AC125 Circuits and Biasing

[buildafriend]

Hi,

I have probably a few hundred AC125 Tungsol Germanium transistors and I have a transistor tester that tells me Hfe, Vbe, Ic, leakage, and all the special stuff.

Now, I need a circuit that I want to build. I was thinking some range masters would be a fun little mission.

So now, I need to figure out how to bias these out properly. It seems like one of the favorites in the rangemaster is the OC44. an AC125 would work too though right?

Could someone point me at the maths needed to rebias the original rangemaster circuit to work with an AC125?

Here is the schemo

http://fuzzcentral.ssguitar.com/rangemaster/rangemasterschematic.gif

and yes I can do voltage dividers and I know about .7v barrier potenial diode drops and ohms law and the capacitive reactance formulas and fun stuff. I just havent had to apply them in real life. I know what the q point is.. saturation.. cutoff blah blah. I learned about it all on a white board in a school building. it was more boring than building fuzz circuits and making rock and roll. But now how to apply this..

Thanks,
-JP

[duck_arse]

builda - head over to geofex - the link is page top - and look for the technology of the fuzzface, and the dissection of the rangemaster. they explain the hows and whys of each circuit.

[buildafriend]

okay cool bread boarded it that was fun.

Now I want to build a two knob tone bender. How would I bias this thing up? Same method on the voltage divider bias resisters?

[duck_arse]

the fuzzes generally use slightly different biasing methods, depending on the circuit type. use the search page here, find the particular tonebender you're after, and there will be a thread here somewhere with loads of biasing notes. here's one now:

http://www.diystompboxes.com/smfforum/index.php?topic=52341.0

there are others about, try home-wrecker/runoffgroove, and "the other place".

[PRR]

> Germanium transistors ..... ...... ...... 0.7v barrier potenial diode

Better re-check that.

And-- why do you think the other part won't drop-in?

Do you have any clue what Q-point the original ran at?

(Hint: I bet different in every one.)

[buildafriend]

the fuzzes generally use slightly different biasing methods, depending on the circuit type. use the search page here, find the particular tonebender you're after, and there will be a thread here somewhere with loads of biasing notes. here's one now:

http://www.diystompboxes.com/smfforum/index.php?topic=52341.0

there are others about, try home-wrecker/runoffgroove, and "the other place".

Excellent advice! thanks so much duck arse! 

[buildafriend]

> Germanium transistors ..... ...... ...... 0.7v barrier potenial diode

Better re-check that.

And-- why do you think the other part won't drop-in?

Do you have any clue what Q-point the original ran at?

(Hint: I bet different in every one.)

I remember my teacher telling us about how germanium had different barrier potential that silicon.. was it 0.3? some google results say i might be right.

Hfe varies from transistor to transistor. the other day I saw BC550 datasheet said it ranges from 420 800 ( I think? ). thats a bit of a hop for the Q point. Right?

http://upload.wikimedia.org/wikipedia/commons/4/45/CornishMussels.JPG
good question.. GOOGLE TIME.

So you think it will simply drop right in? Didn't work for me last time, maybe I screwed up the circuit then. back to the bread board.

I might add I have many NOS Russian germanium transistors too. problem is, they are labelled and described in Russian.

[M23Bomber]

I agree with PRR i think buildafriend is overthinking a bit.

Just bread Board IT, and then kinda change things to until you like it.

M.

[pinkjimiphoton]

unless you have a ridiculously stable batch of transistors all in the same gain and leakage range ((HAA!!!!!)) you're gonna have to bias each one differently for best results.
easiest way is with a resistor on  the c of the q's. look at what the schematic says for a value, and pick a trimmer where that value falls somewhere around half way up. then you should be able to quickly bias stuff.

don't worry about it that much it ain't rocket science. just use your ears. when ya find the sweet spot between utter fuzz bomb with the guitar dimed and a nice compressed "cleanish" crunch with the guitar rolled back, you're there.

si has a .07v drop. ge .03 if memory serves, which i believe was paul's point.

EDIT: MAKE THAT .6 FOR SI AND .2 FOR GE, mah bad!!

[duck_arse]

builda - there's always a thread ..... a geezer here got to know some cheap russian trannies, lookie:

http://www.diystompboxes.com/smfforum/index.php?topic=108054.0

[pjp - you need to move yer decimal point to the right one spot.]

[pinkjimiphoton]

thanks DA, i realized that just now as i was looking at it.

i'm knee deep in a 63 kay vanguard 704 amp... all ge.... which is presently making me crazy, so i was revisiting jack darr's electric guitar amp handbook just this morning.

[PRR]

> Hfe varies .... from 420 800 ( I think? ). thats a bit of a hop for the Q point. Right?

Is it?

Do some sample calculations.

Take the Rangemaster, but change 470K and 68K to 470 Ohms and 68 Ohms. Try hFE from _1_ to 1,000. The standing current will NOT change (less than 2%). For hFE from 10 to 1,000 it won't change enuff to see on your best meter.

That is UN-realistic, because the bias network eats 30 times more power than the rest of the circuit, and your guitar would faint facing a 68 Ohm load. However so is the idea of hFE as low as 1. I do remember when we could not be sure of hFE>10, but those days are long gone. I think the parts you mention we can assume hFE>50. And since 50*3.9K is "larger" (though not "much-larger") than 68K, we might bet that "hFE>50" transistors will all bias nearly the same.

A RangeMaster is _not_ the type of electronic device that, like an iPhone, *every* unit made works *exactly* the same. Favored musicians would go through a whole box of fuzzes looking for ones with tone they liked.