Author Topic: Zonk Machine from anyone build?  (Read 3232 times)


Zonk Machine from anyone build?
« on: April 07, 2007, 01:24:09 AM »
I'm working on a Zonk Machine and Treble Booster that I got from
I'm having a bit of trouble biasing the transistors... and the schematic has R9 listed in 2 places. I'm assuming that's an accident. and I'm not sure what transistor to use on the treble booster side. And which cap to use on for DC blocking on the input.
Anyone have some insight?


Re: Zonk Machine from anyone build?
« Reply #1 on: April 22, 2007, 07:06:10 PM »
I've been trying to build the same thing Dustbro. Mine's pretty much dead in the water. Small Bear electronics sells the 2N4861 jfets that are supposed the be used on the treble booster side. You'll notice the pinouts have "S" "D" "G" for "source" "drain" and "gate" instead of the emitter collector base for a transistor. The build instructions aren't really clear about that fact. Took me a while to figure it out. I originally stuck in a pnp germanium like the others. The 2N4861 doesn't seem to pass any signal. I double checked the pinouts with the fellow from Small Bear who gave me a speedy response. I'm wondering if the build instructions have the orientation wrong?
  I've spent hours and hours trying to debug it. My lack of knowledge doesn't seem to help much haha.The most I've gotten out of it is a quiet sputter. I have a 25k pot in place of the 15k resistor to bias Q3.
  Has anyone here succesfully made the Zonk Machine using the General Guitar Gadgets RTS board? I'm curious to find out how a working one sounds. My transistor voltages read: Q1: (Emitter, Base, Collector) -153mv,-214mv, -9.82V.  Q2: 0v, -107mv, -8.52v. Q3: 0v, -23mv, -9.21v. The jfet measures -7.86v on the source, -7.73v on the drain, and -8.50v on the gate. Weird.



Re: Zonk Machine from anyone build?
« Reply #2 on: April 22, 2007, 08:50:23 PM »
I've built one using the Geofex layout fairly recently, with the treble booster. It's not that wild and crazy, but it's warm and horn-like...the album UFO by the Krautrock band Guru Guru has Zonk machine all over it.

There's a few head scratchers in the layout. But they can all be solved by searching and looking at the schematic (I didn't have to outright guess about anything to get it to work).

I went with a 47uF cap for the input. I think both R9s should be 56k. I used a BF245A in the treble booster, with the legs twisted so the middle leg went in the 'S' hole. I used three AC125 germaniums with almost identical hFEs of 160 (using the tester) and little leakage. I haven't really played around with it too much, to optimise it, but it sounded pretty good straight away.

I hope that helps. Ask me more questions if you have them.



Re: Zonk Machine from anyone build?
« Reply #3 on: April 23, 2007, 03:53:58 AM »

  I tried shifting the orientation of the jfet legs and found if I reversed the source and gate of the jfet it started working. Pretty much opposite what the schematics I've come across say. I switched both the "rz9's" to 56k and the pedal started coming to life. I think its probably more a matter now of tweaking the bias on the transistors. What's the output like on yours stobiepole? I'm getting about unity gain right now. Not a lot of sustain but the tone is pretty awesome. Nice throaty mids and fuzz texture with pronounced pick attack. If I can get some decent gain out of it the Zonk Machine mught find its way onto my pedalboard.



Re: Zonk Machine from anyone build?
« Reply #4 on: April 23, 2007, 04:40:36 AM »
Well, you know, gain's always easy to get, but tone is tricky...I don't think mine's terribly loud, but you could easily put a booster after the pedal (or in the same box) if you need it. I think that if you put a transistor with higher hFE in Q3 and play with the value of R8 you should get more volume. It's a nice pedal with a wah in front.



Re: Zonk Machine from anyone build?
« Reply #5 on: January 31, 2008, 05:21:00 AM »
Thought I would dust this one off. I was never able to get my GGG Zonk machine to bias up as per RG's notes. I decided to do a new layout in Eagle and try it out. At first I was having similar problems to the GGG version. Couldn't get Q1's emitter past -.33 volts... Q3's collector wouldn't go below -9.44. Have a 25k pot instead of the 15k biasing resistor. I was swapping out resistors trying to bias q1.
  I had some 2n404's in there at first. Not sure what their gain/leakage was. I tried some oc75s and started to see a very slight improvement in the bias. On a whim I grabbed some transistors I had tested earlier in the day and put aside to be used as diodes due to their large amounts of leakage. One measured something like 300 hfe with more leakage than I think I had ever seen. I threw them in and the pedal turned into a fat focused tonebender/treble booster. Checked the bias and was able to use the trimpot to bias q3's collector to the recommended -7 V. I checked q1 and saw that the emitter was reading around 2 volts. Close enough. I'll have to play it through an amp instead of headphones tomorrow but I was more than pleased with the sound. Before it was just sounding like a mild booster with some slight misbiasing artifacts. Now its fat, focused and fuzzy with a nice sort of half %^&*ed wah sound.
  If you're having trouble with the Zonk maybe grab some really leaky transistors and throw them in, see what happens


John Lyons

Re: Zonk Machine from anyone build?
« Reply #6 on: January 31, 2008, 11:04:16 AM »
I built mine on the breadboard and then tweaked it to taste.
I built just the zonk and not the combo with the treble booster.
I also used untested Ge transistors, just swapping them out until I got a good gainy one that would bias up.

I went with:
1.8M for R2 (1M)
12K for R7 (3.3K)
47K for R9 (10K)
13K for R8 (15K)

Q1 1.34v
Q2 collector 5v
Q3 C 6.8v

I also got rid of that 2M/56K at the end and put in a .01 to ground off C4 with a 17K or so resistance between the cap and ground as a LPF shelf. From C4 I go to the the 500K level pot. If you keep the 2m2/56K just make the 56K larger, try 100K and you'll get a bit more level out of the circuit.

Here's a quick sound CLIP of what I have with a Strat and then a Les Paul.
The circuit pumps a little with a loud input. I still need to play with it. The input cap I'm using is .002 and it calls for .001...


Basic Audio Pedals


Re: Zonk Machine from anyone build?
« Reply #7 on: January 31, 2008, 02:14:53 PM »
Yeah this time around I left out the treble booster. About that treble booster though. The schematic I've seen has it negative ground. On the GGG board its incorporated with the Zonk on a positive ground board. I've been spending most of my efforts trying to understand regular transistor bias so I haven't dealt much with jfets. What type would work in a positive ground HB treble booster? My Zonk doesn't seem to need a booster but why not give myself another reason to bust out the solder?



Re: Zonk Machine from anyone build?
« Reply #8 on: January 31, 2008, 02:43:39 PM »
You need to do some searching in the archives. There's been all kinds of discussion/controversy/speculation about the Hornby-Skewes treble booster in the past.

I breadboarded a Zonk Machine once and wasn't too impressed. Just sounded like another Fuzz Face variation to me. I've already got an Easy Face so it wasn't worth my time to build the ZM. YMMV...

"I can explain it to you, but I can't understand it for you."