Rangemaster; Trouble finding trannies that bias

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Rangemaster; Trouble finding trannies that bias

Postby HamontheBone » 28 Jan 2017, 20:58

I'm trying to build the RG Keen rangmaster project from GeoFex. Ive built a test circuit using pots for the two biasing resistors. My buddy & I spent hours dropping in Ge trannies & measuring the collector voltage. Of the hundreds of trannies we tested, the only ones we could get to measure in the proper voltage none of the pots fell within the suggested resistance range. When plugged into an amp they sounded like garbage, spitty/gated.
I feel like with all the trannies we tested we should've been able to find at least two that would work.
We measured the trannies first on dmm, low leakage readings, hfe btwn 70-150. We tried Ac128s, GEs, Russians & even tried some sinos from some small bear ff matched sets.
Of the ones we could get to read above-6v at collector we had to set the pots outside of the usable range.
Maybe somebody else can suggest something we weren't thinking of, or knows other tweeks that can be made to the circuit to get us There.
Any advice would be much appreciated.
I'm sure it's something stupid & obvious we are overlooking.
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Re: Rangemaster; Trouble finding trannies that bias

Postby lolbou » 28 Jan 2017, 21:15

Have you been through this? http://www.geofex.com/article_folders/rangemaster/atboost.pdf

It really explains the bias stuff at page 4...

i built some with Ge transistors tested with a peak DCA analyzer... They did leak, but quite reasonnably, and biased well with stock values...
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Re: Rangemaster; Trouble finding trannies that bias

Postby HamontheBone » 28 Jan 2017, 22:20

Yes sir, that link is the exact document that I was referencing. Thanks for posting the link. I followed his instructions to build the circuit with 100k pot for RB1 & 10k pot for RE. He says RB1 needs to land between 27k & 82k when the collector voltage measures between -6.6v & -7.2v. While the RE should not go below 2k7 or above 5k1.
To be more clear my question is: Is it unusual that from all the transistors I tested none of them would fit the bias specs?
I don't think they are bad. I have built some really good sounding fuzz's using them. My buddy has built a lot of stuff with them too.
Is there something else I should check if I am having these results?
Is there a certain brand of transistor or supplier I should check out to find RM trannies?
If they are suitable for fuzzes, have the correct gain range & are measuring minimal leakage on the DMM then why won't they bias properly?
I can't imagine this circuit would've seen the light of day if it was that difficult to select usuable germs.
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Re: Rangemaster; Trouble finding trannies that bias

Postby mwelch55 » 30 Jan 2017, 21:25

I bought the OC-44 Germanium Transistor from SmallBear. The price is $8.95, which is expensive, but it biased perfectly and has relatively low noise. I bought one off of eBay that was unusable.
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Re: Rangemaster; Trouble finding trannies that bias

Postby lolbou » 31 Jan 2017, 11:17

Still at geofex : http://www.geofex.com/article_folders/ffselect.htm

Try your transistors with this and measure leakage. Leakage can definitely alter bias voltage...

Again, if you dig Ge transistors even more, try the Peak DCA Analyser : it really works well, and with other transistors as well...
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Re: Rangemaster; Trouble finding trannies that bias

Postby dv8r601 » 01 Mar 2017, 00:45

I would check that all my parts are right one by one and check voltages. Its always somethings you've over looked like I had a harmonic perc that wouldn't work no matter what and I had the ground in the wrong spot off board. Make sure pots and all are the right resistance etc. don't get too discouraged bc it will make you go insane when it SHOULD BE WORKING. relax take a breather thebn go back at it..
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Re: Rangemaster; Trouble finding trannies that bias

Postby poiureza » 01 Mar 2017, 16:55

Did you test them for leakage according to the GeoFex article ?
http://www.geofex.com/article_folders/fuzzface/fffram.htm

I have bought tons of PNP Ge's from a german seller on eBay and only a handful had manageable leakage.
Too leaky and it won't bias. I've had much better luck with russian parts (MP21 and similar).

You should anyway try to bias your circuit using the "best" tranny you have, even if that means resistor values way different from those mentioned.
Don't be afraid of values, the only important thing is that you manage to get workable voltage readings.
It will definitely not sound as intended but it might still sound good, who knows ...
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Re: Rangemaster; Trouble finding trannies that bias

Postby RareDevil » 01 Mar 2017, 17:54

Have you tried to follow the values from the schematic and to replace only the 3K9 resistor with any 10K potentiometer (trimmer)!
Never had biasing problem with any transistors in a Rangemaster, and I throw in low gain Ge to high gain Si, and then set trimmer by ear.
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