playon wrote:Also, the "driver" transistor is the one with the lower gain, not the other way around.
yeeshkul wrote:They are marked Hfe>25 in the catalogue. As it seems they like to be somewhere between 60-100 ... that's a bit surprising since i supposed they should be either really weak or quite gainy. They are non of that as it seems. Not sure what were the resistor changes for, because this is the range OC75's fit in. The 47k->100k in the Q2 collector should have something to do with gain ... i do not understand the D*A*M guy's statement that they raised the Q2 collector resistance because OC81D had bigger Hfe, it should be other way round. That makes me double-confused. Maybe someone has explanation.
yeeshkul wrote:When i am back from holidays i am gonna put together one unit with 3 really weak trannies just to know how it sounds.
My old unit is L-H-H (low gain - hi gain - hi gain) and i like the sound, but i would prefer more control on the fuzz pot. I guess i will have to try the recomended H-H-L variant, just to know if they differ and how.
Scruffie wrote:And I'm gunna take readings of my OC81's again this evening when I get the chance, 1 of them shows up as 80 on my multimeter which would make sense if it was very unleaky for the reading I got, but one shows up as 200 now I look so the readings I got from the leakage tester could well be wrong as I thought they both showed up okay...
But I will also be building a Tonebender to put them in, leaky or low gain or not, so we can double check yeeshkuls results, perhaps put a 100k/10k on a switch.
yeeshkul wrote:As i said - raising the base res raises the base DC voltage thus opening the BE and CE junctions for larger stretch of the guitar signal than just the tip of the negative swing. That makes signal more efficient thus the fuzz tone more fuzzy.
Scruffy, thank you buddy for checking those rare units for us. Are you sure you know how to get the Hfe out of the R.G's little aparatus?
1. Adjust the trimpot the way R.G. sais or "hand pick" the resistors so they have THE values
2. adjust your DMM for 2V scale
3. apply the 9V battery
now the machine is ready for some mean testing
4. stick the tranny in at the room temperature and let it "breathe" for at least 2 minutes. Even touching the tranny raises up the leakage and will have a hell lot to do with the final result... which is what we do not want .
5. measure the voltage(U1) with the base lifted - wait until the displyed number gets steady
6. switch your DMM to uA and measure leakage current
where 100uA is common
about 200 happens pretty often
>300uA means the device is nothing special
> 500uA means the device really leaks a lot
7. back to the voltage scale - connect the base and get the voltage U2, again - wait until the displyed number gets steady
8. (U2-U1)/100 = Hfe
Scruffy forget about any numbers coming out of your DMM when you try to measure a Ge tranny straight.
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