Spark Gap: Unusual 9V Hybrid Valve Overdrive

[PRR]

I don't see how the voltage regulation on the heaters works, what voltage does it set them to?

http://www.ee.nmt.edu/~thomas/ee322_s06/labs/volt_reg/ee322_lab1.html

[merlinb]

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.

I'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!

[blackcorvo]

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? 

[bcalder]

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,
Bruce

[merlinb]

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:

[bcalder]

Got it now - thank you.

[bcalder]

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!

[bcalder]

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!!

[Mustachio]

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

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

[merlinb]

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...

[stevie1556]

What are you thinking about doing for the redesign?

[Mustachio]

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 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::::::::::::

[Mustachio]

ok maybe this will work

[merlinb]

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...

[stevie1556]

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

[dukie]

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.  
This baby will be in my next build list for sure..

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!

[iccaros]

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.

[asbrooks]

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 
 

[merlinb]

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! )

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!

[rankot]

Merlin, why did you use NPN/PNP bypass circuit, and not switch?