Correct Zener Value for Marigold Fuzz

[fgb1967]

Hey,

I built a Marigold Fuzz with an AC127 trannie, and am having trouble getting the correct voltage.

Not sure if it's because it's not an OC140, but these are the Q3 Collector volts I'm getting with the AC127 :

3.3v zener = 0.53v
4.3v zener = 0.98v
5.1v zener = 1.48v
5.6v zener = 1.63v
No zener = 2.16v

I don't have higher value zeners than 5.6v, but will try and source some.

Just curious if anyone has an idea about whats going on.

thanks

Frank

 

[Sturdag Lagernathy]

Did you use the 4.7 v zener the BOM calls for?
Did you mind the pinouts on the transistors? They appear different..
PICS

 

[fgb1967]

Thanks for reply,

I've checked the pinouts several times, they are fine.
I thought you were mean't to find the zener that gives the closest value to 2.6v at the Q3 collector.

regards

 

[mdc]

What's the voltage on the zener's cathode?

 

[mdc]

Also, it might be leakage from either of the two germanium devices throwing things off? OC140s usually have little to no leakage. If you have multiple germanium transistors try swapping one with lower leakage (and if you can't measure for leakage just try a bunch of different transistors until it sounds cool or measures correctly).

 

[fgb1967]

Thanks mdc,

I built it on vero using this layout :

Colorsound Hybrid Tonebender 2018 Edition

Couldn't resist to draw a schematic and a layout for this interesting mutant of a circuit that just got traced. First two gain stages is a...

dirtboxlayouts.blogspot.com

with a supply voltage of 9.3v

using a 5.6v zener (I don't have a 4.7), I get 5.5v on the zener cathode
using a 4.3v zener I get 3.5v on the zener cathode

could I bias it with an adjustable resistor?
why do they use this method with a zener?

regards

 

[mdc]

I think the zener is there to regulate the supply voltage to that single transistor. Sort of a half-dead battery effect but only for one stage.

 

[mdc]

IIRC, Q3 in mkiii/mkiv designs usually end up around 4.5V on a 9V supply, so ~2.5V on a ~4.7V supply would make sense.
I think the leakage of the two germanium devices can mess with the bias quite a lot; so you might want to try subbing those out or adjusting the C-B resistor on Q3. You could even use a 500K trimmer to dial it in I think? The germanium transistors they use in the Sola Sound versions have little to no leakage and gain in the 80/90 range. I'm not sure where AC127s fall—do you have hfe and leakage readings?

 

[fgb1967]

thanks again mdc,
I have 5 AC127's and they test 115 - 155 hFe with 14uA leakage using my TC1 tester, I'm using a 1N34a diode on base to gnd.

I'll try using a trimmer like you suggest, probably next weekend though!