Hybrid Fuzz Driver

[jay1980]

No. If you need help finding the cause & fixing it, post a request for help in the Troubleshooting forum. Include clear detailed pictures of both sides of the board. But first, do your due diligence and perform a thorough visual inspection checking for incorrect parts, incorrect orientation, bad solder joints, shorts, etc.

Thanks.. Found the problem 1k fuzz potentiometer was garbage.

 

[lespauldoc]

Just curious, what hfe are we shooting for on this Ge?
I have tons of Russian/Soviet mil Ge that can fit this, so may just socket and see which sounds best.... just curious which range skreddy shoots for, and if you all are altering the R10 5.1k at all?

Thanks

 

[Chuck D. Bones]

I did not remove and test the AC127 in the production pedal I bought. All of the AC127s I've come across are leaky. The leakage lowers the effective gain (not hfe) of the last stage. Different Ge transistors will sound different in this pedal. Best bet is to audition a few.

 

[micmac]

Hello Chuck, everybody,

I have the HFD on the breadboard and it sounds good to me. Kind of like this YT video.

I used 2n5088 for Q1 and Q2 on the breadboard. I will order the BC109C transistors for the actual build. I have two AC127 transistors on hand.

I measured these voltages:

Q1 2n5088 E-B-C: 0.86V 1.45V 4.94V Q2 2n5088 E-B-C: 0V 0.57V 0.62V Q3 AC127 (hFE 111, Ie 1.0mA, Ube 108mV, ICE0 64uA, E-B-C: 0.52V 0.62V 3.16V Q3 AC127 (hFE 134, Ie 1.3mA, Ube 118mV, ICE0 0.14mA) E-B-C: 0.53V 0.62V 3.21V

In another thread Chuck wrote Q3's collector should be around 4.5V. But with the two AC127s that I got I only get a little over 3 Volts. Do you think that's a problem?

I can order more AC127, but they're 5 bucks per piece and I don't want to go overboard. And I'm thinking that maybe I only get 3V because I'm currently not using the right transistors for Q1 and Q2.

Kind regards,
Seb

 

[Chuck D. Bones]

Higher gain for Q2 will help a little. Do you have BC549C or MPSA18?

You can also adjust R10 and/or R15. 4V to 5V is the target for Q3-C. Reducing R10 and R15 to 4.7K should get you there.

 

[micmac]

Thanks Chuck!

I don't have these, BJT NPNs I only have 2n5088 left. I'll order some transistors and then I'll recheck.

Kind regards,
Seb

 

[Chuck D. Bones]

BC549C are $0.10 each at Tayda and are a good low-noise, high-HFE Si NPN. I recommend having a quantity on hand.

 

[micmac]

Thanks, I ordered 10. I've seen them mentioned on the forums quite a lot, but I don't remember ever doing anything that would require them myself. I'm in EU and order locally, so mostly retroamplis and Musikding. I get ten BC549C at 0.10 apiece, too. € that is.

 

[micmac]

I have now five AC127 and tested them in RG's tester that I put on vero (DC reversed to account for NPN). Results not very inspiring I think I was able to bring up the collector voltage, though. I needed to go down a little lower than 4k7 for R10/R15.

But seeing the not-so-impressive measurement results for my Germaniums, I decided to try a 2N2222A for Q3 instead. Updated R10/R15 to 8k2. I think 8k85 would be perfect. I'll see if I can find 9k1 in a local shop tomorrow, I need to get some new solder anyway.

It sounds good, the same, a little more grit than with my Germaniums in my opinion. I'll box it up like this then.

Thanks for your help!

Transistor	Leakage	mV	uA	Gain	V	Beta
A		590	238		1.66	107
B		470	190		1.25	78
C		650	262		1.93	128
D		970	392		2.33	136
E		660	266		1.90	124
2N2222A		160	64		1.53	137

Kind regards,
Seb

P.S.: I found it funny that the Si transistor showed up with a little leakage in the tester.

 

[Chuck D. Bones]

Iceo = 64μA is abnormally high for 2N2222A. There is no spec for Iceo. The ones I have measure between 1μA and 5μA, with the majority at 2μA. That amount of leakage makes the device suspect. Where did you get it? A while back, I bought twenty 2N2222A's from a Chinese vendor on eBay and all but 2 were so far out of spec as to be useless. Clearly, they were fugazi and I obtained a refund.

All of those AC127s should be acceptable in an HFD.

2N2222A has more bandwidth than AC127, which will increase the upper harmonics. You can install a cap from B to C to tame the top end if you like. Try 330pF. Some people prefer the hotter sound of Si.

 

[micmac]

Sorry, my mistake. I must have slipped up when I measured the 2N2222A. I redid it, also two 2N2222 metal cans, they all seem fine, low leakage. My 20€ T7 tester also shows no leakage for these (for the Germaniums it does show leakage).

Transistor	Leakage	mV	uA	Gain	V	Beta
2N2222		20	8.09		1.56	154
2N2222		22	8.90		1.54	151
2N2222A		30	12.14		1.47	144

 

[micmac]

Forgot to mention, I don't have the sensation that there's too much top end with the 2N2222A. I have the extra 10k between the 220n cap and the Presence knob if that makes a difference. Presence control works nicely. I like it the way it currently sounds, so I'll leave it like it is.

 

[Chuck D. Bones]

Those leakage numbers are still high. My opinion is that Keen's tester is optimized for measuring Ge transistors and it falls short when measuring leakage below 10 or 20μA. What is the accuracy of your DMM when measuring mV? One final comment: you are measuring voltage to at best 2 significant figures, therefore your calculated current can't be accurate to more than 2 significant figures. The leakage measurements should read 8.1μA, 8.9μA and 12μA.

I rechecked my CCTT's leakage measurements using my Fluke 115 and it indicated <10nA Iceo on a 2N2222A. So even the 1μA to 5μA measurements I reported earlier were high.

That extra 10K resistor makes a little difference in that when you set PRESENCE to 10, it behaves like a stock HFD set to 9.

If it sounds good to you, that's what matters. It's just not an HFD since you modded the design.

 

[micmac]

I'm sure your right on all counts. The DMM I use is my UNI-T UT61E+. During the measurements (mV and V) on the vero the numbers were always jumping around a bit. I didn't care too much, for me they're all ballpark numbers. If you say that for Si transistors the actual leakage will be (way) smaller I have no problem believing that.

I feel a little bit like I hijacked your thread. Apologies for that!

 

[Chuck D. Bones]

It's all good.