Spark Gap: Unusual 9V Hybrid Valve Overdrive

Started by merlinb, January 19, 2011, 12:12:31 PM

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Quote from: armstrom on March 01, 2012, 10:08:02 AM
My question is, did you only need to mess around with the heater voltage on your design because you were dealing with the smaller voltage swings inherent in a 9V pedal design?
Yes, exactly. With larger signal levels like you get in a high voltage design, 6.3V heaters will work fine.

QuoteI'm also considering adding a pot to the shared ground connection for the diodes so that I can lift them slightly and tame the clipping. I'm not sure how well that will work though. Your thoughts?
Should work great! I did almost the same thing in an 18Watt once, but with the diodes+pot strung between the grids of the push-pull output valves. Got some nice distortion down to whisper levels!


Quote from: merlinb on March 02, 2012, 04:18:15 AM
Should work great! I did almost the same thing in an 18Watt once, but with the diodes+pot strung between the grids of the push-pull output valves. Got some nice distortion down to whisper levels!

Wow... now THAT sounds interesting! Care to share a scribble of that, oh mighty wizard of the glowy toobs?  :icon_cool:
She/They as of August 2021


Hi all, I'm at the home stretch with my SparkGap, but have a newbie question.

I'm confused about what goes where on the three center bottom points - I see them on the schematic & layout, but not sure which lugs on the switch correspond to them.

I will probably use a 3PDT instead of DPDT switch. I understand I only use three lugs (thanks Merlin), but not sure which will go to the point that connects to R14 & which to the one that connects to Link2.

Thanks in advance,


Quote from: bcalder on April 01, 2012, 04:11:48 PM

I'm confused about what goes where on the three center bottom points - I see them on the schematic & layout, but not sure which lugs on the switch correspond to them.

Here you go:



Hi all -

I inexplicably don't have any BC327's - I think Banzai sent me two orders of BC337's. Is it possible to sub in a BC337 at Q2?

Thanks very much for the help!


Ha, the bottom of my parts box yielded a BC327 ... I've been in Billy Gibbons land for the past hour and a half.

Thanks to Merlin for a wicked pedal & being an all-around great gent!!


Just seen this thread the other day for the first time! So I ordered eight 6AL5's before I was done reading this thread  :icon_lol:

Cant wait to get started this looks like a really fun build! thanks merlin!



Thanks! Actually I'm thinking of redesigning, but keeping everything in the same position on the PCB so I can just drop it into the existing enclosure...


What are you thinking about doing for the redesign?


Well I got all my parts and dove into making my own boards for the first time this weekend! This is my first build making my own pcb's , I used the positive photo process with mg chemicals products and it went really well!

I will post pics of the build process soon! I'm at the soldering components stage now. When printing the artwork for the pcb I couldn't find info for the dpi , so I just copied to clipboard and played with photoshop settings a few times to get it right. And I found it was right at 254 dpi. Now strangely I found this to be the case with a bunch of other pcb artwork I found in pdf's and gifs etc. and I'm wondering if it has something to do with a standard in one of the formats as to which these are shared(Copy/Paste/Monitor-dpi/Webformats?) , I thought for some reason we where supposed to be using 300-600dpi for pcb artwork.

But it works fine at 254dpi and maybe that will help some one else, I just set to mm or cm to what i could find on the pcb this one 60x80mm and when you change dpi when creating a new file from clipboard you can see the change in work area size. And I did a few test prints on reg paper to double check the sizing for ic socket and it was dead on!

I have been trimming the pcb with some large industrial snips and now filing down the sides and its just a hair away from the copper on both sides and still doesn't fit into a Hammond 1590N1. I mean its a Razor thin margin for fitting but I think ill get it :D just a lil more to file down, but it will be a very tight fit.

I could not find any tube sockets without a solid metal plug in the center so I drilled these out with my press. The center metal plugs actually hold them together and I figured so before I removed them. I used some super glue for now to keep them together. but I'm wondering if I should do something else maybe some Large shrink tubing.

Anyway this has been a really fun project for my first self made pcb build! I love tubes! and if this sounds in the ball park of a tube screamer I know ill love it!

Ill post more pics soon! Thanks again merlin! I love your circuits , I may build your engineers thumb on vero, I was itching to do it on pcb but I seen Harald at sabrotone made a vero of your engie thumb with all 5 knobs and a bright switch ! I would love to do it on pcb but Im not good enough yet to modify the pcb and make it ready to go with all controls.

:::::::::::::My pics keep disappearing was trying to use googles picasa or even the G+ photos to host and it seems to be given a lil issue. ill toss some up on a diff host later::::::::::::




Quote from: stevie1556 on May 21, 2012, 04:01:23 PM
What are you thinking about doing for the redesign?
The problem with the Tubescreamer/feedback diode approach is that it has no compression, it's just a fixed clipper. Personally I think this approach sounds dry and gets boring after a while.

More 'creamy' and touch responsive distortion circuits such as real valve amps and discrete OD pedals use AC coupling, and this leads to self rectifications effects that work a lot like a compressor.

I want to redisgn the spark gap to produce these effects, and get closer to a real valve amp sound. Possibly incorporate some SS clipping just to increase the available grind, with valve OD for colour...


Sounds like a fair amount of work fitting all that into the box! Good luck with it :)


Wow..I think i've been away from this forum waay too long..i was searching for the old "12ax7 in DS-1 mod" when stumbled upon this thread.
I think spark gap is even better than the 12ax7 DS-1 mod! because i can't get any explanation of how it done and nor any schematic of the DS-1 mod.  :icon_mrgreen:
This baby will be in my next build list for sure.. ;D

But Merlin, is it possible 12ax7 be used intstead of EB91? if it possible then what components must be added or changed?
Great build Merlin!

Cheers! ;)


the EB91 is a dual Diode, while the 12ax7 is a dual triode (has a control grid). so the pin out is different
to turn a triode into a diode, I believe you attach the grid to the plate you will have a diode.. But I believe Merlin did not have very much luck at these voltages with that.. see bottom of page 1 where Paul brought this up using 12au7.


Hi Merlin,
I realise that this thread is over a year old, I'm interested in how your redesign is going.
I use a solid state amp and would like to add some of that valve amp driven creaminess to it.
Sounds like it could be a great little pedal!
Looking forward to seeing the results  ;D


I haven't even touched this project again :(   Don't hold your breath people, I may never get round to it! (I am always surprised at the interest this design gets. It's not nearly as good, original or unusual as some of my other pedals! :icon_rolleyes: :icon_lol:)

A 12AX7 will almost certainly not work. It's reverse current and dynamic resistance are both probably too high. I suspect that's why you can't find any schematics for a DS-1 mod, i.e it probably doesn't work!


Merlin, why did you use NPN/PNP bypass circuit, and not switch?
60 pedals and counting!