Spark Gap: Unusual 9V Hybrid Valve Overdrive

[merlinb]

My most recent completed pedal. (I have actually been working on this since the summer, but it takes me ages to complete anything)

It is essentially a "real" Tubescreamer, since it used feedback clipping diodes, except they're replaced with EB91 / 6AL5 valve diodes. I discovered that by operating the heaters at reduced voltage (between 4V and 4.5V) you get much more favourable diode characteristics and true soft clipping.

The first clipping stage is voiced for treble and the second for bass/mids, so I named the knobs "bright" and "fat". This gives you control over what bands of frequencies and boosted / distorted, rather than simply distorting everything and then adding a tone control afterwards.

The sound varies from mild tone fattening at the lowest setting (a rather nice booster pedal) to medium overdrive, to medium/heavy distortion at max, but with a 'warmness' that a typical Tubescreamer can't muster. Clips to follow when possible.

Built-in voltage stabiliser and proper electronic bypass switching. And yes, the LEDs light up the valves when you hit the footswitch, so they act as a status indicator!

[merlinb]

Here are the measured characteristics of the EB91 at different heater voltages. Notice that above about Vh = 5V it actually doesn't act much like a diode as far as small signals are concerned, which is why simply subbing tube diodes into a tubescreamer circuit doesn't normally work. This valve is unusual though, because the whole characteristic can be shifted to the right by reducing the heater voltage- problem solved!

[Govmnt_Lacky]

Absolutely phenomenal work Merlin!     

[ayayay!]

I like your work!  For two reasons:

1)  The tube used.  I've never seen them before (in my limited knowledge of tubes) so it's great to see some tubes that work with such low voltage.

2)  The LEDs.  At first I thought, "Hey, he's not fooling anyone thinking they're 'glowing.' "  But then I saw that's the status indicator, which I think was a GREAT move.  

And thank you for sharing the schem, and the coming soundclips.  (<-- Hint hint) 

Is R1 one optional? 

[thetragichero]

hey, that's neat!

[Renegadrian]

Altough I am not particularly keen on hybrids, I see a great idea behind that pedal! I guess I have to build it sooner or later! Now I'll check those tubes, prices, availability...thx for sharing!

[thetragichero]

Altough I am not particularly keen on hybrids, I see a great idea behind that pedal! I guess I have to build it sooner or later! Now I'll check those tubes, prices, availability...thx for sharing!

i agree that most pedals that run a tube are bleh, but i LOVE the ingenuity of using a tube diode in place of clipping diodes.

you should build a cage to protect the tubies, if you're anything like me (i have a feeling i would either stomp on or kick them)

[Manny]

Great work Merlin, this looks like a very interesting build.
Finally a screamer that's actually tube 

I'm looking forward to hearing the sound clips!

[amptramp]

Nicely done!  Do you intend to add a filament voltage control so you  can adjust the clipping level?

[panterafanatic]

Pretty sweet, a variable heater control would work cool for something like this.

[merlinb]

Is R1 one optional? 

Yep.

you should build a cage to protect the tubies,

There was supposed to be a metal bar that ran over the tubes, but my friend (trainee metalwork teacher) made them the wrong size . I don't mind much though, as they're not easy to step on, all the way at the back and with the knobs in front.

Do you intend to add a filament voltage control so you  can adjust the clipping level?

Right now I don't plan on doing anything. It took me long enough to get this far! However, it is designed to accept both regulated and unregulated power adapters, and you get slightly different heater voltage depending on what you use, and it does affect the sound, so it's definitely a possible mod!

[PRR]

I don't see the sparks?

> it actually doesn't act much like a diode as far as small signals are concerned

What is "a diode"? Two bits in one bulb.

_I_ would say the vacuum diode is the better _rectifier_.

But when building a "clipper", you want an ideal diode PLUS a voltage. The fault of crystal diodes as rectifiers is a large built-in voltage. If this voltage is suitable for your clipper, the fault is a feature.

Cooling the cathode works (I'm sure you know) by lowering the average emitted electron velocity below zero. The high-tail of the gaussian curve can be urged to leave the cathode surface by some "large" positive voltage on the other electrode.

This is also the cause of DC error in tube DC amplifiers. Diff-pairs tend to cancel, certainly good enough for 1.5V VTVMs. More sensitive tube DC amps often sport regulated heater supplies (an awful expense in those days).

> The tube used.  I've never seen them before

Small twin-diode. THE way to demodulate FM. The standard AC-DC converter for VTVMs. Sometimes used in AM/SSB for detection and AVC or for (duh) clippers to tame ignition hash and other impulse noise. I have seen it used for B+ power supply for very-very small gear.

But the exact tube used is NOT a key detail. Take any isolated-cathode twin-triode. Tie each grid to its plate. It is now a twin-diode. Obviously it should cost more; in today's world you can buy 12AX7 on a rainy saturday night in a distant town, or may have a stash of odd-sounding 12AU7es which just might be groovy diodes.

> Do you intend to add a filament voltage control so you  can adjust the clipping level?

Just separate the 9V for the chippy stuff from the heaters, rig two supplies. Replace the heater regulation with a buffered potentiometer.



This may "go too far", the lowest half or third of knob-turn may not do much. Or maybe it will. try it.

[merlinb]

But the exact tube used is NOT a key detail. Take any isolated-cathode twin-triode. Tie each grid to its plate. It is now a twin-diode.

Unfortunately, all the regular double triodes I tested would not work in an application like this, because lowering their heater voltage simply raised the anode resistance rather than 'shifting the characteristics to the right'. It seems you need a close cathode-anode spacing to make it work.

> Do you intend to add a filament voltage control so you  can adjust the clipping level?
Just separate the 9V for the chippy stuff from the heaters, rig two supplies. Replace the heater regulation with a buffered potentiometer.

You could use simple zener stabilisation for the TL074/LEDs and leave the transistor to handle the 250mA heater current.

[merlinb]

The PCB layout I used can be found here:
http://s81.photobucket.com/albums/j207/merlinblencowe/Spark%20Gap%20EB91%20PEdal/

I use Evatron enclosures, but since it's 60mm wide I *think* it would fit in a Hammond 1590N.

[misterg]

Provisional stripboard layout: I have added a 100n cap across the zener out of habit (Cnoise on the layout). The taller electrolytics & Q3 need to lay down if you want it to fit in the Evatron box. Don't miss the track breaks under C1 & C2.



I have built from it, but I work from the schematic at the same time - use at your own risk!

The knobs are only stop-gaps - they're 1/4" bore knobs on a 6mm shaft, so they look even wonkier than they really are. For a bit of bling, I used rainbow colour changing LEDs to light the valves up when the effect is active














It sounds really smooth - a bit like a valve amp 'crunch' that moves seamlessly into a smooth distortion. I lent it to a friend who is a Proper Guitar Player (tm) who liked it. He had a quick go at recording it, but the recording sounded "like a bee in a bottle", despite sounding good in reality.

I have a PCB version on the bench, as well and it is no quieter or noisier than the veroboard version - it's a quiet pedal, noise wise.

If you fancy a great sounding  pedal with major mojo and no high voltage - give it a go!  

Andy

[Joecool85]

Sound clips please!  That is one neat circuit.

[Renegadrian]

thx for the vero!!!  Now I have no excuse!

[Govmnt_Lacky]

Keeping an eye on this one. Waiting for a PCB layout. I would try one myself but, I have 11 projects in work right now! 

[misterg]

Keeping an eye on this one.Waiting for a PCB layout. I would try one myself but, I have 11 projects in work right now! 

See the post above mine.

No excuses 

Andy

[Brymus]

IDK how I missed this one.
Splendid and superb looking pedal!
Thank you for sharing the design,Merlin !