Zonk Machine - why can't I get my OC44 working?

captaincoconut · Post by **captaincoconut** » 21 Dec 2020, 23:20

Greetings! I've built a good number of MKII Tone Bender variants over the years, but had just a handful of successful MKI builds. A while ago I set my heart on building a Zonk Machine and I'm running into a situation that makes me scratch my head. I based my build on the following schematic that was posted by user Jonasz a while ago:

As with all MKI builds I tried a bunch of different transistors. I got really good and pleasing results with a 2N1307 (Q1) and two OC75 (Q2&Q3) transistors. So far, so good.

As some original units used an OC44 for Q3 I also tried a bunch of OC44s I had laying around. Unfortunately, this made the unit sound like garbage and I would like to learn the reason for it. The decay got very fizzy and gated, very unpleasant. I'm aware that the Zonk is a rather bright and somewhat gated sounding circuit, but it's far away from what you'd expect to hear. I measured all the OC44s I have (about 10) with my trusty DCA55 and I got results ranging from 80 to 120 hfe with almost none to zero leakage. Collector voltage at Q3 is about -8.5 volts (should be around -7V according to RG Keens tuning tips) but fiddling with the 15k resistor results in just very slight changes in voltage.

My first guess is leakage, but - and here's why I'm scratching my head - there is at least one unit where Jon from Acidfuzz measured an OC44 with zero leakage for Q3. So they somehow made it work

Any help is appreciated!

theblackman · Post by **theblackman** » 22 Dec 2020, 00:18

component variation and biasing? did you measure the voltages and compare with other people's builds?

CheapPedalCollector · Post by **CheapPedalCollector** » 22 Dec 2020, 03:03

The schematic seems incomplete to me and there should be a resistor between -V and the base or one between the base and collector.

captaincoconut · Post by **captaincoconut** » 22 Dec 2020, 06:05

theblackman wrote: ↑22 Dec 2020, 00:18 component variation and biasing? did you measure the voltages and compare with other people's builds?

As I said in my opening post, Q1 and Q2 (bias) voltages are absolutely fine and what I would expect from the circuit. The only culprit is Q3 which won't bias up properly when using an OC44 transistor. As soon as I replace Q3 with another germanium transistor everything works fine, though. I tried a bunch of other OC75s, some Japanese 2SBxxx series transistors, some USSR ufos and so on. Boom, everything works, sounds great, box it up, right? I'd just like to know why those darn OC44 transistors won't bias up properly while other substitutes work absolutely fine.

CheapPedalCollector wrote:The schematic seems incomplete to me and there should be a resistor between -V and the base or one between the base and collector.

The schematic is correct.

theblackman · Post by **theblackman** » 22 Dec 2020, 06:08

what is the voltage at the base of Q3?

Manfred · Post by **Manfred** » 22 Dec 2020, 08:55

The bias voltage Q3 is generated at the 15 kiloohm resistor with the base collector leakage current.
Therefore, I assume that in this case this leakage current is too small and thus also the base voltage.
In general, it can be observed in germanium transistors that the base collector leakage current also increases with increasing DC current gain HFE.
Do you have the possibility to measure the current gain of this OC44?
Measure the base voltage as suggested by Theblackman.

KingWah · Post by **KingWah** » 24 Dec 2020, 21:19

Just a newbie never built anything and playing guitar most of my life but the idea that you're able to build fuzz boxes that I mean most people would not be able to afford and to build and try all these different flavors of fuzz in the search for your own personal Town fuzz Nirvana sounds fantastic and fascinating

DrNomis · Post by **DrNomis** » 01 Jan 2021, 13:11

I built a Tonebender Mk I, which is very similar to the Zonk Machine schematic you posted in this thread, while in the process of building my Tonebender Mk I, I found out that some of the transistors do need to have some leakage in order to work, and bias-up properly in the Mk I circuit, notice how Q1 and Q3 have one resistor going from their base to ground?, this is because their internal leakage forms part of the biasing along with the base resistor, so Q1 and Q3 are the ones that need to have some leakage, because the OC44 transistor you're using for Q3 happens to exhibit very little leakage, it won't work properly because it isn't being biased-on enough due to it's low leakage.