Tonebender MKII OC75 issues

noiseuoh · Post by **noiseuoh** » 06 Oct 2011, 08:07

hey everyone,
ill just start off by saying thanks to everyone who reads/helps out. absolutely love this forum.
the problem is this. i ordered smallbears OC75 mullard set for the MKII. when i test the HFE's of the trans i get something like 160 for each one. im not sure if its some problem with my multimeter or what. when i test some other PNP germaniums like AC128/122's i get a range like 78, 100... etc like you would expect. i dont know why the OC75s have generally the same HFE and why they are so high.
question number 2. im not getting much response out of my attack (fuzz) pot. there isnt much range in the settings. could this be due to misbiasing?
i also get a hell of a lot of background noise/feedback in almost any attack pot setting, except on very low volumes and the lowest attack setting.
i have added a 10nf capacitor on the input (i read somewhere that it should reduce the noise, and cut out some of the highs) and a 2.2m resistor to cut out the poping. otherwise i have a 100k trimpot on Q2's collector and a 22k trimpot on Q3's (can someone please send me a link or quickly inform me how to test the bias for my trans? i dont know how to do it with my DMM and have been searching like mad today for a simple explanation. i have been testing by ear as of now). i have r1 as 100k instead of the 10k mentioned on fuzz central (the marshall supafuzz OC75) for example.
i am using this layout and schem
http://www.aronnelson.com/gallery/main. ... ewsIndex=1
hope what i wrote makes some sense and thanks a lot for all your help.

noiseuoh · Post by **noiseuoh** » 06 Oct 2011, 08:26

well i figured out the DMM bias testing stuff. just for PNP's i use the red pin on the positive ground and use the black to test. and i put it to the collector yea? thanks

DrNomis2 · Post by **DrNomis2** » 06 Oct 2011, 10:29

If you use a multimeter's Hfe test function to test the Hfe of a germanium transistor, it will tend to show a higher than normal Hfe, this is due to the fact that germanium transistors exhibit something called "leakage", what's happening here is something akin to a water tap with a worn rubber sealing washer, even though the tap is turned off water still comes out of it in drips, when the base-emitter junction-voltage of a transistor is below a certain value, the current flow from the emitter to the collector should ideally be zero,but in reality it is actually a very small amount, silicon transistors exhibit emitter to collector leakage currents in the nano-amps range, whereas germanium transistors exhibit about a thousand times more than silicon transistors, the multimeter's Hfe function interprets the leakage current as being part of the collector current, and thinks the germanium transistor has more gain than it really does, to properly measure a germanium transistor's Hfe you should invest in a Peak Atlas Semiconductor Analyser, because it takes the leakage current into account and can determine what the real gain is....

noiseuoh · Post by **noiseuoh** » 06 Oct 2011, 11:14

thanks,
ive been aware of the leakage stuff. I think its just been misbiased. Too much info on the mkii to condense. I needed the 10k on r1, a 20k on r3 and a 22k trim on r6. Think i got it sorted with the oc75s. Though still not as much response with the attack as i would like

DrNomis · Post by **DrNomis** » 06 Oct 2011, 12:17

noiseuoh wrote:thanks,
ive been aware of the leakage stuff. I think its just been misbiased. Too much info on the mkii to condense. I needed the 10k on r1, a 20k on r3 and a 22k trim on r6. Think i got it sorted with the oc75s. Though still not as much response with the attack as i would like

I actually built a MkII once and the tone was pretty gainy even at minimum attack settings, and it got more saturated when I maxed out the attack...