section vs audio section? Thanks.

lawry
26-07-2014, 09:59 AM
Hi Scott,
I'd forgotten all about this thread. I ended up using a 47u but a 10u would work just as well. It's just the final smoothing and storage cap that feeds the HV section. Consequently it does have to be 100V or higher because you end up with around 70V from the multiplier. As for the audio caps, you can use common old greencaps (metalised polyester) from Jaycar cos they're rated at 100V (you need to block the HV DC). Feel free to use any swanky ones you like, but these work OK in my opinion.

WeirdBits
26-07-2014, 10:17 AM
Thanks.

DrNomis_44
09-01-2015, 10:54 PM
This looks like an interesting little project, I'll have to have a go at making one of these, I'm a member of Freestompboxes.org, a forum for those interested in building their own fx pedals, one member with the username bajaman designed a Tube overdrive pedal based on a well known Tube overdrive pedal, I thought I would post a pic of my build of bajaman's Tube overdrive pedal, note that I have since changed the knobs to some blue ones, anyway, here's a pic of it:1253



Just in case you want to hear what it sounds like, here's a link to a clip I uploaded to soundcloud:

https://soundcloud.com/drnomis_44/lespaulrecord

DrNomis_44
24-01-2016, 08:13 AM
Still bemused by the amazing hieroglyphics.. :)


You're not the only one mate, I've been doing electronics since I was 13 years old and I still get confused by those hieroglyphics, and we all know just how easy it is to confuse me....lol.

DrNomis_44
24-01-2016, 10:06 AM
@ Lawry: I just had a listen to the demo clip you uploaded to Soundcloud and I think it sounds great, might have a go at building one since I've got a spare 9-pin valve socket with a shield in amongst my bits and pieces, I also have a diecast aluminium box I could use too.


If you're after some 47uF caps that are rated for 450V you can get them from Jaycar Electronics, I know that you're only getting about 70V, the 450V DC rating of the caps should ensure that they can handle the voltage with ample to spare.

Have you tried using a 12AT7 or a 12AX7 valve in your pedal design?, the 12AT7 and 12AX7 have different gain factors compared to the 12AU7, but they have the same pin connections and therefore should work fine.

WeirdBits
27-11-2016, 03:23 PM
Finally got a chance to do some tinkering... and now I too can spend some time in Purgatory!

It's definitely a single coil pedal in my mind. With humbuckers it seems a bit too much or too little to me, whereas single coils seem to work nicely. YMMV.

Here's a quick run down on my build. It's Lawry's version 2 of the circuit (thanks Lawry) but with the addition of 12v/9v switching via a 4 pole slide switch and some extra bits (so that I can use a 9v supply if I don't have a 12v supply set up).

Initial layout showing where track breaks need to be:
15280

Track breaks in place:
15281

Working out where I can squeeze the 9v extras in on the board (6v regulator and caps):
15282
15283

Extra track breaks for the new bits (the three on the right) to isolate them from the existing circuit:
15284

cont...

WeirdBits
27-11-2016, 03:28 PM
Links in place, wiggly left one is the common for the 9v parts:
15285

The enclosure I was going to use wasn't as deep as I thought, so I decided to reuse the box from my first ever pedal built several decades ago. The existing holes weren't quite in the places I needed, but I could make it work:
15286

Marking out the new layout:
15287
15288
15289

cont...

WeirdBits
27-11-2016, 03:33 PM
Implements of destruction... needed for the big tube hole:
15290

Holes drilled, filed and hopefully all in the right place:
15291
15292
15293

Quick mockup to make sure everything is mostly right:
15294

cont...

WeirdBits
27-11-2016, 03:36 PM
More mockups:
15295
15296

Components added:
15297
15298
15299

cont...

WeirdBits
27-11-2016, 03:42 PM
Test fitting board in enclosure (this is where the existing holes made things awkward):
15300

A little bracket to hold the board in place:
15301

I'm using a tri-colour LED so I can have one colour for bypass and then a colour each for 12v and 9v active:
15302

Loom prep:
15303

Wiring:
15304

cont...

WeirdBits
27-11-2016, 03:48 PM
Installing the wiring into the enclosure... normal sensible people do this outside the box. Things are getting tight:
15305

Everything is connected, time to add the heatsink to the regulator (double-sided heat tape, then bolted to box):
15306

All internals in place, don't ask me how long it took to get the nut on the heatsink bolt:
15307
The 4-pole slide switch selects between 12v and 9v supply. For 12v the tube heaters are in series via pins 4 & 5 as per Lawry's build and fed directly from the 12v. With the 9v the heaters are swapped to parallel across pins 4 & 5 with a ground added to pin 9 and fed via the output from the 6v regulator circuit added to the board.

Noise maker installed:
15308

Caged up and ready to rumble... hopefully:
15309

cont...

WeirdBits
27-11-2016, 04:00 PM
Amazingly, it worked first time!

Red on bypass (9v or 12v):
15310

Violet active on 9v:
15311

Yellowy Green active on 12v:
15312

Basically, the RGB LED always has the red on and then either the blue (9v) or green (12v) is added when active giving violet and yellow-green respectively.

Done and done. I was going to paint the enclosure when I first planned this, but now I'm thinking I'll leave it... at least for now.
15313

I still need to experiment with the sounds and how it changes with the voltage. It has a little less 'oomph' on 9v but the overall sound isn't that different. I've only tried it on a little solid-state amp so far, but it gave that a bit of tube-ishness which was nice. I used a 12AU7 tube like Lawry's, but may try a few alternatives when/if I get time. Thanks again for the circuit and inspiration Lawry!

DrNomis_44
27-11-2016, 04:10 PM
Amazingly, it worked first time!

Red on bypass (9v or 12v):
15310

Violet active on 9v:
15311

Yellowy Green active on 12v:
15312

Basically, the RGB LED always has the red on and then either the blue (9v) or green (12v) is added when active giving violet and yellow-green respectively.

Done and done. I was going to paint the enclosure when I first planned this, but now I'm thinking I'll leave it... at least for now.
15313

I still need to experiment with the sounds and how it changes with the voltage. It has a little less 'oomph' on 9v but the overall sound isn't that different. I've only tried it on a little solid-state amp so far, but it gave that a bit of tube-ishness which was nice. I used a 12AU7 tube like Lawry's, but may try a few alternatives when/if I get time. Thanks again for the circuit and inspiration Lawry!



That looks wicked, I like the effect you get with the holes drilled in the metal valve shield, gives it a retro-vibe, I'm going to have to have a go at making one of these next year, I still have some spare valves which I can use in my build.

Also, the tri-coloured led is a nice cool touch.

DrNomis_44
27-11-2016, 06:15 PM
And, young Scott, you have mad a deal with the devil. Or at least his Dirty Angel. So be prepared for Purgatory.

Today's phase:
Got me a box and mounted everything inside. (Weird Note: For some reason it picks up more noise inside the box so I'll have to work on that).
However, it's getting closer.

Next step is to come up with some artwork to make the box look perty.
http://www.pitbullguitars.com/wp-content/uploads/2014/06/w498k-image.jpg
http://www.pitbullguitars.com/wp-content/uploads/2014/06/k1y7m-image.jpg


One thing you could try doing to get rid of the noise is to connect the metal of the case to circuit ground, the case will then act as a Faraday Shield for all the internal electronics, also, make sure that any shielded audio wire you use for the input wiring has it's shield braiding connected to ground at one end of the wire, this will prevent earth-loops in the input wiring from inducing hum-noise into the input, the control grid of a 12AU7, 12AT7, 12AX7 operates at a very high impedance, meaning that it takes very little current to drive it, as a consequence it tends to be very sensitive to noise pickup.


I'm going to try bread-boarding the Purgatory V2 circuit over the Christmas holidays and see if I can come up with some cool mods to the circuit, I can think of at least a couple that I'm interested in trying out, one mod is to add a 3-Band passive EQ tone control after the second triode gain-stage, you could either use the Fender 3-Band EQ component values, or, the Marshall 3-Band EQ component values, I'm considering using a Mosfet, such as the 2N7000, as a Common Source amplifier stage to act as a buffer for the second triode gain stage so that the 3-Band EQ doesn't present too much of a load to the second triode gain stage, the result should be a tone that is a bit brighter, the other mod is to add an extra gain stage at the front end of the circuit, again made up of a 2N7000 Mosfet, this should increase the amount of gain attainable from the circuit, I will have to check to see if the mods will work as intended though.

JohnH
17-10-2017, 07:07 PM
Sorry to revive an old thread, but did you ever end up building your modded version Doc? Curious to see how it went if you did. I wouldn't mind having a crack at building this pedal myself

DrNomis_44
18-10-2017, 08:25 AM
Sorry to revive an old thread, but did you ever end up building your modded version Doc? Curious to see how it went if you did. I wouldn't mind having a crack at building this pedal myself


Not yet since I've had a few things on my plate lately, but I'll start working on it next month, actually, thanks for reminding me about it.....cheers.


I managed to find a couple of spare 9-pin valve sockets complete with metal shields, one is made from Ceramic, the other one is made from Teflon, I'm going to use the Teflon one to make a 4-Knob Tube Driver pedal, the other one will be used to make a Purgatory pedal, might even do a build thread so that you can see how I went about building it.


Might need to order some spare 12AX7/ECC83 valves from Evatco soon, since I'm running out of good ones, most of the spare 12AX7/ECC83 valves I have are a bit weak in gain.

Here's the link to the Evatco website again, if you're interested in ordering stuff online from them, click on the Products List menu to see what they have in stock:

http://www.evatco.com.au/

JohnH
19-10-2017, 04:17 AM
Thanks for the link! Will order some bits and pieces today and have a crack at it. If you get the time, a build thread would be great (especially for noobs like me)

DrNomis_44
19-10-2017, 09:11 AM
Thanks for the link! Will order some bits and pieces today and have a crack at it. If you get the time, a build thread would be great (especially for noobs like me)


No worries at all mate, hope the link proves to be of some use to you, I may even order some more 9-pin valve sockets when I get round to ordering some new 12AX7/ECC83 valves, I'm going to be ordering some new valves for my 5 Watt Legacy amp later on this year together with a new Jensen C10Q speaker to put in my old Peavey Backstage Plus amp so I can get it back up and running, building a Purgatory pedal may be a good project to work on over the Christmas holiday season come to think of it.

I've got quite a few good projects on my "to do" list, one project that I'm pretty keen on is the restoration project on my Fender Super Twin amp, I reckon that the Fender Super Twin amp is going to be a killer piece of gear for live gigging, I've had a yarn to a local Darwin repair tech about the restoration project, he seems pretty keen to help me out with it, I also have a speaker cab build project planned for it as well.

JohnH
20-10-2017, 12:56 PM
The Fender Amp sounds like a great project - you should definitely document it here on the forum.

I started getting the bits and pieces for the Purgatory Pedal today, so will have a bit of a play this weekend if I get any spare time.

Out of curiosity, what does the inverter do in this circuit? I've been reading up on them, but to be honest it completely confuses me, and I just ended up reading a lot about oscillator circuits (which was fun, but distracting)

DrNomis_44
20-10-2017, 01:25 PM
The Fender Amp sounds like a great project - you should definitely document it here on the forum.

I started getting the bits and pieces for the Purgatory Pedal today, so will have a bit of a play this weekend if I get any spare time.

Out of curiosity, what does the inverter do in this circuit? I've been reading up on them, but to be honest it completely confuses me, and I just ended up reading a lot about oscillator circuits (which was fun, but distracting)


Basically, all the inverter circuit does is it takes the lower supply voltage from the plugpack adaptor and converts, or steps it up, to a higher DC voltage that the valve can use to run properly, even though a 12AX7 valve can work with voltages as low as 12V DC on it's anodes, it tends to work a lot better if the anode voltages are more in the range of 50V to say 100V, the 12AX7 valve was actually designed to operate with anode voltages up to a maximum of 300V DC, that's where it is able to produce the most voltage-gain, by gain, I mean amplification, if you run a 12AX7 at lower anode voltages, the 12AX7 tends to distort in a characteristic way earlier, of course we could replace the inverter with a 240V AC mains transformer that can supply up to 300V DC, but they tend to be a bit expensive and dangerous, using a plugpack adaptor with the inverter circuit works out to be cheaper and safer too.

JohnH
20-10-2017, 01:36 PM
Basically, all the inverter circuit does is it takes the lower supply voltage from the plugpack adaptor and converts, or steps it up, to a higher DC voltage that the valve can use to run properly, even though a 12AX7 valve can work with voltages as low as 12V DC on it's anodes, it tends to work a lot better if the anode voltages are more in the range of 50V to say 100V, the 12AX7 valve was actually designed to operate with anode voltages up to a maximum of 300V DC, that's where it is able to produce the most voltage-gain, by gain, I mean amplification, if you run a 12AX7 at lower anode voltages, the 12AX7 tends to distort in a characteristic way earlier, of course we could replace the inverter with a 240V AC mains transformer that can supply up to 300V DC, but they tend to be a bit expensive and dangerous, using a plugpack adaptor with the inverter circuit works out to be cheaper and safer too.

Ah, ok, that makes sense.

JohnH
30-10-2017, 06:33 PM
Got some tubes in the mail today, so am finally fully prepped to give this a go. As a heads up, I may have to ask a million dumb questions. Apologies in advance...

JohnH
12-06-2018, 08:04 PM
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.

DrNomis_44
13-06-2018, 10:12 AM
Got some tubes in the mail today, so am finally fully prepped to give this a go. As a heads up, I may have to ask a million dumb questions. Apologies in advance...


Ah that's okay, there's no such thing as a dumb question, they are all valid in my book if the answers help.

DrNomis_44
13-06-2018, 10:13 AM
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
15-06-2018, 10:30 AM
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:

26974


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
15-06-2018, 04:23 PM
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
15-06-2018, 07:00 PM
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:

26974


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
15-06-2018, 07:10 PM
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
06-06-2020, 07:39 PM
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
06-06-2020, 07:46 PM
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
06-06-2020, 07:49 PM
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
06-06-2020, 08:02 PM
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.

35538

I thought it should be like this?

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

JohnH
06-06-2020, 08:03 PM
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

WeirdBits
06-06-2020, 08:39 PM
35539

(tube socket pins)

JohnH
06-06-2020, 08:48 PM
35539

(tube socket pins)

Excellent! In that case I've just wired the socket backwards, so hopefully that'll be an easy fix tomorrow.

Thanks heaps

JohnH
07-06-2020, 06:12 PM
Got this wired up today, plugged it in, and poof! Magic smoke from the IC.

After a closer inspection there's a bit of stray solder joining two tracks so I imagine that had something to do with it. Will try again tomorrow with fresh eyes

jugglindan
07-06-2020, 06:17 PM
Good luck. I have liberated a lot of magic smoke in my time. There's no getting it back in either :(
I hope you have spare chips.

JohnH
07-06-2020, 06:20 PM
Luckily past John was kind to present John, and ordered many chips lol.

JohnH
07-06-2020, 06:21 PM
I forgot how gross the smell is when you release the magic smoke. Found two other half built pedals in a drawer while looking for the 12v power supply, so will look more closely tomorrow and see what they need to be finished

DrNomis_44
07-06-2020, 06:22 PM
Oh the dreaded magic-smoke locked-up inside those components, fortunately I haven't had that happen to me very often....touchwood, although one time I did experience what happens when an electrolytic capacitor explodes, first you hear it go bang, and then you smell the aftermath.

jugglindan
07-06-2020, 06:35 PM
Fortunately I wasn't around when this happened:
35562

It's on a 30w Kustom acoustic amp chassis I picked for $5. I will try to fix it one day. Or raid it for parts. If I could ID the spring reverb unit on it that might make it into my valve amp build if I ever get around to that.

DrNomis_44
07-06-2020, 08:34 PM
Fortunately I wasn't around when this happened:
35562

It's on a 30w Kustom acoustic amp chassis I picked for $5. I will try to fix it one day. Or raid it for parts. If I could ID the spring reverb unit on it that might make it into my valve amp build if I ever get around to that.


Yeah, that looks like a classic example of an exploded electrolytic cap, if you're lucky all you'll need to do is replace that cap and the amp should work again, I've seen an exploded electrolytic cap where the aluminum casing had literally been blown-off, and you could clearly see the insides of the cap that had exploded, it was in a DC regulated variable power supply I had built from a kit I bought from Jaycar Electronics.


If I were you, I'd replace both of those caps with new ones of equal or greater capacitance and working voltage.

JohnH
08-06-2020, 07:27 AM
Fortunately I wasn't around when this happened:
35562

It's on a 30w Kustom acoustic amp chassis I picked for $5. I will try to fix it one day. Or raid it for parts. If I could ID the spring reverb unit on it that might make it into my valve amp build if I ever get around to that.

It'd be fun to see if you can get it going again, but then sometimes those projects are such huge time-sucks

jugglindan
08-06-2020, 07:45 AM
There are two problems with repairing that amp. First I don't have a cabinet or speaker. If I went to the trouble of making a cabinet, I would prefer to use it for a tube amp build. Second, I don't need an acoustic amp anyway.

I may just raid it for parts. There is a $10 op amp on a daughter board just ripe for taking. Plus the spring reverb, pots, knobs etc. More than $5 in parts easy.

Sent from my moto g(7) using Tapatalk

JohnH
08-06-2020, 07:49 AM
Ahhh, yep, the lack of case or speaker is a dealbreaker.

JohnH
08-06-2020, 06:23 PM
Victory!

Fixed the solder bridge, plus a few solder joins that looked problematic, new IC and it all works fine. Can't really test it properly while the kids are in bed, but I'm pretty chuffed. My electrickery skills are very minimal so every time something works I'm equal parts stunned and amazed

jugglindan
08-06-2020, 09:02 PM
Congrats! My electrical skills are somewhat better than my woodworking, but I am still surprised and relieved when things work.

WeirdBits
08-06-2020, 09:13 PM
The Victory Fuzz. Well done.

JohnH
09-06-2020, 11:50 AM
Congrats! My electrical skills are somewhat better than my woodworking, but I am still surprised and relieved when things work.

My son asked why I kept working on electronic projects when they're so obviously frustrating, and I found it very hard to explain the satisfaction of pushing through and making the d**n thing work

jugglindan
09-06-2020, 12:13 PM
Santosh Kalwar said (among other things I am sure):

Never stop just because you feel defeated. The journey to the other side is attainable only after great suffering.

I am pretty sure they were talking about DIY projects.

FrankenWashie
09-06-2020, 05:41 PM
Oh the dreaded magic-smoke locked-up inside those components, fortunately I haven't had that happen to me very often....touchwood, although one time I did experience what happens when an electrolytic capacitor explodes, first you hear it go bang, and then you smell the aftermath.

Mmmmm....aftermath.....smells like opportunity.

jugglindan
09-06-2020, 06:09 PM
I do love the smell of aftermath in the morning. Much better than the smell of pure math, which hurts my brain.

McCreed
09-06-2020, 06:36 PM
I'm not sure if was Magic Smoke or Idiot Dust that came out the time I tried to test our electric horse fence (7000 volts) with my multimeter... :o

D'oh!!!

JohnH
09-06-2020, 06:45 PM
I'm not sure if was Magic Smoke or Idiot Dust that came out the time I tried to test our electric horse fence (7000 volts) with my multimeter... :o

D'oh!!!

Woah! That is very many volts

FrankenWashie
09-06-2020, 06:48 PM
Woah! That is very many volts

One would assume it is very low Amperage? I’d hope?

McCreed
09-06-2020, 06:59 PM
One would assume it is very low Amperage? I’d hope?

Yes, that's why it doesn't kill you, but will deliver a helluva a thump. (still kills a multimeter though :D)

Not one of my brighter moments btw. Not sure what I was thinking...

Cliff Rogers
09-06-2020, 07:52 PM
I know about electric guitars but I didn't know there were electric horses. :confused: ;)

jonwhitear
16-06-2020, 08:06 PM
I think I'm going to have a go at making this pedal - looks like a fun project. Can anyone recommend part numbers for the 40106 (Jaycar have a 74C14 Hex Schmitt Trigger which looks like it should be OK) and the mosfet (Jaycar have a 2N7000 N-Channel FET, but it's only 60V)?

Cheers, Jon.

JohnH
17-06-2020, 05:45 AM
Hey Jon,

I thought 74C14 and 40106 were the same, but I haven't looked at the datasheets to confirm:

http://electronica.ugr.es/~amroldan/cursos/2014/pcb/modulos/documentos/smd/Surface%20Mount_archivos/chip/74C14.html#:~:text=74C14%2040106,voltage%20from%20 5v%20to%2015v.

I'm not sure where you're putting the 2N7000?

DrNomis_44
17-06-2020, 01:04 PM
Here's a simple circuit for generating the high voltage needed to run tubes, like the 12AX7/ECC83 etc, properly, it's basically a 60kHz square wave oscillator made from a CD4049, or CD4069 Cmos Hex Inverter IC, two 33k resistors, and a 330pF capacitor, and a voltage multiplier made from five 1N4007 1000V PIV Silicon Power Diodes, and six 33uF/50V electrolytic caps.

Inverters D,E, and F are just used to buffer the output of the 60kHz square wave oscillator, formed by inverters A,B, and C, the two 33k resistors, and 330pF cap, so that it has enough current-output capacity to drive the voltage-multiplier circuit.

I remember doing some breadboard experiments with this circuit, and found that I could increase the output-voltage, simply by adding more 1N4007 diodes and 33uF/50V caps, I was able to easily generate +90V DC from a single 9V battery that I used to power the circuit.

Here's a pic of the circuit for those who are interested in checking it out:

35974


One great thing about this circuit is that you can easily power it off a +15V DC supply, which means that you can then also run the tube heater off a DC supply as well, which will reduce hum.


The only caveat with this circuit is that the working voltage of each of the 33uF caps needs to be at least 50V DC or greater, 33uF/63V caps should work fine, I've used some 1uF/100V caps in the circuit instead of 33uF/50V and they worked fine.


As a side-note, if you build the high voltage power supply circuit as I have posted above, you will get about +45V out of it, to increase the output-voltage you need to add more 1N4007 diodes and 33uF/50V caps.

jugglindan
17-06-2020, 01:48 PM
Interesting. I have seen charge pumps using the ICL7660S followed by a similar diode/cap multiplier, but not with the CD4049.

jonwhitear
17-06-2020, 08:30 PM
I thought 74C14 and 40106 were the same, but I haven't looked at the datasheets to confirm:

Thanks John.


I'm not sure where you're putting the 2N7000?

DrNomis_44 added it in his modifications, earlier in the thread. I've drawn up the circuit, which shows it.

36002

jonwhitear
17-06-2020, 08:33 PM
The only caveat with this circuit is that the working voltage of each of the 33uF caps needs to be at least 50V DC or greater, 33uF/63V caps should work fine, I've used some 1uF/100V caps in the circuit instead of 33uF/50V and they worked fine.

Would there be any changes to the original circuit (with your mods) required with this power supply? I gather the original makes about 70V, whereas this one's about 90V.

jugglindan
17-06-2020, 08:49 PM
That's a beautifully drawn schematic! Lovely.

JohnH
18-06-2020, 05:45 AM
DrNomis_44 added it in his modifications, earlier in the thread. I've drawn up the circuit, which shows it.

36002

Ahhh, yep. Missed that, sorry.

DrNomis_44
18-06-2020, 06:35 AM
Would there be any changes to the original circuit (with your mods) required with this power supply? I gather the original makes about 70V, whereas this one's about 90V.`


Not really, as far as I can see, all you would need to do is just replace the original +70V power supply with the CD4049 high voltage supply circuit I posted, the rest of the Purgatory Pedal circuit would remain the same, you might need to experiment with the 1N4007 diodes and 33uF/50V caps and see how many of each you need to add to get about a +70V output, the high voltage power supply circuit I posted will actually generate about +45V DC with five 1N4007 diodes and six 33uF/50V caps when powered off a +12V DC supply, so to get about +70V you might need to add maybe three more 1N4007 diodes and 33uF/50V caps to the voltage-multiplier section, I'd breadboard the high voltage power supply first and then tweak it to get +70V out of it.

You might need to substitute a different Mosfet for the 2N7000, maybe an N-Ch Power Mosfet that's capable of running on about 100V or maybe higher, since the 2N7000 is only rated to 60V maximum there's a likelyhood that running it off a +70V supply might destroy it.


Since the N-Ch Mosfet is connected-up in a configuration called a common-source amplifier, or source-follower, it actually produces a voltage-gain of 1, or slightly less than that, what it does do is it converts the high output impedance of the tube's plate/anode to a lower value so there's no loading effects.

jonwhitear
18-06-2020, 08:32 AM
That's a beautifully drawn schematic! Lovely.

Thank you!

DrNomis_44
18-06-2020, 08:36 AM
If any of you guys manage to build a fully-working version of the modded Purgatory Pedal schematic with the Marshall tone-stack, please post an audio demo of it cause I'm keen to hear what it actually sounds like....cheers in advance.

jonwhitear
18-06-2020, 12:08 PM
If any of you guys manage to build a fully-working version of the modded Purgatory Pedal schematic with the Marshall tone-stack, please post an audio demo of it cause I'm keen to hear what it actually sounds like....cheers in advance.

I'm planning to, depending on availability of parts (and time.) I haven't come across charge pump power supplies before. Would you recommend I go with Lawry's original power supply design, or with the one you've just posted? What are the advantages of the latter? Cheers, Jon

Simon Barden
18-06-2020, 02:48 PM
The higher the voltages, the more you'd need to be careful with the wire used, to make sure its insulation was adequately rated. You'd need to use something that had a stated rating, rather than use anything you had lying around of dubious provenance. Some cheap wire is only good for 30v or so.

DrNomis_44
18-06-2020, 03:15 PM
I'm planning to, depending on availability of parts (and time.) I haven't come across charge pump power supplies before. Would you recommend I go with Lawry's original power supply design, or with the one you've just posted? What are the advantages of the latter? Cheers, Jon


If you breadboard the circuit first, you could try either of the two high voltage power supply circuits, the main advantage I can think of for the circuit I posted, is that it will run off a DC supply, that means you can also run the tube heater off a DC supply as well, doing so will reduce unwanted hum.

DrNomis_44
18-06-2020, 03:17 PM
The higher the voltages, the more you'd need to be careful with the wire used, to make sure its insulation was adequately rated. You'd need to use something that had a stated rating, rather than use anything you had lying around of dubious provenance. Some cheap wire is only good for 30v or so.


Good point, not sure what the insulation of the wire that I normally use, is rated for.

jonwhitear
18-06-2020, 04:18 PM
If you breadboard the circuit first, you could try either of the two high voltage power supply circuits, the main advantage I can think of for the circuit I posted, is that it will run off a DC supply, that means you can also run the tube heater off a DC supply as well, doing so will reduce unwanted hum.

Ah, yes. I had missed that rather obvious difference. Thanks.

DrNomis_44
18-06-2020, 09:17 PM
Thanks John.



DrNomis_44 added it in his modifications, earlier in the thread. I've drawn up the circuit, which shows it.

36002


That 250k pot for the midrange control really should be a 25k pot.


I've got a copy of VeeCad installed on my studio desktop pc, VeeCad is a small app for designing Vero/Strip-board layouts, anyway, I'm going to see if I can create a Vero-board layout for the modded Purgatory Pedal circuit I posted in this thread, so I can have a go at building it, I think I've got a spare 12V DC plugpack adaptor in amongst all my bits and pieces, I'm also thinking of expanding the basic design so it's like a 19 inch 1U rackmount guitar preamp with channel switching, lots of potential there.

jonwhitear
20-06-2020, 04:24 PM
`
You might need to substitute a different Mosfet for the 2N7000, maybe an N-Ch Power Mosfet that's capable of running on about 100V or maybe higher, since the 2N7000 is only rated to 60V maximum there's a likelyhood that running it off a +70V supply might destroy it.

Something like this?

ZVN4310A - Power MOSFET, N Channel, 100 V, 900 mA, 0.65 ohm, TO-226AA, Through Hole

https://au.element14.com/diodes-inc/zvn4310a/mosfet-n-logic-e-line/dp/9524991

DrNomis_44
20-06-2020, 04:34 PM
Something like this?

ZVN4310A - Power MOSFET, N Channel, 100 V, 900 mA, 0.65 ohm, TO-226AA, Through Hole

https://au.element14.com/diodes-inc/zvn4310a/mosfet-n-logic-e-line/dp/9524991


Yeah, I reckon that should do the job fine.

DrNomis_44
20-06-2020, 04:41 PM
For those of you wanting to build the modded Purgatory Pedal circuit, the values of the three pots used in the Marshall tone-stack that I added, are as follows:

Treble: 250k B.

Mid: 25k B.

Bass: 1M A.

jonwhitear
21-06-2020, 07:27 AM
I've updated my drawing to reflect the correct tone pot values, the uprated mosfet, and your DC HV supply. I'd be grateful if you would check the supply - I wasn't clear quite how it's connected up.

36090

DrNomis_44
21-06-2020, 09:44 AM
I've updated my drawing to reflect the correct tone pot values, the uprated mosfet, and your DC HV supply. I'd be grateful if you would check the supply - I wasn't clear quite how it's connected up.

36090


Yep, I just checked it out and as far as I can see it looks all correct to me, all the 33uF/50V caps in the DC HV power supply look like they're oriented properly, as are the 1N4007 diodes, so it should work properly, one small mod to the circuit that I would make is to put a 10 Ohm resistor in between the tube heater (Pin 4 of U2) and the +V supply for the whole Purgatory Pedal, and then change the supply voltage to +15V DC, this will enable the tube's heater to run at about 12.6V.


The 10 Ohm resistor can be a 5 Watt ceramic wire wound type.


I definitely need to have a go at building the modded Purgatory Pedal circuit one day cause I'm curious to find out what it actually sounds like.

Simon Barden
21-06-2020, 06:51 PM
I may be showing my ignorance here but I thought that caps started to de-rate in value if subjected to more than roughly half the rated insulation voltage. So maybe a slightly bigger value for C7, or a higher voltage rating if you are pushing for 70+ V.

It's also not drawn for true bypass, so you'd need to modify it if you wanted that as well.

DrNomis_44
21-06-2020, 07:11 PM
I may be showing my ignorance here but I thought that caps started to de-rate in value if subjected to more than roughly half the rated insulation voltage. So maybe a slightly bigger value for C7, or a higher voltage rating if you are pushing for 70+ V.

It's also not drawn for true bypass, so you'd need to modify it if you wanted that as well.


Good point, maybe use a 47uF/350V cap for C7, or maybe even a 47uF/450V cap.

JohnH
22-06-2020, 06:03 AM
Good point, maybe use a 47uF/350V cap for C7, or maybe even a 47uF/450V cap.

The only downside to the 450V cap (from jaycar at least) is it's size - I'm still pondering how to get my circuit into an enclosure, ha ha

jugglindan
22-06-2020, 08:02 AM
I may be showing my ignorance here but I thought that caps started to de-rate in value if subjected to more than roughly half the rated insulation voltage. So maybe a slightly bigger value for C7, or a higher voltage rating if you are pushing for 70+ V.

It's also not drawn for true bypass, so you'd need to modify it if you wanted that as well.

It looks like true bypass to me, as SW1B connects the input directly to output in the bypass position. But it's not the bypass wiring I normally use (http://stinkfoot.se/archives/2233). I suggest using a 3PDT instead of a 2PDT so you can also ground the effect input (effect send) in the bypass position. From the perspective of the effect circuit this is the same as winding the guitar volume all the way down. Apparently this helps prevent popping when turning the effect on and off, although R11 should help prevent popping as well.

My other comment is that the power indicator LED current limiting resistor value is quite a lot lower than what I would use. At 12v, the 1k CLR will give 12mA through the LED which will tend towards the bright side. On my solid state pedals, I aim for closer to 3 or 4mA which I find to be bright enough. On solid-state pedals, 12mA for the LED could double the power draw of the pedal. Clearly with tubes that won't be the case, but it still might be too bright. A 3k CLR will give 4mA from 12V.

Of course, it highly depends on the mcd rating of the LED, which can be very variable. Jaycar sell 5mm LEDs ranging from 8mcd (very dim) to CREE LEDs at over 60K mcd. Which would be interesting. You could use the pedal indicator as stage lighting!

DrNomis_44
22-06-2020, 10:01 AM
Depending on what kind of led you use, you might need to use a current-limiting resistor as high as maybe 47k for one of those very bright 3mm blue leds, best thing to do is do some breadboard experimenting with different resistor values first.

DrNomis_44
22-06-2020, 10:07 AM
The only downside to the 450V cap (from jaycar at least) is it's size - I'm still pondering how to get my circuit into an enclosure, ha ha


Yeah that's true, I think the 22uF/450V caps are a bit smaller than a 47uF/450V cap, so if you put two in parallel you'll effectively get a 44uF/450V cap.

jugglindan
22-06-2020, 10:24 AM
Depending on what kind of led you use, you might need to use a current-limiting resistor as high as maybe 47k for one of those very bright 3mm blue leds, best thing to do is do some breadboard experimenting with different resistor values first.

Yeah, there is a huge variation in available LED brightness. I always breadboard the resistors since my box of LEDs vary quite a bit in brightness by colour, and also to a lesser extent within a colour. Blue tends towards the bright side. I suspect it's due to the complex shenanigans required to get a blue colour.

Simon Barden
22-06-2020, 04:09 PM
'True bypass' avoids any connection of the input signal to the circuit, and on this pedal as shown, the signal is still connected when in bypass mode. It may or may not not alter the sound, but for some true bypass freaks, it won't be acceptable.

Take an extreme case of the circuit simply consisting of a capacitor connected to ground (like the tone circuit on a guitar but with the pot set to 0). Then the capacitor would still affect the tone of the signal in both 'in' and 'bypass' switch positions. You'd need the input signal and the output signal to be disconnected for the circuit not to affect the tone.

The circuit as shown doesn't disconnect the input from the circuit, so isn't even the 'basic true bypass' of the linked article. It's just a bypass. Semantics maybe, but still important as you are reliant on the design of the circuit not to noticeably affect the tone in bypass. And there could also be knock-on effects with respect to increased ground loop noise with some circuit designs.

For this circuit, you've got a 1M ohm resistor down to ground that's certainly connected in bypass. This will be in parallel to the amp's input impedance, so assuming that is 1M, you've now got 500 k Ohm. Still pretty much OK. But assume there are several other pedals with a similar bypass design. You can easily drop down to a 250k ohm or below equivalent input impedance, which will start loosing top end if there's no buffered circuit upstream.

So I'd definitely recommend using a 3PDT switch so you can call it true bypass.

jugglindan
22-06-2020, 05:06 PM
OK, I see your point and gracefully concede :)

/me goes back to check my bypass wiring ...

Yeah, the scheme I use connects the effect send (input) to ground, and leaves the effect return (output) disconnected in the bypass state.

Simon Barden
22-06-2020, 05:35 PM
A bit of a mental exercise for me as well. I can now see why people insist on either a good buffer or a true bypass switch.

jugglindan
22-06-2020, 05:39 PM
I prefer true bypass and separate buffer pedals. That way I avoid the issues caused by too many buffers (I watched a video where they hooked a bunch of Boss pedals together, all buffered - the audio result with all pedals bypassed was not pretty) or not enough, and I get to choose where the buffer goes.

It's also why I use a trusted wiring diagram each time. I don't need to rethink this, just follow the instructions.

Simon Barden
22-06-2020, 05:56 PM
Some of the Boss designs haven't changed since they were designed in the 70s/80s, and buffer design has moved on considerably since. There's no reason why a good modern buffered pedal shouldn't be totally transparent in bypass.

JohnH
21-05-2021, 08:53 AM
It's embarrassing it's taken so long to finally box this up, but I thought I'd share the final product with you all...

I definitely recommend building one of these - it's a lot of fun to play

40627

WeirdBits
21-05-2021, 12:28 PM
Looks great.

jonwhitear
18-07-2021, 02:43 PM
Well, I've just placed an order with Tayda, so my tube driver build is now officially underway (as opposed to being unofficially underway, as it has been for the last year or so ;-)

jonwhitear
31-08-2021, 07:32 AM
It's embarrassing it's taken so long to finally box this up, but I thought I'd share the final product with you all...
I definitely recommend building one of these - it's a lot of fun to play


John, I love your idea of using a U-Bolt to give the tube some protection - I'll be doing the same. Do you recall what size bolts you used? In non-pandemic conditions, I'd take a tube to Bunnings and eyeball them, but that's obviously not possible in Sydney at the moment.

Also, do you have any photos of the internal layout of your pedal by any chance? I'm trying to work out how to fit everything into the enclosure, and I'm currently planning to include the full tone stack, so I'll have three extra pots as well.